Hello guys! I've recently purchased an 02' F250 SD with a 5.4 in it. The cooling system seems to get air bound on the engine side of the thermostat, which will then cause the engine to "overheat" and the heater to blow cold. I thought that I had a Head Gasket issue, but I'm not burning, leaking or losing any coolant. The only coolant I lose is when the system gets itself air bound and it forces coolant from the Degas bottle cap. I removed the air bleed check valve from the thermostat to allow any air to escape easier. This allows to drive the truck at least, still pushes coolant though. The pressure check of the system was great. The first check was done cold, held 17 lbs. for approx. 1/2 hr. When the engine was warm I had the same results. I started the engine and the pressure never built up pass 16psi. Now Im just wondering if I have a large air pocket caught in the heater core or something, and when the engine gets warm the air expands and forces the pressure to rise which then forces coolant from the cap. I have bled the system multiple times with the same results. The waterpump, thermostat, and fan clutch are all new. Please help.

 

This sounds like a stuck t'stat to me---even though its new doesn't mean its working properly.

If you have the time and energy remove the t'stat completely and observe if the same thing happens.

Air pockets tend to work themselves out to the overflow reservoir during driving when the engine is at full operating temperature. I've rarely seen trapped air being so problematic to remove.

Another test for t'stats not yet installed is placing them in a large pot of water and bring it all to a boil. It should open around 195* or so. IF you do this with household cookware make sure to scrub them afterward very, very thoroughly!!!

 

I've actually cut out the little brass rivet in the t stay vent hole. It doesn't overheat anymore but it still push anti freeze past the degas cap. Before doing that I put the t Stat in a steel bowl on a scientific hot plate and used on of our calibrated thermocouples to verify the opening point.

 

I'd be curious if there's not a head gasket beginning to fail?

Has a leakdown test yet been performed?

 

I agree. I had this problem occur on an older engine and after five mechanics looking at it I just took it home and tore the top off myself. I found a slight crack in the head gasket and it was leaking to a coolant passage. The crack was so small, even a leakdown would not show. I did not think to do a compression test, but that would show it immediately. Try that before you do a teardown. It might save you some time.