Did I get a bad thermostat?
 

I installed a cummins thermostat about 4 months ago and its worked fine until recently. In the mornings when I let it idle for about 10 min and once Im moving the gauge will go up to 1/2 or 3/4 once I start going and drop back down to the 1/4 and stay there all day long. It drops slowly unless I downshift and then it drops fast. It hasn't used any coolant since I changed it so I don't think its a HG issue. Is it possible that I have a bad thermostat?

My dash guage runs between the first mark and the second mark on a regular basis in all kinds of weather. My aftermarket guage moves 2 or 3 degrees at the most. It has done this for years without any over heat problems.

Bob

You may still have some air trapped in the system.

Mine never warms up at all

My 92 is the same way, cold blooded Dodge.

My new cummins stat does the same thing.

Well mine never even gets to the cold mark I think my thermo is stuck open or some crap and it makes it so the tranny won't shift into overdrive :(

Probably right, mine does warm up quicker than it used to now.

I think it might just be the stat. It didn't act like that at first but I don't overheat either. I ran it pretty hard today through some decent hills pulling my trailer and the temp stayed steady in 85f weather. I may try a napa thermostat which is what it had before. I don't want to risk it getting stuck closed or something.

maybe your heater core is flush, mine was on my silverado and it was constantly overheating even with a fail safe thermostat. better go get it flushed...

Boil the tstat and use a temperature gauge. You can watch it open when it's suppose to hit the certain temperature. If it's not then it's bad.

I had an aftermarket stat in mine and it did all kinds of stuff. Mostly take forever to warm up and constantly open and close once it was warm. This is with a new radiator, new heater core, new hoses and a flushed block. I went and got a new cummins one and it did the same thing to a lesser extent. Still opened and closed all the time but took 2-3 times longer to cycle. I cleaned up my factory one which was in really good shape ( I only replaced it because I had the system open and as preventive maintenance) and put it back in and it runs just like it always did. Warms up faster and temp stays steady. I really think the best stat is an original one.

This is hilarious...I was going to type out almost the exact same response as Sooty did....[laugh]

I just went through all this as well. New Cummins T-stat, gasser T-stat, gasser T-stat with small hole, Cummins T-stat with jiggle pin(s) removed, and eventually cleaned my original and installed. Actually after the cleaning and testing on the stove in a pot of water it now functions even better then before.

So then what is the true fix? and do you have an idea as to why mine runs too cold?

T-stats fail.
The OP stat is sticky, and opens late, then all at once, then settles down to normal operation. It's failing and eventually will stick shut. This same symptom is also seen if a stat is installed the wrong way, but it would occur right away.

Usually when a t-stat is subject to overheat, it'll fail stuck open, and the engine runs cold unless it's driven hard.

If a stat sticks shut, you boil over soonly.

Many other things also affect the engine temperature, from radiator condition to engine tune.

And yes, they're a mechanical device and eventually wear out.