My 2000 5.9L Durango doesn't reach normal operating temp. I've replaced the thermostat but the heat in the cab is marginal. I suspect some kind of thermostatic switch since the electric cooling fan for one of the radiators is on as soon as you start the car. The car is cold when this fan comes on. Shouldn't the engine be warmed up before this electric fan turns on? Thanks, Bob

 

The mechanical fan will always run on start up, just the way the fan clutch is designed. Mine always had a hard time reaching operating temp in winter. Had to cardboard the whole radiator. Then last summer the fan clutch went bad causing my motor to almost overheat one day so I replaced it and voila heat. Engine warmed up and didn't need card board anymore. So thats where I would start.

 

The electric auxiliary fan should only come on if the a/c compressor is running or if the engine is over 210 degrees. The a/c compressor will run in max a/c, a/c, bi-level and defrost. The only settings it doesn't run in are head and feet iirc.

 

if your electric fan is always on my first guess would be a bad e-fan relay in the PDC (simple and cheap to check)

 

Thanks for the suggestions. Bob

 

Bob,
I have this exact same problem in my 99 5.2L. So far all I have done is just like you and replace the thermostat. After running for a long time, the highest temp seems to only be around 170 or so with barely warm air pumping out in the cab and nothing but cold air blowing out of the rear vents. I am on to a control arm issue right now but please post if you find your issue as I will be getting back to investigating this in the next couple weeks. Thanks!