Volume 3 Issue 3 - Diesel Articles

I haven’t been this excited about a new truck model since the Duramax engine debuted in 2001. Back then, the new GM diesel powerhouse represented an exciting leap forward in torque, horsepower and reliability. GM market share leapt forward as well, from an abysmal three percent to 30 percent. Looking for improvements in the new 2011 LML Duramax vehicles, we might expect to find them in the engine and transmission. The truth is that while there are some significant improvements to the powertrain, the Duramax engine has become a very refined engine with little refinement necessary. Why am I so excited then? The chasis!

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The chassis of the 2011 GM 2500 and 3500 “Heavy Duty” trucks are markedly stronger - and stiffer – than previous years. © GM Corp

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Pretty well all of my complaints of the old chassis have been addressed. And GM has again, in my opinion, left Ford and Dodge behind with plain intelligent engineering. Yes, I live and breathe GM products, but there is a reason I am a Chevy man! Perhaps someone over at the competition will wake up and realize what “unsprung weight” is and get rid of the clunky floating axles that they insist on using in their four-wheel drive heavy-duty trucks.

A little explanation:  unsprung weight, or more accurately, unsprung mass, is basically the mass of the suspension components that are in contact with the road.  When a wheel approaches a bump in the road, it attempts to accelerate these components upward.  The heavier those components are, the more resistant to this acceleration force they are.  However, they have to yield to the force applied by the road surface and move upward.  When the bump levels off, the acceleration force applied levels off, but now these components have momentum and their upward motion has to be stopped by the springs.  The heavier the suspension components, the stiffer the spring has to be to keep the components under control.  Naturally, the stiffer the springs, the more of that force is applied to the chassis and eventually to the passengers, contributing to a rougher ride.

That is not the only downfall of a heavy full axles in the front suspensions of a Ford or Dodge – imagine how much more impact force is applied to the steering knuckles in order to make all that axle weight track the road surface.  Therefore, the ball joints that support the steering knuckles need to be very stout to resist premature wear.  In practice, the rougher the road, the harder it is on the front ends of a Ford or Dodge heavy duty 4WD pickup.  Here in the northern oilfield country, most mechanics will agree that a Ford or Dodge front end will wear out just as fast, if not faster, then the more sophisticated double A-arm independent front suspensions underneath a GM truck.  And that is to say nothing about how much better the GM setup rides and handles.  It doesn't take much imagination to realize how much less mass is involved in the front suspension of a GM truck.  This besides the other wheel-control and steering angle stability advantages that an independent suspension has over a solid-axle setup.  In fact, the advantages of the GM front suspension are inarguable – there is a reason that GM continues to produce this more complex and more expensive front suspension system.  Ford and Dodge have long since abandoned the solid-axle in their half-tons.  The only good reason I can think of that they continue to use the solid-axle in their heavy-duty trucks is because it is so much cheaper to produce.

At the GM dealership where I work, we have run a demo 3500HD Silverado through the fall, towing various loads in a fairly large trailer. Our experience with the truck has been primarily towing a race-truck that we built over the summer. It has been a trouble-free, comfortable runner. If you thought your current truck handles loads well, think again. These trucks are engineered and built for hauling, period. In fact, you need to adjust your expectations before you test-drive one of these new trucks in order to properly appreciate the changes. Adjust, how exactly?

Built for a Purpose

In the past, those of us who wanted a daily driver with a GM diesel under the hood stepped up to a 2500-series truck because the half-ton had no diesel option (and still does not). We might occasionally pull a good-sized trailer, but most of the time the truck was used simply as a glorified half-ton The 2500s were always built and marketed as “Heavy-Duty” and compared to the half-tons this was a fair designation. While the 2500 rode a bit harsher than the half-ton, it was not too uncomfortable. Some drivers even changed out the shocks for a softer valved unit to improve the ride a bit. This paradigm, however, has vanished with the new heavy-duty trucks. The old ambiguities between the half-ton and the 2500 suspension have been decidedly resolved. If you are considering using one of the new heavy-duty trucks as an unloaded daily-driver, I suggest taking it for a test-drive first. The improved suspension and chassis in these new trucks were designed with one purpose in mind: hauling a payload, not running around empty.

These trucks ride stiff. I mean the kind of stiff that you would expect to find in a truck with such a prestigious nameplate as Peterbuilt. Driving a crew-cab standard-box 3500HD around without any load, I found myself wishing for a good chiropractor or maybe an air-ride seat. Now I should put that statement into context – these trucks are considerably more compliant than a Kodiak/Topkick medium-duty truck and quite comfortable by comparison. And they certainly do not rearrange your internal organs like the Ford F-450 I had the displeasure of driving a few weeks ago. But they ride noticeably stiffer than the previous generation of heavy-duty trucks. The rear springs are significantly wider (3-inch as opposed to the old 2-1/2-inch), longer than the older trucks and the shock valving is also more restrictive to control sway from heavy payloads. These factors accommodate the higher payload rating of the new trucks. Add in the much stiffer chassis and we have the makings of a chassis ‘beaming’ problem. Just what is chassis beaming?

