f150 4.6 overheating

[JoeB64]

I have a 1997 f150 4.6l. The truck overheats if it does not warm up at idle for 20 minutes before driving. The first sign is loss of heat - temp gauge spikes and then blows antifreeze out of the overflow. If warmed up first, the truck seems to run fine. Changed tstat and pressure tested the system - no loss or gain in pressure while at idle. Why would the truck only overheat if not allowed to warm up? We live in New York and this problem only started after winter arrived. Any ideas....

[CJ0717]

Usually a sticky thermostat would cause this but you have already replaced this with a new one. Have you inspected the drive belt for the water pump? Could be that when very cold this may be slipping. The only other place where I have run into this is when I had a harmonic balancer pulley outer ring not bonded to the inner housing. Slipped alot when cold and then when warmed up the friction was better but not 100% so it would drive the belt. Never heard any squeeling at all. The only way to verify that would be to remove the belt and try to turn the pulley with your hands. It should not budge.

[Davescort97]

Welcome to AF!

Check the antifreeze to see what temperature it is protected to. If your antifreeze is freezing in the radiator it could be slowly melted at idle, whereas if you run the vehicle it will get to temperature before the antifreeaze slush melts in the radiator and will make the engine overheat.

[dfordski]

Originally Posted by JoeB64

I have a 1997 f150 4.6l. The truck overheats if it does not warm up at idle for 20 minutes before driving. The first sign is loss of heat - temp gauge spikes and then blows antifreeze out of the overflow. If warmed up first, the truck seems to run fine. Changed tstat and pressure tested the system - no loss or gain in pressure while at idle. Why would the truck only overheat if not allowed to warm up? We live in New York and this problem only started after winter arrived. Any ideas....

Air trapped in the coolent system. Burp it!