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Discussion Starter #1
Been reading over the forums...

Just bought this 2012 Crew 4x4 a few days ago. Outside temps when I bought it were warmer (60+) and had no dash lights or codes. Runs great. Driving it home at 70 the DIC showed 21.5 MPG, everything was perfect.

Drove it last night when the temps were under 50 and got an engine light... well $^&#!

Put my phone on it with Torque Pro and got a P0128. Figured it was thermostats. Noticed the coolant was a bit under Min so thought I'd start with the KISS principle and top it off then took it for a test drive and monitor temps. The gauge hits the first tick mark which I figure is the normal operating temp of 185ish and under some full throttle runs it would creep over that mark a touch then come back down instantly... however.. Torque never showed more than 154 under WOT and settled around 143 to 147 under normal driving. There seems to be a disparity there between the dash and Torque.

If it was thermostats I'd expect it to be more consistent, again assuming the gauge is correct..... however, after reading a few threads here it seems GM hasn't perfected an accurate temp gauge and it should be hitting the 210 mark for a true 185ish op temp.

https://www.duramaxforum.com/forum/...ain/123916-210-normal-operating-temp-lml.html

IF that is the case then yeah, I'd say thermostats and the gauge is inaccurate. The attached pic shows where it is on the gauge under normal throttle, etc.

As I'm new to the Duramax and this particular truck anyone care to verify my diagnosis a bit before I start tearing it apart to replace thermostats (and hoses and belts while I'm at it).

Thanks!

Brad
 

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Discussion Starter #4
ECT gauge is not accurate. It will probably read 210 when the real temp is about 185.
That's what I'm reading so I'm going to grab some AC Delco thermostats, upper hose, clamps and belt since I have the hood open.

Needing to baseline this thing so fuel filter, oil/filter and what not will get changed as well.

Just wanting to double check my 'diagnosis' with folks that have more Duramax exeperience that me....
 

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2012 3500 hd srw
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My ect dash reads210 about 10 minutes after start and stays there. Even when t-stats went bad. Dash gauge is no better than idiot light. Read somewhere they did that so wouldn't have to here about gauge movement until warm.
 

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Of course your problem could be thermostat related, but it could be sensor related, the sensor itself sending incorrect temp info to the ECM. Just keep in mind when you are assessing the problem. Test, don't guess.
 

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Of course your problem could be thermostat related, but it could be sensor related, the sensor itself sending incorrect temp info to the ECM. Just keep in mind when you are assessing the problem. Test, don't guess.
Agreed. However, as it seems to be a common issues me thinks I'll replace them anyway, not that big of an expense or time to do.
 

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Swapped out the thermostats today. Yep, that was the problem. They both, especially the 180, seemed very weakly sprung but I didn't do a boil test on them.

Runs up to 185 gets no higher (50 here today) and yeah, the gauge sits at 210.

At some point I'll do a DPF/DEF delete and will welcome an H&S or whatever for more accurate monitoring...but for now I'll get Torque Pro squared away with the necessary gauges and use it to watch things.

This forum helped immensely and this YT vid did as well.

Although, and I think this was mentioned at the end of the video, you do NOT need to remove the box on top of the engine, the oil filler tube nor the AC compressor. Pretty simple.
 

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I made this comment on the YT page above and thought I'd share it here as I did things a bit differently and made it a touch easier to do:

"This was helpful for sure and as you stated in the video, you do NOT need to remove the box on top of the engine, the oil filler tube nor the AC compressor. Take the top radiator hose off first, makes it all much easier. The bolt at 1 o'clock on the thermostat housing was very close to two hoses and made it hard to get the 13mm socket on and harder to get it off (long needle nose pliers). The two bolts for the aluminum hose mount are indeed the most difficult to remove and reinstall but not hard by any means. I didn't need to use a screwdriver to remove the sensor on top of the thermostat housing, just levered out from the front very easily. I lost very little antifreeze afterwards as I had siphoned out the overflow, released the clamp on the top hose and squeezed on it some which put some more antifreeze in the overflow... altogether less than a gallon and that much back in it. "
 

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Had the same problem last year with my 2011 although I got a few different codes along with the P0128. But just having factory guage showed normal temp like you had. I suspected the ect sensor but it checked out with correct resistance. Found the little harness that runs over the ac line right behind compressor to have worn through a few wires. It was a ground and the low voltage reference circuit for a few sensors including the ect. After I fixed this only got the P0128. Pulled the trigger on both OEM thermostats and that was it. The 185 thermostat was weak and the 180 was partially stuck open. Engine was never getting to temp and causing regen more often. Guage then read 210 but installed edge insight and all is well. Glad you found the issue as well.
 
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Thanks for sharing your solution to your issue. Good to find out what solved problem.
 
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