Think of the chassis itself as a big spring. There has to be some flex in all that steel and there is. The chassis will tend to bounce when flexed: the stiffer the material the higher the frequency of the bounce gets. The new chassis is fully boxed, with a larger cross-section in various areas. It has less total flex, but it does still move a bit. With a stiff suspension providing the kick to bounce the chassis, you can get quite a judder when driving unloaded over seams in the pavement, potholes and such. To GM's credit, they have installed specific hydraulic-style body mounts to improve cab isolation, a very good move that allows for more comfort in these trucks than they would have without them. However, I've driven quite a few configurations of these trucks unloaded, gas or diesel and they all manifest a degree of chassis 'beaming'. The smoothest riding of the various configurations is the 2500HD, and it is still noticeably stiffer than the old model. The 3500HD is much less compliant. But when you are engineering a truck to handle payloads, it is a necessary compromise to create the 17% higher GVWR, on average.  Of course, when the truck is loaded, it minimizes the chassis beaming and has a much more agreeable ride.   These trucks were not built simply to run around unloaded.  If you want to do that, think seriously about a half-ton or, at least, a pre-2011 Duramax. However, having said all that, you will not find a more capable heavy-duty truck around, in my opinion. These trucks really come into their own when you start loading them up – which is what they were designed to do.

Front Suspension

Moving along to the front suspension, we find major changes designed to beef it up for the increased payload capacity. A prominent feature is the much longer spindle that increases the distance between the upper and lower ball-joints. This increased distance creates a better load distribution across the front suspension components and frame. Also, the upper control arm is now made from cast iron, improving wheel tracking stability due to its increased rigidity. GM is now using several different specific torsion bars, which allow different spring rates depending on the application, in an effort to improve front end ride quality for each GVWR configuration. The steering linkage, tie rods, tie rod ends, pitman, idler arm and center link are all much beefier. Expect these front end parts to last much longer than the old ones, which had about a 100,000-mile lifetime when maintained well.

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The new front differential assembly appears much more robust than the old one. I have found that the new rubber mounts make it much quieter in operation as well.

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A major complaint of the older trucks was the fact that the power steering was just barely powerful enough at times to turn the wheels on dry pavement. Not a problem anymore. The enlarged power steering box and pump are all set up to allow the driver to turn the wheels on dry pavement, parked, with the front axle at its full weight capacity. Honestly, I would have expected that of the old trucks. At any rate, this complaint has now been addressed properly.

The front driving axle in the four-wheel-drive models is radically different than the old one. Gone is the old two-half aluminum housing. Now we have a single-piece housing casting and a more conventional differential cover. It certainly looks tougher but uses the same size 9.25-inch crown gear. It is also better mounted and isolated. Four-wheel-drive operation is quieter and produces less vibration than the older trucks.

The rear driving axle also has several minor improvements, including larger hubs and wheel-hub bearings. These upgrades handle the higher gross vehicle weights of which these trucks are capable. The old 11.5-inch rear differential was always plenty to handle the torque output of the powertrain and essentially remains unchanged inside.

Whoa I Say, Whoa!

The most pleasant surprise for me with these trucks is the new brakes. I had to laugh when I found out that the front brake rotors on my Grand Prix GXP actually weighed more than the front rotors on my 2005 K2500 Silverado. I’m serious – my car weighs less than half as much as my truck does, does not carry any payload other than passengers and it has more braking power! Give me a brake.. and they did. Whoever finally made the decision to put some real brakes in these trucks needs a promotion and an all-expense paid vacation to some magnificent Caribbean island. Someone needs to be congratulated here. The brakes were by far my biggest complaint with the old trucks. Actually, they have been my biggest complaint with all GM trucks for the last 30 years. In fact, I am still researching viable braking improvements for the old trucks.

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The new truck REAR rotor is on the left. The old truck FRONT rotor is on the right. There is simply no comparison in the brake capacity.

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Thankfully, there is some serious iron in the new brake rotors and the calipers. They are now 14 inches in diameter as opposed to the old 12-inch units. You will note that the calipers on the rear rotors are now intelligently placed behind the axle instead of in front of the axle where they used to pick up every bit of rock, mud and debris on the road. The brake feel is substantial and positive. You have no problem gauging braking force. The old brake calipers would flex a bit, creating a non-linear brake feel. The new brakes give you the feel that the force you are exerting on the brake pedal is translated proportionally to the brake rotors.

