I have a 91 f250 with the 7.5 engine, and a few months ago I let my friend drive it. It was left idling in a parking lot for like 15 min and it over heated so bad the plastic side of the radiator exploded. I was pretty sure the thermostat got stuck closed but decided to flush out and overhaul the whole cooling system while I was at it just cuz its old and might as well. Opened up the plugs in the block and flushed it, new radiator, thermostat, water pump, and various hoses.

So now I am trying to burp the system. I think I got most of the air out with the truck parked on a hill nose in the air and radiator cap off. Only problem is, the truck keeps overheating while I'm trying to burp it. Temp gauge keeps hovering over at the R and M on normal, it gets over to the A and I get scared and shut it down. Before it has never went past the O ever except while stuck in traffic in 95 degree weather.

So I was just turning the key on to see if it was cooled down yet, and noticed the gauge was moving with the engine off!?! Like bouncing slowly between the O and the R, moving about as far as the entire diameter of the O!

So maybe it is not overheating? How the heck could it be bouncing around like that with the engine not even running? Would air still trapped in there cause this?