Fan clutch or water pump?

[Bowesmobile]

1997 expy. with 4.6L 4x4 rear air and heat. 200k. Heater core replaced at around 100-110k. Thermostat, water pump and fan/fan clutch its unknown if its ever been replaced. Antifreeze level is good. Now to the problem.

I get decent heat when I'm driving on the expressway or any time the truck is moving. But when I come to a stop at a light the heat slowly disappears. Then when I start moving again the heat comes back.

So in my mind this either means the impellers on the water pump are bad. Or the fan clutch is not disengauging and its over cooling the motor.

Last night I popped the hood when I got home and the engine compartment wasn't warm like I would think it should be following a 50 minute drive with most of the time spent on the expressway. The upper radiator hose was warm to the touch. But not hot. The heater core lines were the same.

The truck warms up steadily when I start driving. Which leads me to believe the thermostat is working properly. I'd like to have great heat without throwing parts at it. Any help is appreciated!

 

[Big White]

It's not the fan. Assuming the temp gauge is where it is supposed to be, I am guessing you need to back flush the heater core. Carefully.

 

[rtbrjason]

It's not the fan. Assuming the temp gauge is where it is supposed to be, I am guessing you need to back flush the heater core. Carefully.

Will having the coolant flushed at a quickie lube oil change place accomplish what the backflush will?

 

[Bowesmobile]

I dont put much faith in the factory temp gauge. But it sits half way up when its warm.

 

[stamp11127]

Things to check:
Fan clutch - should be able to spin the fan with some resistance. If locked up as you suspect then will only turn when running.

Coolant temp: If possible get an temp gun and read the upper and lower rad hoses and or upper/lower portions of the radiator on a hot engine. Thermostat may be stuck open. Would take longer than normal to heat up on cold mornings.
Depending on the thermostat, temp should run between 190-210 on the upper hose.

Blend door: Not sure if it controlled by vacuum but will close at idle if it is due to low/no vacuum.

Water pump: If the vanes were shot then it should overheat due to the lack of flow within the engine. It would also heat up quick - basically standing coolant inside the engine. (Been there, done that)

 

[Bowesmobile]

I'm thinking I'm going to back flush the heater core and go from there. Next up will be the thermostat. Then water pump or fan clutch after that.

The clutch has resitance and rotates when the engine is off. I might use this as an excuse to get electric fans. That and better fuel economy.

 

[toms89]

If the fan clutch sticks on it will typically sound like your driving a 747 jet. Even so the thermostat should keep the coolant in the block until its about 190.

I would consider a stuck open thermostat a good possibliity as it would run cooler at speed due to the extra airflow over the radiator. The motor should still warm up as normal but would take longer. The factory temp gauge is no good for determining actual temps. Its only good to tell you if its cold, overheating or somewhere in between.

Other than that I would consider issues with the heating and air system itself.

 

[splintrcel]

How do u back flush your heater core. Mine def has a bunch of crap in it. I can see crap in the main radiator hose so its safe to say its also spread throughout the system.

 

[Country Boy]

I have a 99 expedition with the 5.4 and im haven the same heating trouble and have put new thermostat and back flushed the heater core and it still does the same thing. Im thinking its the clutch fan hanging in and over cooling the the radiator and hoses.

 

[expy2001]

"i think he was on to something too because when my truck was hot the radiator cap did not hiss when unscrewed and also the main upper hose was too easy to squeeze.

it's still kinda weird. my truck at idle does not get real hot. once you up the rpms even a little bit you can instantly feel it get warmer. to me that is a flow issue and not the water heating up from the extra load."

If you don't have heat unless you step on the throttle or your upper radiator hose is easy to squeeze, I bet you have the same issue I had.

Water Pump. Don't waste your time with the heater core, thermostat, etc if you have a weak upper coolant hose. The water pump cannot create enough pressure unless at a higher rpm (too much blow by).