And the base brakes are not the only thing providing stopping power. After running a turbo-vane based exhaust brake since 2006 in the Medium-Duty trucks, GM has finally figured out that maybe they should do that with their pickups. I am almost embarrassed that it took so long for them to introduce this feature when they had the capacity for it that many years. All whining aside though, with a simple software-based modification, we now have a very effective exhaust brake with no more moving parts than the truck had previously. A switch on the dash enables or disables the exhaust brake. The grade-braking feature of the Allison has been re-calibrated to make use of this extra stopping power and is very well integrated. An exhaust brake that works as well as this one may make a guy wonder why he towed without one for so long!

The Mighty LML Duramax

While Ford and Cummins seem to have been tripping over themselves to produce completely new engines, the basic design of the Duramax itself remains unchanged. It is now rated at 397 HP @ 3000 RPM and 765 lb-ft of torque @ 1600 RPM. These numbers are not “class leading” hype but real world testing. Experience shows that GM is at least reporting honest and consistent numbers. Some of the chassis dyno work I have seen with these engines shows a variation of only about one HP from run-to-run. For those worried about having class-leading power numbers, don't be too quick to fully believe the 420HP/800lb-ft designation on the Ford -   Diesel Power Magazine published a test that is pretty conclusive and quite embarrassing for the Powerstroke! There have been minor incremental improvements to help with block structure, continuing Isuzu's tradition of heavy over-engineering. The block structure before was not really a problem in my mind, but these changes do help with reducing noise and vibration. The oil pump is now larger and main bearing size has been changed. The connecting rod and piston design have also been changed to improve strength and reduce rotating mass.

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Somewhere under all that is the familiar Duramax we all know and love, with a few modifications.

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Two of the most significant changes are external to the engine itself. One is a much more complex EGR valve system. It includes a bypass passage to reduce oil coking and soot buildup in the intake system – a positive change that probably would have been nice to have some time ago. On the pickup trucks, dual coolers are involved to allow for higher EGR flow as well. The EGR valves themselves are driven by stepper motors with sensors that are positively attached to the valve and read true valve position. These changes, subtle to us, allow the EGR system to operate more precisely and keep itself clean. Increased EGR flow is one way to reduce NOx emissions during the combustion process itself.

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At the bottom left is a glimpse of the new EGR solenoid and EGR system. Note the Orange wrapping on the wiring - that is to warn of the high voltage wires therein used to trigger the fuel injectors.

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The other major change is the new piezo-injectors. These injectors are more precise and much faster than the previous solenoid-based injectors. You will notice this change right away while the engine is idling. It sounds different and is even smoother than they previous LMM engine which was pretty smooth in its own right. They operate at higher pressures, up to 29,000 PSI. You may notice orange markings on various wire harnesses under the hood – this is to indicate high voltage. The new injectors require a trigger of around 160 volts and use a hold voltage from between 140-250 volts. The applied current is quite high, around 20 amps. Not a good idea to punch into one of the injector wires during operation! Also, a new fuel injection pump is used that is specially timed to allow the pump output pulsations to align with injection events. Two fuel pressure regulators are used to allow for more precise control of fuel pressure. It is also worth mentioning that this new fuel system is now rated as B20 diesel compatible.

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Cutaway of the new LML Duramax Fuel Injector

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The Ninth Injector

While we are talking about fuel injectors, we must include mention of the new post-injector. It is a separately controlled fuel injector that places fuel directly in the exhaust stream for regeneration events. This is a far superior method of regeneration than the previous LMM engines. The LMM’s used late main injection and post-injection events to add raw fuel to the exhaust stream. There are two major problems with that method: One is the reduction of engine mechanical efficiency, basically not only are you throwing fuel out the exhaust manifolds, the fuel that you are using to run the engine is not being converted to mechanical energy very efficiently either. The second problem is fuel-wash on the cylinder walls themselves, contaminating engine oil and potentially contributing to cylinder and/or piston ring wear. Installing an injector in the exhaust stream after everything including the turbocharger eliminates both these problems. Now the fuel used for a regeneration is used efficiently and the engine can continue to operate efficiently with no fuel contaminating the engine oil.

An interesting addition is a fuel filter restriction sensor. With this sensor, the ECM can now determine if there is an excessive restriction on the fuel supply side, usually caused by a plugged fuel filter. This extra sensor and the warning it triggers may help reduce potentially expensive failures caused by fuel filters that haven’t been serviced properly. Of course, it is up to the owner to properly maintain the fuel filter in order for this feature to be useful.

