I have a 99 Ford Expedition 4.6 that overheats a lot. I changed the T-stat and checked for leaks and found none. The temp would get real high especially on a hot summer day. When i'm on the highway it goes up to about 80% of the max temp. After a while it would drop back to the regular temp, then it repeats this behavior. I topped of the radiator with a 50/50 mix. The problem is a lot worst during the summer, during the winter it only overheats when at standstill. Can anyone help me?

 

Out of curiosity does your radiator have a cap to fill it or are you filling through the reservoir?

Anyhow, it sounds to me that when you opened the cooling system to change the t-stat the pressure was lost and it sounds like you have an air bubble in the system. When the motor got hot, the sensor was sensing the air pocket which read very high. As soon as it passed, coolant dropped the temp gauge on the dash.

Or, is it possible you installed the t-stat backwards? Just a thought...

Open the hood and check the reservoir level when the engine is cool. I'm thinking it just needs more coolant.

Good luck

 

The radiator definitely sounds plugged and there's a chance the fan clutch is no good.

With the engine OFF, you should be able to turn the fan by hand, but there should be resistance. If it spins freely, the clutch is no good and is the cause of overheating when standing still.

 

You should also be able to hear the fan. If the fan clutch locks properly, the fan moves a lot of air, you should hear it when accelerating from a stop when the temp gauge is high.

 

You may have an air pocket in the system and have to bleed it out and if it's been doing it for a long time you could be doing serious damage to the engine as they don't accept overheating very well.