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Duramax Second Generation: 2004.5-2005 (LLY) Discuss the second generation (2004.5-2005 LLY) of the 6.6L Duramax diesel engine & associated components. 2006 LLY engine discussion belongs in Third Generation forum due to the differences of the 2006 LLY and the 2004.5-2005 LLY. Engine related discussion ONLY.


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Old 08-11-2019, 04:47 PM   #1 (permalink)
BrennanK
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Bad New Thermostats?

I replaced both thermostats in my truck this winter when I replaced my water pump (Merchant). I put in two new OEM (Merchant) thermostats. My truck would always run 181*f (based on OBDII readings) unless it was above 80* or so outside then it would get to 191*f. It almost never wavered from those 2 temperatures when cruising. I have never seen my truck above 210* even pulling a heavier load and it would only be there during acceleration - never cursing.

I replaced them in the middle of winter and right away my trucks normal temperature was 185*. I thought that was probably normal given that the old t-stats were probably not closing fully or something. I just ignored it. Once the summer weather happened my truck will run around 195* around town with no load. Higher than I'm used to - but nothing crazy.

Now 2 weekends ago I pulled my first trailer since replacing the thermostats in the winter. I was pulling a 12ft u-haul enclosed probably around 4,500 lbs for 1000 miles. I was doing 70-75 MPH and it was 80-90* outside. Nothing too extreme in my mind. My truck would climb to 215* almost instantly and the fan would engage and bring it down to 197* or so and then within 15-20 min I was right back at 215* with the fan again. I got awful economy because the fan was on for what felt like 40% of the drive.

I am just perplexed. I replaced my thermostat and my truck acts like it's on the brink of overheating (fan always engauged) all the time with a trailer now. Is it possible to have thermostats that don't open fully? I never had a hot running truck before and the only things that changed are the water pump and the thermostats. I get these temps are a far cry from truly overheating, but it's much different than I'm used to.

I just want to get others thoughts. Am I over-analyzing this or should I look into testing my thermostats? I really hate having my truck act like it's running so hot with such a small load compared to what I've done in the past. I get wind, trailers, and whatnot all play a role, but I have towed 5-10 times now and it's acting this way consistently now.

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2005 Black GMC Sierra 2500HD SLT - CCSB
Installed:
AUX Radio Input (iSimple ISGM535) - High Idle Mod - Head Gaskets (June [email protected]) - OEM Cab Lights - LBZ Mouth Piece - All On Headlight Mod - Puddle and Footwell LED's - OEM Towing Mirrors - Uniden BCT15x Scanner - Gauges in Trippin' Overhead mount - Rigid Industries Dually Backup Lights - UC6 radio with LUX amp
-Stock tuning and staying that way

Future:
LBZ door badges, Kennedy lift pump, aux CAT filter

Last edited by BrennanK; 08-11-2019 at 04:52 PM.
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Old 08-11-2019, 11:26 PM   #2 (permalink)
pj951
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I installed new thermostats last weekend as a part of other work. Mine now does the same as yours. Old one(s) was defective, even threw a code once in a while. Truck now indicates 205 on the instrument cluster all the time, running empty. Haven't towed yet. It used to run much lower.

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Last edited by pj951; 08-11-2019 at 11:27 PM.
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Old 08-12-2019, 07:53 AM   #3 (permalink)
BrennanK
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If you are are seeing 205 or so on the dash gauge you are likely fine - assuming you are not experiencing other symptoms of running hot. The dash gauge is very inaccurate, it will show 200-210 for anything between 180-220 or so. You need to monitor actual temps via OBDII.
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2005 Black GMC Sierra 2500HD SLT - CCSB
Installed:
AUX Radio Input (iSimple ISGM535) - High Idle Mod - Head Gaskets (June [email protected]) - OEM Cab Lights - LBZ Mouth Piece - All On Headlight Mod - Puddle and Footwell LED's - OEM Towing Mirrors - Uniden BCT15x Scanner - Gauges in Trippin' Overhead mount - Rigid Industries Dually Backup Lights - UC6 radio with LUX amp
-Stock tuning and staying that way

Future:
LBZ door badges, Kennedy lift pump, aux CAT filter
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Old 08-13-2019, 10:42 AM   #4 (permalink)
dave-lly
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Did you clean the stack when you replaced the water pump? Perfect time to atleast spray some water from back to front...
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Old 08-14-2019, 10:14 AM   #5 (permalink)
BrennanK
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Cleaned the stack about 20,000 miles ago (about 1 year). It wasn't a full pull out the radiator cleaning, but I did get in there. I plan on doing a full tare-down cleaning soon, but I just cannot imaging that it got that dirty in 5 months.

I didn't install them myself, so I am wondering if they maybe installed them 180* rotated with the holes in the wrong direction. Maybe they accidentally flipped the 180* and 190* thermostats. I don't know...
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2005 Black GMC Sierra 2500HD SLT - CCSB
Installed:
AUX Radio Input (iSimple ISGM535) - High Idle Mod - Head Gaskets (June [email protected]) - OEM Cab Lights - LBZ Mouth Piece - All On Headlight Mod - Puddle and Footwell LED's - OEM Towing Mirrors - Uniden BCT15x Scanner - Gauges in Trippin' Overhead mount - Rigid Industries Dually Backup Lights - UC6 radio with LUX amp
-Stock tuning and staying that way

Future:
LBZ door badges, Kennedy lift pump, aux CAT filter
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Old 08-19-2019, 07:01 PM   #6 (permalink)
BrennanK
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Definitely something funny going on.

I was in Atlanta this past weekend and I was doing 70-75 unloaded in 95* heat with the AC on and over the 60 miles I drove the fan was on fully engauged over a 1/3 of the time. ECT were around 202 the whole time.

Then today I went to fill up with fuel. Left it idling for the AC after pulling off the highway with the ECT at 198. After pumping fuel in I got in and the ECT was 207 and the AC was almost not cold.

Rather frustrating to have a truck that can't keep itself cool when it's unloaded let alone loaded.
__________________
2005 Black GMC Sierra 2500HD SLT - CCSB
Installed:
AUX Radio Input (iSimple ISGM535) - High Idle Mod - Head Gaskets (June [email protected]) - OEM Cab Lights - LBZ Mouth Piece - All On Headlight Mod - Puddle and Footwell LED's - OEM Towing Mirrors - Uniden BCT15x Scanner - Gauges in Trippin' Overhead mount - Rigid Industries Dually Backup Lights - UC6 radio with LUX amp
-Stock tuning and staying that way

Future:
LBZ door badges, Kennedy lift pump, aux CAT filter
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Old 08-19-2019, 08:35 PM   #7 (permalink)
pj951
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Easy enough to delete those thermostats temporarily. No re- seal needed.
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