Allison – THE Diesel Transmission, Period.

Again, other manufacturers have tripped over themselves to produce a half-decent automatic transmission to go behind their diesels. We have Allison. Strong and steady since 2001, only minor incremental changes have ever been made to this transmission. Even the five-to-six-speed transition had been planned, in an attempt to improve fuel economy while they made minor changes to deal with the increased torque output from the Duramax. Allison built that transmission as a four, five or six-speed combination depending on what the manufacturer ordered up. The only real change is in the valve body and the control software. Incremental changes have been made to clutch packs and valving in order to refine the transmission further and to allow it to deal with more torque output. For 2011, not much has changed other than some refinements to the control strategy to deal with the new engine-braking feature.

The New Chemical Conversion Plant

I’m not kidding about that. The exhaust system is the most complex piece of emissions hardware I have ever seen, compete with its own computerized sensor suite. The regular catalytic convertor is still there. The diesel particulate filter is still there. Behind the diesel particulate filter is something new, another type of catalytic convertor that is used to lower the NOx emissions. Before we get into the technical details of how the extra catalytic convertor operates, it would be good to brush up on the exhaust system on the LMM Duramax in this article.

The Achilles heel with any diesel engine is its propensity for producing NOx. High cylinder pressures and temperatures cause atmospheric Nitrogen to bond with Oxygen, creating various oxides of Nitrogen. Oxides of Nitrogen are a major cause of smog and acid rain and other health concerns. Prior to 2011, Tier 1 standards required that the maximum limit for producing NOx was 1.2 grams per brake-horsepower per-hour. Now, the standard is 0.2 grams per brake-horsepower per-hour. To get a diesel engine down to that level requires considerable engineering gymnastics. By comparison, gasoline engines have a relatively easy time of reaching that emissions level.

One of the major boons for emissions reduction since the 1970s has been the catalytic convertor. For a diesel engine, it is a simple device that uses a ceramic bed coated with the precious metals platinum and palladium. These metals chemically assist unburned hydrocarbons to combine with free oxygen producing harmless carbon dioxide and water vapor. The unit also assists carbon monoxide molecules to find free oxygen and combine to form carbon-dioxide. A side effect of the reaction is heat. This is the primary function of the catalytic convertor located at the front of the LML Duramax exhaust system. The heat generated is put to good use to regenerate the diesel particulate filter located immediately downstream. Under normal operating conditions, a ‘passive’ regeneration is always present due to the heat output of this catalyst. However, as the diesel particulate filter inevitably plugs with soot, the ECM commands the special fuel injector mentioned above to provide fuel to heat the first catalytic convertor further, burning down the soot in the diesel particulate filter to fine ash. Together, the catalytic converter and the DPF take care of several of the major pollutants from a diesel engine: unburned hydrocarbons (raw fuel vapor), carbon monoxide and fine particulates (soot). If only it was that simple to reduce oxides of nitrogen.

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Basic Diagram of the new LML Duramax Exhaust System

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Enter yet another catalytic convertor. Gasoline engines can use a catalyst with palladium and rhodium that works quite efficiently for NOx reduction - if the engine is able to maintain a specific fuel/air ratio. Unfortunately for diesels, we do not have any simple combination of metals that will “magically” facilitate a chemical reaction to reduce NOx to oxygen and nitrogen. However there is a solution that sounds simple but ends up being rather complicated to implement. A diesel engine can use a process known as selective catalyst reduction, or SCR. If the chemical makeup of the engine exhaust itself is altered, a specific type of catalytic convertor can reduce the oxides of nitrogen to simple oxygen and nitrogen. So how do we change the chemical composition of the exhaust?

Would you believe – diesel exhaust fluid? Honestly, if someone even 10 years ago were to tell me to go down to the local Esso and buy diesel exhaust fluid, I would have wondered what kind of joke he was trying to pull. Laugh as we may, that is exactly what we are dealing with now. Diesel exhaust fluid comes in different forms, but in the case of the Duramax it is composed of a mix of ammonia and pure water. The ammonia mixes with the exhaust, allowing a catalytic convertor with another group of precious metals to take the ammonia and oxides of nitrogen and chemically convert them to water vapor and free nitrogen. Sounds simple enough, but it’s not that simple in practical application.

In order to deal with diesel exhaust fluid, we need a tank, a pump, level sensors, fluid quality sensors, sensors to tell us how much NOx is in the exhaust before and after the SCR and a couple more exhaust gas temperature sensors. And just to make this interesting, we have to remember that this mixture of ammonia and water we call diesel exhaust fluid freezes at 12*F. Of course, it is inherently obvious that these trucks will often be used in temperatures colder than this. So now we need a whole group of heaters to make this thing work. And we are going to need a whole lot more computer power to control and monitor this whole operation. For that reason the LML Duramax is equipped with a new Bosch E86 ECM.

