Engine/Transmission (1994 - 1998) Another temp flux question

[RyanStewart]

Hey all. This'll be my first post, but I've been hiding in the woodwork for quite a while now.



Like most trucks the engine temp in mine goes back and forth between 150 and 190 while I'm going down the road, and from what I've found in searches this is normal, and not bad for the engine, but I've got something else going on that kind of worries me. Before the thermostat opens for the first time, after the truck has been sitting long enough to cool off, the temp will go up to 200 - 210 before the thermostat opens and the temp will drop to about 145. After that it cycles between 150 and 190. From what I've read, I guess cycling between 150 and 190 won't shock the motor too much, but it seems to me that first shock in the morning, when the 200 degree engine is getting hit with 40 degree coolant out of the the radiator, can't be a good thing. What do you think?

 

[GOlsen]

It seems like your thermostat is sticking. I had a temp fluctuation problem before my thermostat got stuck open. Once I replaced it, no more fluctuation. I don't know if I got an "updated" thermostat or what. I got from the Cummins dealer for about $30. It doesn't take that much work to change it out either. Probably not a bad idea IMHO.

 

[AlFischer]

Saw a similar prob on a 96. Had replaced the thermostat with a NAPA part. (did this strictly as a 100 K PM original was working fine) Fluctuated much like you see but did not do the 200 degree thing. Replaced with a new part from Cummins. No more fluctuation. Runs about 180.

 

[24valver]

I had the same problem with a NAPA t-stat. After I put in a Cummins stat, it doesn't "spike" in the mornings like it used to, but it still moves from 160-to 180ish, which is normal.

 

[RyanStewart]

So I got the latest thermostat from Cummins, and I drilled a couple holes in it as per WDixon27's instructions. It sits steady now, although if I'm driving slow around town it runs cool, around 150 - 155. Once I get on the freeway it comes up about 180 and stays there. The cool running is because of the holes allowing too much cold (radiator side) coolant access to the element, causing it to open too much? I donno, but its good enough that I won't bother opening it up again. Most of my driving is on the highway anyway, and as long as its up to temp and the motor isn't getting any harsh temperature shocks, I'm happy.