hello I have a 94 cummins 12v 5 speed with 143k on it, I just bought it about 2 weeks ago, came with lots of paperwork for work being done. It says it has a new water pump, hoses and thermostat. When I drive 65mph down the highway my temp will sit at 150, then rise to 190 then fall back down to 150, you can watch it rise and fall, not sure if it's the thermostat? The truck sat for a year before I bought it. Thanks!

 

if u search the forum u will find out this is typical. Hope this helps

 

Yep, more do it than not, it has to do with the overkill cooling system and the action of the thermostat.

Won't really hurt, but if you want it to be relatively stable, get a genuine Cummins thermostat in there.

 

i agree with pat, for some reason after market thermostats just don't work like the cummins. the cummins t stats will cost more, but are well worth it!