This does have implications for the consumer. Diesel Exhaust Fluid (DEF) is now going to be a regular part of life for the Duramax. Fortunately, in optimum conditions, the tank of fluid will last around 5,000 miles and is relatively inexpensive. The capacity of the DEF tank is about 20.1 liters, or 5.3 US gallons and costs around twenty bucks for two gallons. Even so, for some of us it will prove to be an annoyance to have yet another consumable fluid to worry about. The other, less obvious, consumer implication is the added complexity of the system. When you add components to any system, you add more possibilities for failure. And because this is such a critical piece of emissions hardware, any failure at all will result in a major inconvenience for the customer while he waits for repairs. The ECM is programmed with several modes that can eventually shut the vehicle down to 4 MPH with a failure of the DEF system. In other words, if you run out of DEF many miles from nowhere, you better have an extra jug on board to get you back home in a reasonable time frame.

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Top view of the rear part of the new LML Duramax Exhaust system. You can see the NOx sensor, the EGT sensor and the DEF injector and line coming into the middle of the SCR/DPF assembly.

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We also should talk about the tamper modes. GM is well aware of the aftermarket’s desire to remove exhaust emissions equipment. So they have introduced tamper modes that will eventually shut down the truck should any of the DEF system components be altered, removed, or the diesel exhaust fluid itself substituted with something sub-standard. It operates on a “warning level” system. (see chart) Even if you have the ability to clear DTC’s, these tamper modes latch in and will not clear until the ECM can see that the component in question has been restored to proper working order. Modifying this system, or even installing a power program on the LML Duramax, has become a whole new issue covered in another article.

Driving Impressions

We have all heard the expression: “that truck pulls so well I didn’t even know the trailer was there.” These trucks take that statement to a new level. As mentioned before, the brakes are fantastic. Fade free, there is little to worry about even with tremendous loads behind. And then we have the exhaust brake. Actually the exhaust brake on the LML Duramax is a bit friendlier than the one on the LMM and LLY medium-duty trucks. The medium-duty trucks had an exhaust brake that was either all-on or all-off. The LML has a much more linear exhaust brake that is load sensitive. Driving empty, you may not hardly know it is on. Put the transmission in tow-haul mode, and it becomes more aggressive and the downshifts come more often, but it is still not obtrusive. Now put a load on or hitch up a trailer and this thing holds back hard! Using the hydraulic brakes on the highway becomes almost completely unnecessary. You have to appreciate the thoughtfulness that went into the integration of the exhaust brake and the transmission.

As far as the actual power is concerned, it is comparable to the older LBZ and LMM with a mild power-program. Accelerator pedal feel is a little different – the LBZ’s in particular would pound you in the seat with mild pedal application. These tend to be more linear and requires more pedal if you need all that 765 ft-lbs of torque. I don’t mind that at all – in fact – this is one of the things that I would modify with my custom tuning because cranking all that power in the first 30% of pedal travel would make big-power tunes hard to drive.

The sound of starting the engine cold gives me a bit of an eerie feeling - it tends to clatter a bit like an older diesel, only with a sharper tone. It will take me a bit of getting used to. It just doesn’t quite sound like a Duramax at that point. Warm the engine up however, and you get a very quiet and smooth engine. In point of fact, I started a warmed-up LML Duramax one dark morning here and thought I was driving a gas engine for a moment. All you feel in the cab is an ever-so-slight vibration that lets you know that the engine is in fact running.

Normal steering feel is solid and similar to the older trucks, slightly numb on-center. I’ve never really liked that. I actually custom-aligned my old truck to take some of that away and make it feel tighter and more responsive. This is no real major change from the older trucks, other than the fact that there is no additional effort required to steer the wheels when stopped on dry pavement. Load the truck up, on the other hand, and the steering feel remains much more consistent than the older trucks. That’s also a benefit of the much longer distance between the upper and lower control arms. The camber and caster angles do not change as much with a load on, again, giving the impression that you really don’t have much of a load on.

The interior is not much different than 2010, only with the addition of the exhaust brake enable button and some extra driver information center messages pertaining to the diesel exhaust fluid system. Otherwise the truck is quite comfortable with a trailer or load on, allowing you the luxury of ‘forgetting’ it was there. Let me put it this way: If Mercedes-Benz built a heavy-duty pickup truck, this is probably the way it would drive and handle a load. Forget it Ford and Dodge – you will need to get your truck suspension and chassis out of the 1950’s to keep up with these new GM offerings!

