Have a 2002 F150 4.2L. Replaced radiator with a double core radiator, new fan clutch and thermostat and cylinder head temp sensor. Refilled system with coolant thru top hose hoping to eliminate air. Temp gauge still will randomly spike all the way up to HOT but there are absolutely no signs of actual overheating under the hood. Turn it off and in a couple of minutes crank up and temp is sitting back at the halfway mark. And there are absolutely no signs of a blown head gasket either. Have never had to add one drop of coolant to the radiator over all the months this has been happening. Does not leak or lose a single drop. Also I do not have a engine coolant temp sensor..........only a cylinder head temp sensor which when disconnected will put the dash temp gauge flat down on the bottom under cold. Anybody know what is happening? I know others have had same problem and I have yet to read a solution to this problem.

 

Sounds like you still have an air pocket in the system? when you first added the new coolant and warmed up the engine, did you leave the rad cap off? (or the cap off the coolant tank).

I do that to let the system 'burp' itself when it's running at full operating temp. Not driving it, just letting it idle.

This was on all my vehicles thru my 13 F150 from way back in the 70's so I have always let my coolant 'burp' over all those years.

Good luck!

Mitch

 

^x2

I like the feature on my 2000 Dakota with the 4.7l, it's got a neat feature of a coolant burper. It's an allen head screw you take out to get all the air out of the system. Once it's done burping, you reinstall it. Very nifty.