just did a motor swap in 98 1500 2wd. put a 96 motor in it
takes a long time to build up heat in motor,
wont go over 150deg. put a new 190deg.thermostat in it still wont heat up. trip to work barly hit 150 20 miles, out side temp 30deg.
old motor would get to 180deg. everything else works fine
thanks for any info carey

Are you going by the factory gauge or aftermarket?

How is the heater working? Same as old engine or weaker?

sounds like thermostat is stuck open. got a digital infrared temp gun? check temps with gun at upper and lower hoses, and on top of manifold.

I had same problem years ago. Mine was a faulty thermostat. It was the expensive stant thermostat. Went back to a low buck and problem solved.

I'm thinking out loud here, but IIRC Magnum Beer Barrel manifolds have the sending units in different positions depending on year. Also, I think there is a coolant temperature sensor (black 2 two spade conn.)and a separate coolant sending unit (grey 1 spade conn.) Make sure the connectors aren't switched around and that either one is not broken. They do break easily and they are cheap to replace.

Search RockAuto for pictures.

factory gauge, heat not as good as old engine
have not used heat laser on it yet
thermo is new could still be bad, nothing changed from old thermo to new one
I used the 98 intake whitch has only one sending unit in it 2 wires the 96 intake had 2 sending units
it is in my brothers truck he put cardboard in front if rad. today covering all of it. got to 180 and had more heat
will check it w heat gun tomorrow

I think your viscous is screwed. It's locked up and pulling air all the time.

Yes, it is.

Do you have another fan off your old engine?

yes I still have the fan
will try switching that this weekend