My Opinion

If you are looking for a new heavy-duty truck to tow or haul massive payloads, give these trucks serious consideration. Even with the 6.0 liter gasoline engine, the chassis itself is far more capable of heavy hauling. The jump in capability is quite noticeable, from the brakes to the steering and even the stiffer frame. It adds up to increased safety and ease of use when dealing with large payloads. To describe the difference of these trucks to the old ones, it would be like somewhat like comparing the hauling capacity of the older Kodiak/Topkick medium-duty trucks with the Sierra/Silverado pickups – only these new 2011 pickups are quite a bit smoother and friendlier than the old medium-duty trucks. Still, they are heavy-haulers and not really that suitable for just running around. If you are looking to buy a Duramax and you don’t have heavy-hauling in your agenda, it might suit you better to wait for a half-ton diesel. I do expect that GM will turn its attention to the 4.5 Duramax project at some point, they have too much invested in it to just leave it laying on a shelf.

Again, the chassis is the real story here, even though we are dealing with some significant changes to diesel emissions systems. The engine itself is evolutionary, building on the strengths of the Duramax engine that we all know and love. Now, it has a chassis that is beginning to match it’s capability. Driving it at its near 400 horsepower level feels similar to an older LMM with a minor power programmer in it. It is at the point now where power is really not a big worry anymore, at least in my mind. Sure, I love more power as much as the next guy, but you may not find yourself really needing it or worrying about it. For the loads that this truck is meant to carry, 400 horsepower and 765 lb-ft of torque will do the job beautifully.

Yes, we do have to the addition of Diesel Exhaust Fluid. In all honesty, the increased complexity of the emissions systems does result in a real improvement in tailpipe emissions. Diesel emissions are now low enough that they produce the same emissions, per horsepower, as a gasoline engine. So when we look at this from a big-picture standpoint, there is a benefit. However, we, as consumers, now need to be aware of the inconveniences and operational changes that we need to accept in order to happily own a 2011 Duramax powered truck. It goes back to the expectations discussed in the introduction – set them in the right place and you will be more than happy with the sum of the improvements with these trucks.

DEF System Warning Messages

The 2011 LML Duramax adds a whole new dimension about it that the customer needs to be aware of. The Diesel Exhaust Fluid (DEF) system is complex and requires a certain level of customer interacting. Fortunately, your instrument panel display will let you know exactly what is needed. The following is an excerpt from the Duramax owners manual. And yes, if you let this system go without responding to the messages you may end up being speed limited down to four MPH!

Exhaust Fluid Low Warnings

A full DEF tank will last for several thousand kilometers (miles), depending on vehicle usage. As the exhaust fluid level drops, warnings will be displayed in the Driver Information Center (DIC). There will not be any DEF range DIC messages with more than 1600 km (1000 miles) of DEF range remaining. DIC messages will begin with approximately 1600 km (1000 miles) of range, and as the tank becomes empty, vehicle speed limitation will result.

To avoid vehicle speed limitations, the DEF tank should be refilled at the first opportunity after a low warning indication. If DEF is added before the EXHAUST FLUID EMPTY message appears, it may take several km/miles for the DIC message to update.

If the vehicle speed has been limited and DEF has been added, it may take up to 30 seconds after engine start with the vehicle stopped for the EXHAUST FLUID EMPTY message to clear. If the vehicle is driven prior to the DIC message clearing, the vehicle speed will still be limited. If the DIC message clears while driving, the vehicle must be completely stopped to remove the speed limitation.

If DEF is added under freezing conditions, additional time may be required to remove speed limitations.

A DIC message EXHAUST FLUID RANGE: XXX will be displayed at approximately 1600 km (1000 miles) of fluid range remaining. This message will appear again at approximately 500 km (300 miles) of range remaining before the exhaust fluid tank becomes empty.

Below 500 km (300 miles) of range remaining, these messages will appear every time the vehicle is started.

If these warnings are ignored and the DEF tank becomes empty, a DIC message 89 km/h (55 MPH) MAX SPEED UPON RESTART will be displayed. There will also be a warning light and chime. The vehicle will be speed limited to 89 km/h (55 MPH) at the next restart.

When adding DEF to an empty or very low tank, always add at least 3.78 liters (one gallon) of fluid to release the vehicle from speed limitation. The capacity of the DEF tank is approximately 18.9 L (5 gal).

If the vehicle continues to be driven without filling the DEF tank, it will be limited to 7 km/h (4 MPH) after the second fuel fill.

Exhaust Fluid Quality Poor

Use only exhaust fluid that is GM approved or fluid containing the API certified or ISO 22241 label.

