Hello, lurked here for years and have found some excellent info!
I have a 2001 f150 with 4.6 v8. After about 10-15 min highway speed, overheats, blows coolant out and then returns to normal. I tried several times to fix the issue with no luck. Took it in and had it put through the ringer. Low flow through Heater core and Radiator. Also suspect air getting trapped. They tried a full flush, new thermostat (I'd done this several times) and tested for gasses/leaks that would point toward Head gasket issue. The head gaskets fine. Water pump fine. Had both Radiator and Heater core replaced. Good flow now and have heat but it goes cool and engine gets hot again before heat returns. Still acts like air getting in the system. Now the weird part (maybe not)..if I remove the thermostat, it runs cool but can drive hours with no issues. I'm wore out with this and spent plenty on it! Any suggestions? Any help is appreciated. Thanks in advance

 

Does it have a Motorcraft 195 series stat ? If not , that can cause a slue of issues. I'd stick with 194-196 AFA margins and just to be safe.

 

Dont beleive its Motorcraft. It's a 195 though. I'll check tomorrow

 

Doesn't have to be Motorcraft, it's the numbers that are important.

 

Did they take the water pump off and look at it, because all of the impellers were gone on mine except for one, and it was as thin as a razor blade. The water pump was not leaking.

 

They didnt remove the water pump. I'd asked that and they felt it had good flow and there wasnt a reason to.

 

Check your fan clutch and like the poster said above the water pump maybe doing great setting but under a load not being up to snuff.

Don't know what part of the world you are in but here in Florida there is a very simple way to tell if the fan clutch is doing its job. We turn the ac on, mind you the temp is up and place a pick term in the drivers vent by your right hand with ac max check the cool temp and then have someone pour water over the little rad in front of the engine rad and see it the ac gets cooler than a few deg. If it does then the fan clutch is going out.

 

bad coolant reservoir cap? won't hold pressure? just 2 cents

 

I hope this helps to clarify fan clutches in general.
www.napabeltshose.com/~/media/napa/documents/fan-clutches-standard-vs-heavy-vs-severe-duty.pdf

 

Hey yall, thanks for the ideas and input. Took it back in and now head gasket test failed. Kinda figured that was it. Strange how it was negative last time.
Anyway, thanks again