Low coolant temps?

I'm having an unusual issue I just can’t figure out. My coolant temp will hardly ever get over @ 160*. I initially thought my 5 year old thermostat was stuck open so I replaced it Friday with a new Cummins 3924489, no change. I replaced the water pump with a new Cummins Water Pump in October. If it wasn’t pumping I’d think I would see high temps. Both radiator hoses get hot so I don’t think it’s a circulation issue. I lasered the thermostat housing after driving 40 miles and it showed 168*. I’d say I have a sensor or gauge issue but my tranny temps are low, too, and read the same as my ECTs.

Ambient temps in the lower 90’s, high humidity. City traffic and interstate driving empty. Fan clutch is functioning properly.

Tell me what you think.

I'd maybe stick a manual gauge on it to see what the temp really is. I assume this is the 1998 from your signature.
Maybe the fan is running too much, but I'd think you'd still see the correct (higher) engine temp when you work the truck.

By the way, the part number you mentioned as a thermostat is an oil pan. ??

Did you check the thermostat in boiling water?
I had this happen on my cousins 98 12v. Seems the stock thermostat is a 160° when he bought from the auto parts store. We ordered a 190° Cummins from Geno’s and tested in water before installation. This was 7+ years ago.

The # 3924489 you referenced is an oil pan.

Unless you mistakenly meant to type out "Cummins 3946849"
which is a 58mm 24v stat with the perimeter seal - it will let coolant past even when closed, causing a cold operating condition on a 12v.

The 63mm 12v second gen stat you need on your '95 engine is # Cummins 3928499, with inner & outer seals # 3925466 & 3927305.

BigPapa said:

I initially thought my 5 year old thermostat was stuck open so I replaced it Friday with a new Cummins 3924489, no change.

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The thermostat number is 3928499. Typed it wrong in the original post.

Pull it & check if the rubber shroud got loose & stuck in the opening, keeping the stat open.

I've seen that a couple times.

I can watch my gauge and see my t stat working. It's like that needle is just waiving at me lol. It eventually stabilizes.

Isn't it ironic that this time of year the posts are normally about running too hot?

Check out this old thread I made:
Thermostat Fluctuation? - Competition Diesel.Com - Bringing The BEST Together

I had a normal tstat for many years, then it got stuck open. Did the same thing as you, tried a legit Cummins one, and it stayed at ~150F. I ended up trying a Stant 45479 and not it operates properly and stays at 180F. It is smaller, but the correct diameters and I haven't had any overheating issues with heavy towing.

bateman said:

I can watch my gauge and see my t stat working. It's like that needle is just waiving at me lol. It eventually stabilizes.

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Yes, that's what I'm used to, also, so I really don't know when this began. My regular period glance at the gauge just registered as normal fluctuation. Then, I noticed, every time I looked it was down there and now I see it's staying there, not swinging like it normally does.