All DEF has an expiration date. If the system detects poor quality, contaminated or diluted DEF, the DIC will display EXHAUST FLUID QUALITY POOR. SEE OWNERS MANUAL NOW. Adding fresh DEF to the system may resolve the problem, depending on several factors. If the DIC message persists, see your dealer.

After 200 miles of driving without correcting the problem, a DIC message 55 MPH MAX SPEED UPON RESTART will be displayed. There will also be a warning light and chime. The vehicle will be speed limited to 55 MPH at the next restart.

If the problem is not resolved after 500 miles of operation, speed will be limited to 4 MPH after the next fuel fill.

Service Exhaust Fluid System

If a problem occurs with the DEF system, the DIC will display SERVICE EXHAUST FLUID SYSTEM SEE OWNERS MANUAL NOW. In some cases this message will clear itself, indicating that the DEF system was able to correct the condition. If the DIC massage persists, see your dealer.

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You do not want to see this message come up on your dash!

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After 200 miles of driving without correcting the problem, a DIC message 55 MPH MAX SPEED UPON RESTART will be displayed. There will also be a warning light and chime. The vehicle will be speed limited to 55 MPH at the next restart.

If the problem is not resolved after 500 miles of operation, speed will be limited to 4 MPH after the next fuel fill.

LML Duramax Current Service Issues

One of the very first questions we asked GM engineering as technicians was how well the new Diesel Exhaust Fluid (DEF) system would handle sub 10°F temperatures. Essentially, they made this reply: “Don’t worry about it at all. Extensive cold-weather testing has been done on this system, it is engineered to allow for freezing and thawing cycles. The tank has heaters in it and the lines and pump have heaters to enable operation within minutes of a cold-start. You can ease your mind, tell your customers this system will be trouble-free in cold weather.”

Baloney. I’ve heard a story of this nature before – sometime before the Titanic sank.

I can see they put at least moderate effort to engineer the system to operate in colder temperatures but they sure didn’t work out the bugs before these trucks hit the showroom. Our first bout of sub -10°F weather proved that. We had brand-new trucks right on the lot that hadn’t turned a wheel suddenly coming up with the infamous ‘service exhaust fluid system’ messages. What’s going on? Essentially, a whole series of errors.

Stuck “Exhaust Fluid Low/Empty” Warning Messages

The most troublesome problem we have had is an issue where the ECM will get stuck reporting a low DEF fluid level even though the customer has filled up the DEF tank. This happens more frequently in colder climates, with the failure frequency increasing the colder your average ambient temperature is. Interestingly, GM Engineering is currently working with one of the Northern Alberta, Canada dealers that has huge numbers of the 2011 LML Duramax trucks out there to do real-world cold-climate testing on. Too bad that didn’t happen before this glitch came up. Of course, when the ECM gets stuck reporting a low DEF fluid level, it will work its way down through the warning levels until the truck gets limited to 55 MPH and eventually 4 MPH even though the DEF tank is actually full.

The problem? A glitch in the ECM’s operating software. That’s no easy task to fix as practically the whole operating system has to be re-written to solve the glitch. GM has been releasing calibration updates to help minimize the frequency of the problem occurring, however these updates cannot actually solve the basic problem. GM Engineering (as of February 2011) is currently working hard to re-write the operating software to eliminate this problem. So what does a customer do? If you ever experience this issue with your truck, by all means go in and get the current calibration updates installed, but be prepared to wait a while to get your truck back. The dealer has to go through quite a series of steps, thawing the tank out, draining and ensuring the DEF tank is fully empty, reflashing the ECM with a special calibration, refilling the DEF tank, verifying the ECM reset its “latch” and then reflashing the ECM again in order for this repair to work. In Canada, this takes a while as most any tank we encounter with this condition is frozen solid. After this process is completed, the frequency of this problem occurring again is reduced, however it can still occur again in the right combination of conditions. GM has promised that the new operating software should be available by April of 2011.

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A DEF tank removed from the truck that needs thawing out. Notice the frost formed on the outside of the tank.

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If you haven’t experienced this issue and you do not live in a cold climate that sees regular temperatures below 10°F, don’t go in for the update – wait for the new operating software to come out. GM will most likely introduce a recall campaign at this point, which may also include some other hardware upgrades for the DEF tank itself. When the new operating software comes out, the “thaw, drain, reflash, fill, reflash” process will not be necessary and programming in the new operating software will not take long at all.

