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I already picked up the 2 thermostats - NAPA cross referenced for the Cummins 3928639. What about the 3rd one - the external bypass thermostat?

Head gaskets??? YIKES!! I cringe at the thought. As to the thermostat, there are 3 thermostats on these, so which ones would cause heating problems - the two internal ones or the external one?

Well as the truck warms up and hits around 190 or so, I'm guessing that the thermostat is opening since the tube that connects from the thermostat housing to the upper left of the rad gets hot, and the heat does seem to work well right about then, but as the temp gets to around 200 on the gauge...

Thanks for the replies so far. I've only had this truck since June, and she was my first MV.... popped my cherry with a 5 ton! Anyway, it didn't have heat at first and after reading posts on here I checked the petcocks that send the flow to the heater core, and as I read they are often closed...

Hey there fellow SS'ers! New 'private' here with a heating/coolant issue with my 5 ton. She's a 91 BMY, 8.3 Cummins 6CTA, Allison 5 speed auto. Suddenly over the past few days, once the temp gauge hits 200, I lose my heat. Maybe bad thermostats? I'm guessing they are opening because the truck...