Thermostat Fluctuation?

Tate said:

This.

My 12v gauge always says its running cold. The previous owner installed a separate water temp gauge under the dash. When I first bought the truck, it would run from 170-190 for hours on end, always up and down. A big rad will do that. That t-stat died on me about 2 years ago, got another one from Cummins, this one hold temp real steady, usually cools down when I'm coming off the highway slowing down in the winter.

I wouldn't worry about it. Your truck isn't the first to do it, and it won't be the last.

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I did check my tstat housing with a laser temp gun before returning the cummins tstat. I know that's definitely not the most accurate way, but it was easy and free. It did stay cool (130deg-ish). I should go check it again with the new one from Napa that stays a lot closer to 190deg.

haloman said:

What Tate said. Got to keep in mind these truck were designed to keep cool in 100deg heat while at max capacity.

Custom cardboard insert with a hole cut in the middle always did the trick for me.

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If you put cardboard in there, it will take a ton of the efficiency away from the intercooler though. I played around with it a few years ago. I went and bought 1/4" insulation board, then cut it into four lengths that I could sit inbetween the radiator and intercooler and my mileage went from 22 to 19mpg. I didnt feel a difference in warm up time really and no difference in power, but the mileage was the only big change. Maybe I'll mess around with the idea again. I also want to go to an electric fan because I have a feeling that my fan is stuck on.

If your fan is locked up, that'll kill your mileage more than a blocked off rad. When did you do this test? Winter fuel gets switched in around October where I am, and I always take a decent hit in mileage on winter fuel. If its cold enough where you are worried about your truck not getting warm enough, I wouldn't be concerned about intercooler efficiency.

Put the cardboard between the intercooler and the rad.

I've always done in front of the radiator. Never had any change in egts that I picked up. The fan and winter fuel were things that concerned me.

I just did the test a few days ago (thermostat heat test). I had the thermostat out or was switching it four times in the last week and a half. I am not sure what they do for fuel around here since I just moved to Denver area, but I do know that'll hurt mileage.

Hmm, looks like I have some fun experimenting infront of me.

EDIT:
Just dawned on me that you were talking about mileage test with the cardboard.
I did it two years ago, it was the middle of winter. I was driving from the UP of Michigan to Detroit with it on, and then back without it, and the weather conditions were very similar. That drive was about 300 miles of 70mph expressway, and about 250 miles of 55mph highways one way.

Opened.

Just a couple of updates. I ended up trying out a Stant 45479.
It has the exact same dimensions as the Cummins one except less depth and it operates at 195F instead of 182F.
Stant 45479 (195deg) next to stock 1997 12V Cummins (180deg) T-Stat - Album on Imgur


I've also read that the Gates 33039S is an option but I haven't tried it myself.


With the Stant, my gauge stays exactly at 190F while driving, and I actually get head out of the vents. It also seems like it fluctuates less, but the first crack open goes to probably 205F.
I have towed with this and had no issues with heat.

The best part is, this tstat is $8, instead of $50+ from Cummins.

I've tried almost every thermostat in my truck, and it always ends up the same. Its not in a dodge, but its a 95 dodge motor in my 88 chevy, and I can put in a thermostat, and it will work great for a month or so, maybe two at 180 or so, then all of a sudden it will start running at 150. once in a great while it will go back up to 180 but doesn't stay there very long.
I've tried oe cummins, parts store cummins, and now it has a 77 chrysler 440 one in it. but they all start running 150 after awhile. its a mystery to me. Might try a 24valve one next.

K204DR said:

I've tried almost every thermostat in my truck, and it always ends up the same. Its not in a dodge, but its a 95 dodge motor in my 88 chevy, and I can put in a thermostat, and it will work great for a month or so, maybe two at 180 or so, then all of a sudden it will start running at 150. once in a great while it will go back up to 180 but doesn't stay there very long.
I've tried oe cummins, parts store cummins, and now it has a 77 chrysler 440 one in it. but they all start running 150 after awhile. its a mystery to me. Might try a 24valve one next.

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I've tried the 24V ones a while back and I'm 99% sure they're a different size and don't fit without modification.

My last one did what you're describing and it was a Cummins one. It would fluctuate around 170-180, then just stayed at 150 after a couple weeks. My original one in the truck lasted to about 230k. I've had this Stant one since summer and have been happy with it.

K204DR said:

I've tried almost every thermostat in my truck, and it always ends up the same. Its not in a dodge, but its a 95 dodge motor in my 88 chevy, and I can put in a thermostat, and it will work great for a month or so, maybe two at 180 or so, then all of a sudden it will start running at 150. once in a great while it will go back up to 180 but doesn't stay there very long.
I've tried oe cummins, parts store cummins, and now it has a 77 chrysler 440 one in it. but they all start running 150 after awhile. its a mystery to me. Might try a 24valve one next.

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Whats your coolant condition like? If they keep failing, sounds like its something causing them to stick open, and not just faulty thermostats.

CorneliusRox said:

Just a couple of updates. I ended up trying out a Stant 45479.
It has the exact same dimensions as the Cummins one except less depth and it operates at 195F instead of 182F.
Stant 45479 (195deg) next to stock 1997 12V Cummins (180deg) T-Stat - Album on Imgur


I've also read that the Gates 33039S is an option but I haven't tried it myself.


With the Stant, my gauge stays exactly at 190F while driving, and I actually get head out of the vents. It also seems like it fluctuates less, but the first crack open goes to probably 205F.
I have towed with this and had no issues with heat.

The best part is, this tstat is $8, instead of $50+ from Cummins.

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I didn't read the thread you posted so I might be covering what's already been said, but if you want to stop the fluctuation, drill one or two 1/16" holes in the thermostat rim. Place the holes towards the top of the t-stat housing so the air bleeds out better when you refill the coolant. This allows a tiny bit of coolant to flow right past the thermostat sensor and seems to allow it to control temperature much more evenly.

Glad to hear your using the 45479 STANT. That's the only aftermarket one I've tried that's held up over time.

Sent from my SM-G870W using Tapatalk

Tate said:

Whats your coolant condition like? If they keep failing, sounds like its something causing them to stick open, and not just faulty thermostats.

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its in pretty good condition. nice and clean looking. its been a couple years since I've replaced it completely. motor has 400K on it, so it could have rust in the block as I got it at 275K. but I've not really seen any junk floating around in it.