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Thermostat gauge not fluctuating

2K views 21 replies 7 participants last post by  Common 
#1 ·
I have a quick question, i used the search bar and all i could find were threads about how its normal for a coolant temp gauges to fluctuate, but my problem is that my thermostat appears not to be fluctuating at all once it reaches operating temp. It runs around 180*, or 3/4 of the way to the 190* mark, i dont understand if its the thermostat, or a dead giveaway for something else, my water pump was leaking a miniscule amount of coolant when looked under the hood, and by miniscule i mean enough for the block to look a little wet in the area underneath the pump, my rad hose is hard and hot when the truck is up to temp and there are no signs of a head gasket leak.
I read somewhere that in 98' they replaced the fluctuating gauge with an "idiot gauge" because of the complaints.
Looking into the pump..But i just wanted to share and figure it out before i start throwing the cash
Thoughts?
 
#2 ·
For a long time I never saw my gauge fluctuate. Then looking at the right time I saw the fluctuation happen.

If the truck takes forever to get up to temperature perhaps the t-stat is stuck partially or completely open.

On ebay there is (or was) a t-stat that would not fluctuate. Perhaps your truck has one of those?
 
#3 ·
No need for concern, sounds normal to me. They don't all do the fluctuation thing. Keep an eye on that water pump though, might be time to replace it.
 
#4 ·
it will fluctuate barely and when it does the needle barely moves an 8th of an inch. Its usually when i drive on the highway. Would it be worth it to buy a new cummins oem thermostat just to see?
 
#6 ·
Does it ever get to 190 or does it stop at 3/4 the way up?
 
#9 ·
Not trying to highjack here but I have the same issue, mine always seems to climb 3/4 of the way then hang out there all day. And every now and then it will spike up or down and bounce around for a bit then back to the 3/4 ish mark. Im guessing that means a thermostat is needed?

Would you guys recommend a cummins one or any replacement?

Thanks
 
#11 · (Edited)
Again, you don't have a problem. They're not *supposed* to fluctuate. The large cooling system can sometimes cause it. I forget exactly which it is, but I know there is a thermostat available that stops the fluctuation and keeps the engine at a constant temp.
 
#12 · (Edited)
#13 ·
The same author of that post has said numerous times that if his gauge didn't fluctuate, he'd think something was wrong.

OP's truck is not getting up to temp - unless the gauge is off or something.

I also remember reading that the *fixed* thermostat still cycles.

I know if mine quit fluctuating, I'd look into it.
 
#14 ·
Mine fluctuates when the air is cool. I doesn't fluctuate when it is hot. Normal temp on my gauge is about halfway between the 140 and 190 marks. It was the same when the truck was brand new. If your t-stat was stuck open the temp would never come up and if it was stuck closed the engine would overheat. Leave it be.

One more thing. I've replaced my share of t-stats, always with Cummins parts. One didn't fluctuate, but I never considered it to be faulty or special.
 
#15 ·
You always have a load behind it, correct?

I've never driven a Cummins that at least didnt get to 190 then drop back down.

When my tstat's rubber ring got stuck - the truck would only reach about where the OP says his temp is.

If his truck at least hit 190 then it subsided to whatever he normally sees, I'd say leave it alone.
 
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