put a new cummins thermostat (#3802968) in my 1990 d 350.it will start to put out heat about 3 miles down the road.put the same cummins thermostat(#3802968) in my 1989 d250.it will not start to put out heat for about 12 miles.both are installed correctly.both heads have been cleaned up with a die grinder as per dodge service bulletin.what's the problem with my 89? this has bothered me for the last two weeks.i can't come up with an answer.

 

You'd just have to test the thermastats side by side and see which opens first, but all i can think of is maybe the one in the 89 might be a dud or somthin. i know my 92 used to take a long time to heat up and changing thermastat fixed that.

 

I'd try a different thermostat. They seem to be quite inconsistent.