Hey guys names Jim and glad to be here I’ve lurked and read through some threads here and seems like a really great forum with knowledgeable 1st gen owners. Anyway let’s get to the questions, truck reaches temp after about 10 minutes driving and everythings normal except heat never gets hot always stays lukewarm barely defrost windows or anything. Could it be a stuck open thermostat? Would clogged up heater core cause that? Or could it be the water valve near the firewall? This is my first diesel and I’m slowly learning about these things.

Welcome to world of Cummins , I have 2 of the beasts , bought one new , one a retrofit into an early truck . Heat is an issue . You could not make one of the beasts run hot in the summer sitting at the Equator with no coolant fan . As suggested look to determine if the obvious problem is there . I’m beating you are going to need the winter radiator cover = a piece of coraplas ( corrugated plastic sheet ) with a few 2 in holes punched in it . Placed in front of the radiator , after you will enjoy warmth with out overheating issues . Even with my radiator covered as suggested , the head temp is 208 * and return to the engine from radiator is 107* . I know your goin to say something is wrong , well it’s a fresh rebuild , new pump , new radiator from a Power Wagon with a 225 cu in gas engine . These engines are very efficient and move lots of air through , sitting still the temp will still noise dive to the sub thermostat opening temp . Look for the suggested issues , if you don’t find any problems try the Hill Billy Thermostat , temporary and see if it’s better before you decide on parts replacement .

Edwinsmith...my temp gauge stays about in the middle once warmed up.

Well that’s a positive indication . OEM gauges are not the best or accurate , but in the middle indication is a good thing . Have you checked to determine the core and heater box are not plugged up with debris ? This is common also . A laser temp gauge is an almost have to have to track down easily what is happening . I really suspect heater control valve or plugged box and outside of the core .