92 F150
302 v8
130k miles

This one is not making sense to me. Here is what I see as the symptoms:

Temp gauge comes up at the expected speed and holds in a normal position. Just about straight up or slightly right of center.

Heater hot air if you let it idle.

At highway speeds the temp gradually goes down to luke warm. But the temp gauge holds the same temp.

Was thinking blending unit not working right but it would not seem like it would be a gradual heat loss from the heater. Seems like it would drop temp fairly quickly.

Thought about stuck open thermostat but temp gauge should not hold the same temp.

Thought about bad water pump but would seem even if weak it would pump more at highway rpm than at an idle and move more water.

Thought about clogged heater core but would expect it to work better with higher water flow at highway rpm than at an idle.

I normally have it blowing through the dash vents but temp is the same through dash, defrost or floor vents.

I have noticed that the rig cools down way too fast. For example.. I am running with the temp gauge on the M in NORMAL. Stop for gas and by the time I pay inside and fill up the temp has dropped to the N. No way it should cool down that quickly.

Anyone else run across this one or have ideas?

Thinking I will replace the water pump and thermostat to start with but really in my mind have no justification for that other than no other more likely places to begin.

 

Low on coolant?

 

Thermostat. And you can't believe the temp gauge in these trucks, they are notoriously inaccurate. Mine can read the same and the actual temp can vary by more than 20 degrees as shown on a real gauge.

 

Coolant is full, topped radiator off while at high rpm and filled overflow. Will try the thermostat first and see what happens.. kinda of put on hold for a little bit because the alternator quit on it... if it ain't one thing its another.

 

Just an update. Ended up replacing pretty much everything but the radiator to make sure the problem was fixed. Heater core ($26 for a brand new one), water pump, thermostat, every hose for the cooling system and ran probably 10 gallons of water through the radiator to rinse/flush it out a bit.

Heater works now and best bet as to what was going on was that the heater core was lightly plugged and the water pump was weak. There was some kind of reddish almost rust like particles everywhere throughout the cooling system. I have been told that can happen when you mix the red and green antifreeze but never seen it before so can not say for sure that is what caused it.

A coolant system flush would likely have fixed the heater problems for at least a while. But the local shops want $130 for something like that and since it has been about 20 degrees or lower not gonna hook up the hose and flush it here in the driveway. Parts to replace everything with new were cheaper than a flush and now everything is brand new so is a better long term fix.

 

Good deal. You were smart to start with fresh components. It is proper maintenance and eliminates guess work.

 

It's just rust from the block and a lack of proper coolant changes.

If you mix dexcool and green glycol antifreeze you get "glop" that will *really* plug up the system.

 

Yeah, mixing coolant types generally results in snot........glad you found the issues, nothing sux worse than a truck with a crappy heater LOL