LLY Overheating: The GM Solution and Beyond

Volume 1 Issue 3 - Diesel Articles

Article Index
LLY Overheating: The GM Solution and Beyond
Exploring Solutions
LLY Underhood Airflow
Aftermarket Solution?
The GM Solution
Facing the Killer
Killer Hill Vanquished
All Pages

My first experience towing with my 2005 LLY Duramax was a thrill – and a disappointment. In stock form, other than some gauges and a power program that added about 100 crankshaft horsepower and roughly 180 foot-pounds of torque, there was no disappointment with the power – it was awesome! I could yank around a 27-foot fifth-wheel travel trailer and almost forget that it was there. I had all the power I needed beneath my right foot and the ability to accelerate on any hill I encountered.

The disappointment was my cooling fan: it engaged far too often for my satisfaction. It wasn’t even hot outside. The ambient temperature stood at a meager 65°F: I wasn’t towing particularly hard either, running at about 65 MPH on mostly flat ground and only a few smaller hills. A stock Duramax would pull just as well on this route and generate the same amount of heat – for the most part – I really was not into all that much extra power.

After towing for a few hours, I could very easily predict the behavior of the cooling fan. With every short hill I pulled, the fan clutch would engage for about three to five minutes near the top or after I crested the hill. To improve my lot, I got into the habit of running every hill: accelerating before the base of the hill and backing out slightly as I climbed. It did not help much.

As a technician, I remembered that a few LLY owners had complained about their cooling fans running too much. Not having experienced the aggravation for myself yet, I had told them the fan was doing its job. Now that it was happening to me, I realized that the cooling fan was running way more than I would have expected. While traveling, I mused over possible explanations for the undesirable behavior of my cooling fan. I noted on my digital gauges that the engine temperature was running between 198 and 220°F, a significant jump from the typical 190 to 194°F when running empty. My cooling fan was engaging at the right temperature, but I couldn’t figure out why, at 65 MPH, there was not enough airflow over the radiator to keep the engine temperature low enough to keep that fan quiet.

My expectations made things even worse. I used to tow with a 1980 GMC one-ton dually with a warmed-over 454, making nearly 400 horsepower. She was fully-loaded with air conditioning. The fan clutch on that old gasser would never engage at speeds above 50 MPH, even in the hottest ambient temperatures and towing the heaviest loads. Above that speed, there was always enough airflow over the radiator to keep the engine quite cool. We had little mercy on that truck and it always handled whatever we threw at it. Now, here I was with the most advanced diesel pickup engine on the face of the planet, fully designed for towing, and the cooling system seemed barely adequate and certainly annoying.

Before resting the entire blame on the cooling system, I needed more information. Was my radiator, intercooler, or A/C condenser restricted with debris? I hoped that something other than the cooling system itself was at fault. Before our next trip, I gave it a close inspection. My hope for a scapegoat was dashed: no debris or restrictions. We headed out for a camping trip in southern British Columbia. There I was, growing more nervous with each mile that passed. High altitudes test the limits of any tow vehicle and long hills are much more frequent in the Rocky Mountains. If I was struggling in tamer terrain, what would these conditions do to my truck? On our route, we encountered a long pull we call Killer Hill. It boasts some eight percent grades with an average climb of five percent up to a peak of 6,810 feet, running a little over three-and-a-half miles. Ambient temperature that day was about 80°F. As we climbed the hill, my worst fears were realized: I watched as every alarm on my gauges fired off. Exhaust gas temperature (EGT) was peaking over 1450°F, engine coolant temperature (ECT) was over 235°F. What made this more annoying was that I ran the bottom of the hill at about 75 MPH and backed out of it to make the climb. I ended up crawling up Killer Hill in low gear at about 35 MPH – defeated; and, even at that speed, the cooling fan roared all the way! My 1980 GMC would have flown over the top of that hill, with my right foot on the floorboard at 70 MPH, towing the load we were pulling, no problem. We had climbed that hill with her pulling heavier loads in hot ambient temperatures with nary a peep out of the fan. It seemed bitterly ironic to me that a significantly modified 454 from 1980, using a stock cooling system, would perform so much better than a 2005 Duramax diesel making about the same power. 25 years had not seemed to do much for cooling system technology. Disappointing? I could hear that old GMC laughing at me, taunting me over my conversion to diesels. And my LLY DMax made no reply. Something had to be done.

At this point, I was also trying to figure out why I had not heard even more complaints about overheating at the dealership. There were a couple of owners who had bad experiences, but they seemed to be isolated, more extreme cases. More research, however, revealed that overheating under these circumstances was, in fact, a significant problem – even more so in hotter climates. We have a more moderate climate, and the hills in our area are generally quite mild, at least in comparison to runs like Killer Hill. Also, I think that the owners with whom I interact are more likely to think that an overheat during a hard pull in hotter ambient temperatures is simply to be expected. But I had experienced better: that 1980 GMC one-ton. It never overheated once, under any condition.


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