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-   -   Bad New Thermostats? (https://www.dieselplace.com/forum/63-gm-diesel-engines/40-duramax-second-generation-2004-5-2005-lly/977882-bad-new-thermostats.html)

BrennanK 08-11-2019 05:47 PM

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.

pj951 08-12-2019 12:26 AM

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.

BrennanK 08-12-2019 08:53 AM

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.

dave-lly 08-13-2019 11:42 AM

Did you clean the stack when you replaced the water pump? Perfect time to atleast spray some water from back to front...

BrennanK 08-14-2019 11:14 AM

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...

BrennanK 08-19-2019 08:01 PM

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.

pj951 08-19-2019 09:35 PM

Easy enough to delete those thermostats temporarily. No re- seal needed.

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