Can anyone tell me how to purge air from the cooling system on a '97 Expedition? After the thermostat opens, I get good heat for about 5 minutes, then it gradually cools to stone cold over 2-3 minutes. Then suddenly it gets hot again, and at the exact same time, the engine temperature gauge spikes to max, and the oil light comes on. That lasts for 8-10 seconds, then the temp gauge drops quickly back to normal, and everything is fine. Only does it once each time it runs, doesn't last long enough to kick into 4-cylinder mode.

My guess it that I have an air bubble that gets into the heater core, then gets pushed out and hits the coolant temperature sensor (and since air heats up faster than water, it appears to be overheating) then it passes and it's fine. Does that make sense? Replaced the coolant overflow tank recently, may have trapped air then.

 

Did this just start out of the blue or have you recently drained and refilled the cooling system?

Just driving around will usually take care of the air pockets. For the real stubborn ones that don't want to come out, I usually just park on a steep hill facing up and leave the engine running for a bit.

Of course you could also have a bad/sticking t-stat and not any air pockets.

 

it helps if you take the cap off it will push the air out by itself but make sure the engine is cold when you remove the cap or you will get burned an then crank the engine let it run for 20-30 mins that should push all out

 

You would be correct if there was a radiator cap, but there isn't. These systems only have the plastic overflow bottle for accessing the coolant. By rights, the air should be getting left behind in the bottle, but I guess it's not making it that far. Of course there's still the bad t-stat possibility.

 

Started after a heater core flush (was completely clogged, got it flowing) & overflow tank replacement. Can a bad t-stat open, close, & open quickly?

 

Did the crud that was causing the clog get flushed into the engine block or onto the ground? If it went into the block it could be causing blockages.

 

you have a radiator cap its on the bottle and it holds pressure and thats called a coolant reservoir an if you just look real close you will find it has a radiator hose hooked to it too but it could be a t stat and when fail they stick open 90% of the time but on the other hand after 15 years of being ASE certified ford mechanic but what do the difference between a coolant reservoir and a radiator cap an by the way i have seen that the water pump impeller will rust off the shaft or the fins just rust away completely causing some of the same problems your having

 

Heater core gunk went into a bucket.