So on my trip over to the folks my temps ranged from 140- over 190 up the passes. I am wondering if my thermostat is going out. Outside temps ranges from a high of 46F to a low of 18F with an avg of about 27F.

I know the factory gauge isn't dead on but I seem to remember my truck staying between the left side line and 190.(about 165-190 according to the gauge).

My service manual says the range for the 190 thermostat is 182F-198F.

Did I just answer my own ? or will the engine temp drop to 140 with outside temps in the 20's?

Thanks, always.

 

Under steady throttle 140 would be unusually low, but when coasting down a long grade without any kind of front cover, I could see it.

I think the real test is how quickly the needle starts to move when starting the truck after it's been sitting overnight. If it's failed open, then it'll take forever.

 

the purpose of a thermostat is to keep eng temps within a desired range. actual air thru the rad should have no effect on temps if thermostat is working properly. in theory you could and likely do have subzero temp of the coolant in the rad, the thermo only opens to regulate the coolant temp in the block, so it does not matter how cold the rad coolant is...
now if your coasting down hill there is no fuel being burned and cylinder heat is almost nil, so ineffect it cools the coolant in the block, even though the thermostat is closed.

using a winter front helps hold heat in the rad, so when the thermostat does open it is using warmer coolant to cool a hotter block, a properly working thermostat should keep your temps around 190 all the time, unless your coasting or idling as there is not enough heat created in the cylinders to heat up coolant

...another reason for larger injectors and a performance chip/box

 

I understand what you're saying, and I agree--in theory a front cover should only prevent the radiator from cooling. However in practice that's not the case, at least in my experience.

 

if you were not pulling anything it sounds to me like the water is not moving out of the engine, if it runs cool when traveling on flats i would say your water pump is working but no exiting the engine when it reaches it's opening point. my vote is thermostat. get a genuine Cummins, there better.

or maybe it is simply low on coolant, check the jug and the radiator when cold

 

Thanks guys, the coolant is topped up and the overflow is at its regular level.
The temp sender is on the block so if its reading 140 I am thinking the therm is stuck open.
I have a cardboard rad blocker that I slip in front of the rad when it gets this cold out.
I have noticed in the past that the temps will drop off when descending a few of the grades in the winter travels but gets back up under load. It just wasn't getting to 190 this trip and I was watching it very closely because of this.

I guess its new thermostat time.

 

The thermostat has a rubber gasket built into it that often fails and gets stuck in the seat. This causes the thermostaat to be partially open all the time. Summertime probably not a problem but in the winter it is. Many of us have gone with winter fronts probably several winters before finally changing 'stats then finding everthing is more or less normal.

I have a picture of the one I took out of mine and that's what happened. Talking around I found that this is the common failure.

Just replace the 'stat with a Cummins one for good quality.

With a new 'stat you will probably find that right after start up the temp will go up to about 200 before dropping back down to 180 or so. I think this is normal. My streetrod does this everytime I drive it. I've had 6 'stats of various mfg and they all do it.

 

Mine holds steady with a Cummins tStat and clean radiator. It fluctuates with aftermarket TStat & runs hot with puke bottle covered radiator.

 

THanks for all the help!

The Cummins t-stat I put in had a rubber cup over the bottom of the stat and didn't look like the one I took out at all. The old one had two check ***** in the rim and the new one doesn't have any. Do you think the old one was a non Cummins one?

 

What was the temp rating on it. I 180 does the same thing when the temps are below 50 degrees. I put 190 in winter to keep it warm.

 

I ran the suggested 190 for my model year and engine #.

The previous one must have been a 190 because it read the same on the temp gauge as the new one does.

A couple sensors I have replaced in the pst looked different from the originals but were Cummins parts and work with the connectors so I expected the stat not to match perfectly.

 

Do you have any part numbers on the different Cummins thermostats? I just ordered one at $60 from Kenworth and never thought to ask what temp rating it was. Cummins said the part number was #3946849 for my 2001. I am hoping it’s a 190.

 

that it should be a 190

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