Constantly fluctuating coolant temp

[kas83]

My truck had a new Cummins thermostat put in it in September, can't tell you the part number off hand. With it below 40* outside, it's constantly bouncing between about the 150(line above 140 on my 95 gauge) mark and just over 190, and you can feel the difference in heat out the vents. My old 97 would fluctuate from one side of 190 to the other, about 2 needle widths. Any 24valve I've driven seems to hold right on 190 while going down the road, although they cool off quick when sitting.

Anyways, do you think I have a faulty thermostat? I thought I had seen a TSB about this, but I can't remember. Should I try a Napa brand or something else? I can't see the constant swings being good for the motor, always hot/cold. Thanks for any and all help.

 

[PRattenbury]

No, mine does the same thing.

 

[zstroken]

I think it is the location of the coolant temperature sensor. 12V is right by the thermostat. Water heats up, thermostat opens.
My 97 used to do the same thing.

 

[ACpwr97]

The thermostat and temp sender are in opposite corners on a 12v

 

[golfersmith]

Cummins has an upgraded tstat to cure the swinging temps for our trucks. I have it but did not make any difference. Doesn't hurt anything though. Under load towing the temps are steady.

 

[bigstacks95]

Never seen one that doesn't do it. (not saying there isnt any that dont)

Freaked me out the first time I saw it.

 

[1sik12v]

i have the upgraded tstat and it does the same thing!! junk!!!!