Low Operating Temp - No P0128 Code - Chevy and GMC Duramax Diesel Forum
01-04.5 LB7 Duramax Powertrain Discussion of components that are directly involved in the power production and all that is needed to get and keep the truck moving . Engine , Transmission Ect

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Old 11-13-2012, 07:11 PM   eBay Motors  #1 (permalink)
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Low Operating Temp - No P0128 Code

So I have been scouring the board over the past couple of days and posting to get some answer with this issue. It seems that I am not the only one that is having the low operating temperature issues now that the temperature is dropping as we head into winter.

A quick run down of what I have happening is that the truck will not hold a normal operating temperature when the temperature is below about 60 degrees. It typically runs at about 176 degrees and doesn't take abnormally long to get up to that temperature. It will struggle to hold the 176 degrees though and fluctuate between about 170-180. When the temperature gets into the 180 it will drop like a stone as soon as I get off of the throttle.

My first thought was to throw the winterfront grill cover on the truck which has had a negligible impact on the temperature. So I am left with believing that one of the thermostats is stuck open. However, I am not getting a P0128 code which I should be getting if that was the case.

I'm not sure If I should just replace the thermostats anyway to be sure that they are not the problem. What other issues could be causing the low operating temperature?

I know that the LB7s are quite the cold-hearted engines but if the thermostats are set to open at 180 and 185 then running at 176 with the thermostats functioning correctly would mean that coolant is not circulating at all. However, the upper radiator hose is always warm after driving, not scalding hot but warm and the hose itself isn't hard.

Any ideas or thoughts, should I just change out the thermostats and bleed the system to check for air pockets. Or is this just a normal LB7 in the winter time?
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Old 11-13-2012, 07:19 PM   eBay Motors  #2 (permalink)
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i just got back from the duramax school in atlanta a few weeks ago and we talked about thermostats and if you replace them make sure they go in the proper location, there are two and it makes a difference which one goes where. one of the side effects of improper locaton is slow to get to operating temp due to the coolant bypassing when it should be holding in the engine. now i also know that when in the shop we can let a duramax run till your tired of hearing it and it will never reach operating temp to run some test that we have to run, so yes they are slow to heat at low power settings.
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Old 11-13-2012, 07:22 PM   eBay Motors  #3 (permalink)
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I don't know how cold you are, but I never had this issue with my LMM, or LML. Only when it was -30 -40 -50c would idle temps drop. LML is much better. But I'd drop down to 75c with leather front.


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Old 11-13-2012, 07:43 PM   eBay Motors  #4 (permalink)
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Quote:
Originally Posted by cherokeepilot View Post
i just got back from the duramax school in atlanta a few weeks ago and we talked about thermostats and if you replace them make sure they go in the proper location, there are two and it makes a difference which one goes where. one of the side effects of improper locaton is slow to get to operating temp due to the coolant bypassing when it should be holding in the engine. now i also know that when in the shop we can let a duramax run till your tired of hearing it and it will never reach operating temp to run some test that we have to run, so yes they are slow to heat at low power settings.
I wouldn't know if they were installed incorrectly until I took it apart which in that case I would replace them. Do you know what triggers the P0128 code exactly? I know its coolant temperature not coming to temp but what exactly is the trigger because I am not getting that code.
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Old 11-13-2012, 07:46 PM   eBay Motors  #5 (permalink)
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I found my answer:


The ECM is calculating an expected Coolant Temp given the fuel that has been burned and the time that it has been burning it. If the coolant temp does not meet or exceed the calculated temp, you get the code.

Check for proper coolant level.

Perform Coolant System Leak Test.

Inspect fan for proper operation.

Inspect the fan belt to ensure it is in good operating condition?

Remove thermostats and verify proper operation. Bring to boil in a pot of water and see what temps they open and close at. Start opening ~ 160 Fully open at ~ 180 Degrees F roughly.

Could also be a bad ECT Sensor. You can do a resistance check at various temps. At 0 degrees F Res = 9420 ohms At 50 degrees F Res = 973 ohms at 100 degrees F Res = 177 ohms, at 150 degrees F Res = 47 ohms.

This condition could also be caused by air in the coolant system. That could possibly be an indicator of a bad head gasket. After shutting the warmed engine off, and letting it set for several hours, check the upper Rad hose to see if it is hard. If it is hard, you probably should test for a headgasket failure.
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Old 11-16-2012, 09:08 PM   eBay Motors  #6 (permalink)
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I just wanted to add some closure to this thread. I swapped out the thermostats tonight and immediately my truck held 181 degrees while driving.


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