“Service Exhaust Fluid System” Warning Messages

If you get this message coming up, you will also have a Service Engine Soon lamp illuminated. This is generally a result of a DTC P204F Reductant System Performance. With the initial calibrations, an operating condition comes up that the engineers for some reason didn’t anticipate. If the truck is started at or below -22°F, the DEF system is disabled. However, if the ambient temp comes up a couple of degrees, say -18°F, the DEF tank, pump and line heaters turn on. Now, if the truck has been operating for more than 90 minutes, the DEF system will begin to operate. At this point, not enough DEF is actually thawed out in the tank for the system to pump it properly and maintain pressure, therefore the DTC sets. This condition happened quite often around here and caused considerable grief. The newer calibrations provided to this point provide a fix to this, extending the run time to 270 minutes to allow for more thaw time before DEF system operation was allowed in these cold temperatures.

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Breakdown of the Diesel Exhaust Fluid tank, located on the outside of the passenger side frame rail. 1-Wire Harness. 2-DEF Pump Assembly. 3-Lock Ring. 4-Seal. 5-DEF level sensor/heater assembly. 6-Electronics Module. 7-DEF tank.

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There are a number of other DTCs in addition to P024F that have been encountered in the field, too numerous to mention here, that have been addressed by calibration updates. The only problem with doing any calibration update at this point is that the dealer has been obligated to go through the “thaw, drain, reflash, fill, reflash” procedure, regardless of what kind of ECM update is required, due to the operating software glitch that can cause a Low Exhaust Fluid Level warning to latch. Suffice to say, if you are not encountering issues at this point, don’t worry about it until GM releases the new operating software.

Another thing that can cause issues is using a winter front other than the one that GM supplies. Turns out that if you cover up the small bumper holes, you can induce DEF system trouble codes because this affects it’s ambient temperature measurement.

So, how “extensive” was GM’s cold-climate testing? You be the judge. To their credit, it would seem that GM’s software engineers were working overtime trying to resolve all of these strange issues. We have had calibration updates practically once a week, sometimes more often, in an effort to reduce the frequency of all these DTCs. We have a couple of customers out there running their trucks full-time, and they appear to be having no problems after the last batch of calibration updates from the end of January 2011 at this point, but time will tell.

Chart: 2010 vs 2011 GM Heavy-duty Truck Specification Comparison (all units in pounds and assume Duramax/Allison powertrain, other truck configurations available, table is for comparison purposes only)

	2010			2011
2WD 2500 HD	GVWR	Ball Hitch	5th Wheel /Gooseneck	GVWR	Ball Hitch	5th Wheel/Gooseneck
Reg Cab/Long Box	9,200	13,000	15,800	9,900	13,000	17,800
X-Cab/Standard Box	9,200	13,000	15,500	9,900	13,000	17,500
X-Cab/Long Box	9,200	13,000	15,300	9,900	13,000	17,400
Crew/Standard Box	9,200	13,000	15,300	9,900	13,000	17,400
Crew/Long Box	9,200	13,000	14,500	9,900	13,000	17,200
4WD 2500 HD	GVWR	Ball Hitch	5th Wheel/Gooseneck	GVWR	Ball Hitch	5th Wheel/Gooseneck
Reg Cab/Long Box	9,200	13,000	15,500	9,900	13,000	17,500
X-Cab/Standard Box	9,200	13,000	14,500	9,900	13,000	17,200
X-Cab/Long Box	9,200	13,000	13,800	9,900	13,000	16,500
Crew/Standard Box	9,200	13,000	13,900	9,900	13,000	16,700
Crew/Long Box	9,200	13,000	12,700	9,900	13,000	15,600
2WD 3500 HD	GVWR	Ball Hitch	5th Wheel/Gooseneck	GVWR	Ball Hitch	5th Wheel/Gooseneck
X-Cab/Single Wheel	9,900	13,000	15,400	11,200	13,000	17,300
Crew/Single Wheel	9,900	13,000	15,100	11,400	13,000	17,300
X-Cab/Dually	11,400	13,000	16,300	13,000	16,500	21,500
Crew/Dually	11,400	13,000	16,200	13,000	16,500	21,500
4WD 3500 HD	GVWR	Ball Hitch	5th Wheel/Gooseneck	GVWR	Ball Hitch	5th Wheel/Gooseneck
Reg Cab/Single Wheel	9,700	13,000	15,400	11,400	13,000	17,400
X-Cab/Single Wheel	9,900	13,000	14,900	11,600	13,000	16,900
Crew/Single Wheel	9,900	13,000	14,800	11,600	13,000	16,800
Reg Cab/Dually	11,400	13,000	16,500	13,000	16,000	21,700
X-Cab/Dually	11,400	13,000	16,100	13,000	17,000	21,300
Crew/Dually	11,400	13,000	15,900	13,000	17,000	21,100

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