Randyp

I had my rad flushed and now it will not build up to operating temp and I have very little in cab heat. After the rad flush they put a Napa thermostat in and I have recently changed it and put a Cummins thermostat in thinking it might help, but still having the same problem. Any suggestions. Even when outside temperature was in the 80’s the engine would not get up to 190 unless I was pulling a load. Please help, thanks.

AlpineRAM

I'd test boil the t-stat.
It's not unheard of that even a new one can be bad.
(The good thing is that they usually fail open instead of closed)

Another cause for this can be the t-stat installed in the wrong direction or without the correct gaskets and seals.

patdaly

You should at least see it come up to the 190 temp and then the temp fall back, if not, then you know the thermostat is sticking open or something is allowing the coolant to bypass, and believe me, it doesn't take much for these things to stay too cold.

I have to put by winter front in if I expect decent heat at all........

SIXSLUG

No heat in the cab could be an air bubble in the heater core or a sticking blend door....

The temp gauge not going into the 190 range is most likely a thermostat problem as listed above. I would attempt checking this first...

Randyp

Thanks for the input. I have another new Cummins thermostat in and the engine is still not building temperature. Do you guys have any other suggestions? Could it be my clutch fan?

nothingbutdarts

That would have to be a call you make, it sure is pretty easy to hear the fan cycle in when it gets hot, how does it sound when cold or warm?

Midnite

A failed fan clutch wouldn't cause the symptoms you describe.

I've heard it's possible to install a thermostat upside down, but I've never actually seen that. Could it be possible that you've installed the two thermostats upside down?

Desert5600

Maybe the temperature being read is incorrect. Have you read the temp of the block or radiator with an infrared gun? Bad gauge or sender?

Busboy

First ensure that the cooling system is completely full and that the heater core has been bled, a cooling system not completely full can cause the HVAC to blow cool or cold and if sufficiently low enough can cause the thermostat to act irrationally and also give a false reading on the temp gauge. As has already been mentioned a winter front is needed in cold weather especially if the radiator is now not clogged with residual oil and dirt because of the CCV.
IF the clutch fan is not operating properly it usually causes an over heating problem... BUT if the bearing is starting to seize or has seized this will lock the fan up causing the engine to not reach operating temp, this is not a very common problem but I have seen it on occasion... BUT you should be able to hear the fan locked and engaged. With the engine stopped and cool try to move the fan blades it should have smooth resistance but not be locked or jumpy. The fact you have no heat in the cab sort of rules out temperature indication.

Desert5600

For some reason, missed the no heat in the cab.

SIXSLUG

Pull the water pump and see if it has any fins left....I'm stumped on this one.

Is this truck new to you? Are the heater core hoses hooked up, or bypassed?