OK, your probably asking yourselves "What the he?? is JJ doing in our backyard? http://www.dieselplace.com/forum/smileys/HiHi.gif Yeah, the most "meck-anickel-ally" challenged D/A (that stands for Duramax/Allison versus dumbass) owner on the DP forum is at your door!


I hope you have been following the overheating situation several of the LLY owners have been experiencing. Some can tow heavy with no problems, others run high temps with minimal loads. The subject has gotten quite a bit of traffic, but no solution has been found. Several are pursuing GM to buyback their trucks. Can't blame them for that much money!


I have a question about the thermostats in the LLY's. I understand there are 2 of them with different temp settings. I read the marketing article for the LLY, but I don't totally understand how they work. If they were put in the wrong location, like the one for the bottom mistakenly installed to the top and vice versa, could this cause the overheating?


Thoughts? Other ideas? JJ http://www.dieselplace.com/forum/smileys/Big Smile.gifEdited by: JJs DuMax

there side by side. dont think thats the issue.
are any of these over heating lly have mods....overheating with no cooling systen issues my be from over fueling.....lly has different pcm. and sensors and program. I think it is over fueling..

Several of these are stock LLY's. "There side by side", meaning located on different sides of the block? Are they set to open at the same temperature?


The LLY marketing page states the LLY has increased flow water pump and new thomstat settings. Puzzling why some are overheating. JJ http://www.dieselplace.com/forum/smileys/Big Smile.gif

JJ: I wonder if there is any correlation between the trucks that are running hot and those that are getting poor fuel mileage. RickDLance has posted some notes on his two LLY trucks stating that they are getting poor mileage and run hot. I got the impression that the mileage was worse on LLY #1 and as I recall, he said LLY # 1 also ran hotter than LLY #2.

If the truck is getting poor mileage and it is not smoking, meaning the fuel is being burnt, then I think the burning of the additional fuel would generate more heat.

I am also wondering if the new computer reflash may help somewhat since it does address fueling issues.

BH, interesting? I'll check the postings on the overheating LLY's to see if there is any correlation between the two. Will advise. JJ http://www.dieselplace.com/forum/smileys/Big Smile.gif

THE T-STATS ARE NEXT TO EACHOTHER AND ONE OPENS EARLIER THEN THE OTHER.i STILL FEEL ITS OVER FUELING.....

Getting intel from owners of high temp/overheating LLY's. Will advise. JJ http://www.dieselplace.com/forum/smileys/Big Smile.gif

Several LLY owners with overheating issues have responded. You can read their posts on the 2004.5 thread, "engine temps" and "overheating". Consistent low mpg's and high engine temps. One LLY is being reflashed today, though I'm not certain it is stock.


Any of you know exactly what the reflash does? I wouldn't put it past GM to detune the LLY's to reduce hp/tq and reduce engine temps and increase fuel mileage. Is it possible GM tried to squeeze too much out of the LLY without compensating with greater cooling capacity? JJ http://www.dieselplace.com/forum/smileys/Big Smile.gif

Patrick, I still have a question about the thermostats. If one is set to open before the other, that is at lower temp, any idea what would happen if they were inverted and put on the wrong side? JJ http://www.dieselplace.com/forum/smileys/Big Smile.gif

nothing I think they are side-by-side and share the same area of coolant.hard to explain unless you see it.The service manual states to put them in correct location,but I cant see why?both sit in a hole and coolant is on one side at the water cross over. the other side goes to the upper radiator hose.all for a half flow full flow idea. Kind of...patrick

Patrick, that does sound odd. One would think the coolant would flow through the entire engine before exiting the outtake thermostat to the radiator to be cooled? The more I think about it the more puzzled I am!http://www.dieselplace.com/forum/smileys/Confused.gif Hang on, JJ's going out on a limb, again!http://www.dieselplace.com/forum/smileys/HiHi.gif


http://www.dieselplace.com/forum/smileys/Geek.gifThe coolant flow is going to follow the path of least resistance. If the thermostats are co-located on each side of the engine the path of least resistance would be to flow directly across the manifold and exit through the thermostat to the radiator, versus flowing through the engine chambers to absorb heat and then exiting to the radiator for dissipation. Whew, this tech stuff is mind draining. http://www.dieselplace.com/forum/smileys/Dead.gif


Did I make sense? JJ http://www.dieselplace.com/forum/smileys/Big Smile.gif

JJ repost the stat info from the other thread here. It will help those you ask for tech explanation.


