Need help with weird overheating/no-heat problem - F150 5.4 content

Hey guys, A friend from work is having a strange (to me) issue with his 2000 F150 4wd/5.4. After a cold start and about 5 minutes of driving the temperature gauge will go into the hot zone for about 10 sec then it goes back to normal for the rest of the drive. However, once it has done that the heater air will blow cold. If you turn off the heater for a couple of minutes it will blow hot again temporarily, but it doesn't last long. Ideas? Traveling bubble in the system? Clogged-with-only-a-trickle heater core?

Air bubble.

The air turns to steam and rockets the gage when it passes the sensor. The erratic heat is the same air bubble. The water pump won't pump air, resulting in the heater core not seeing hot water.

Fire it up, let it run with the heat on and keep filling while squeezing hoses. Be patient. Keep an eye on the overflow bottle, keep it full.

If you're running straight water, don't. Go with a 50/50 mix.

Dan

How is the anti freeze ? Ice or slush could cause this.

Is it full of coolant?

He reported normal coolant levels and consistency. An air bubble sounds like a likely culprit.

He reported normal coolant levels and consistency. An air bubble sounds like a likely culprit.

Get the front end on ramps or jacks stands. run it while filling until all air is out. Quit when thermostat opens and try again when cool.

If that fails look for a bleeder point. Running with air in the system can cause hot spots. If you are sure there is no air and it still does it suspect a head gasket that only leaks when cold.

I don't think there's a bleed point on those, but I don't think it's an easy one to bleed either. Get the front end up. It's an expansion tank system(no overflow bottle), and I think it has a heater core valve like most fords, so make sure the heat is all the way to hot.

With expansion tank systems, if the little steam line on the top of the radiator is higher than the engine, you can pull the steam line off the radiator and fill the air bubble through that hose.

We had regular issues with Panther bodied cars which involved the heater becoming plugged up, I regularly removed both heater hoses at the firewall and used a hose through both ends of the heater radiator until clean water flowed both ways, then reinstall the hoses and top off the system.

Not sure if the F150 is the same but it was the Mod engined vehicles, which is why I mention this.