Then ask why boilovers normally occur in single thermostat vehicles, does the stat fail open, closed, why do failed thermostat contribute to overheating (or not)?

Here's stat info. Hope this helps. JJ http://www.dieselplace.com/forum/smileys/Big Smile.gif


Thermostats (6.6L Diesel Engine)


The thermostats are coolant flow control components. The purpose of the thermostats are to regulate the correct operating temperature of the engine. The thermostats utilizes a temperature sensitive wax-pellet element. The element connects to a valve through a piston. When the element is heated, it expands and exerts pressure against a rubber piston. This pressure forces the valve to open. As the element is cooled, it contracts. This contraction allows a spring to push the valve closed.


The 6.6L diesel engine requires two thermostats for correct coolant flow. The front thermostat is a dual purpose thermostat. The front thermostat controls the coolant flow to the bypass port and to the water outlet. The rear thermostat only controls the coolant flow to the water outlet.


When the coolant temperature is below the rated thermostat opening temperature, the front thermostat valve remains closed to the water outlet and is opened to the bypass port. The bottom portion of the thermostat is raised off of the bypass port while at the same time the top portion closes the coolant flow to the water outlet. The rear thermostat also is closed to the water outlet during engine warm-up. This prevents circulation of the coolant to the radiator and allows the engine to warm up quickly. After the coolant temperature reaches 82°C (180°F) the front thermostat primary valve opening temperature, the front thermostat primary valve will start to open. The coolant is then allowed to circulate through the thermostat to the radiator where the engine heat is dissipated to the atmosphere. As the engine coolant reaches 85°C (185°F) and more coolant demand is required the front thermostat secondary valve begins to close the bypass port and the rear thermostat begins to open coolant flow to the water outlet. The thermostats will continue to control the coolant flow by opening and closing. The front thermostat will be fully open when the coolant temperature reaches 95°C (203°F) the rear thermostat will be fully open when the coolant temperature reaches 100°C (212°F). The thermostat also provides a restriction in the cooling system, even after the it has opened. This restriction creates a pressure difference which prevents cavitation at the water pump and forces coolant to circulate through the engine block.

I have a feeling that the thermostat locations are made so that the stats cannot be physically swapped, does anyone know?


But if they were swapped more lower temp coolant would be going to the radiator, an efficiency hit, but unless this swap would affect system pressure, I don't know how it would be significant, though the block would have a hard time staying heated since the rear stat has no recycle (bypass back to block). OH, maybe that's it, the front stat would be in perpetual full recycle, until a higher temp than design, allowing the block to get hotter, while the radiator is only seeing lower temp (than design) water from the rear stat (reduced efficiency). More heat retained in the block, lower cooling efficiency at the radiator=reduced operating envelope. The boilover could be like a block induced volcano. COOL.


Ok, I'm back. Who knows really

Is there any way to verify whether an LLY is overfueling? Several LLY owners that experience high engine temps also posted they are getting less mpg's than others on the site.


When the state does a vehicle inspection they insert a probe into the exhaust. Is there anyway a probe can sense excess fuel?


JJ http://www.dieselplace.com/forum/smileys/Big Smile.gif

Bumping this back up to you tech's! Now you guys wouldn't be ignoring ole JJ now, would you?http://www.dieselplace.com/forum/smileys/Disapprove.gif


Update on the overheating LLY's. Several guys have noticed the upper radiator hose was flat when they experienced running hot or overheating. Now that has to jump up and grab you by the short ones! http://www.dieselplace.com/forum/smileys/HiHi.gif


We need technical help, ppppppppllllllllllleeeeeeeaaaaaaaassssseeeeee! http://www.dieselplace.com/forum/smileys/Hug.gif

Air in system. sucking hose flat.maybe this is a issue of the new designed waterpump.

That's kinda what we're thinking! Maybe the new waterpump is drawing coolant too fast for the thermostats to keep up, or the radiator can't dissipate the heat fast enough. HHHHHMMMM? http://www.dieselplace.com/forum/smileys/smiley24.gif


Check out the LLY overheating post on the forum for more intel. I'm still not convinced that several LLY's had the thermostats installed improperly. Not all LLY's are experiencing this problem.


Appreciate any help. JJ http://www.dieselplace.com/forum/smileys/smiley4.gif