I have 93 f150 v8.

I just replaced the thermostat, nothing wrong with the old one, I was just doing a basic tune up on the whole truck.

I've burped the radiator...etc...

Here's the problem, I can have the truck run at idle and the temp runs normal.... great heat..

I start cruising 55mph and the temp rises...it is not a faulty temp gauge...the truck is getting hotter......

It gets up to about the L on normal and sits there.....Here is the funning thing...if I take the overdrive off or increase the RPMS by blipping the throttle, the needle almost immediatly makes it's way back down to the N on normal. It is like the thermostat will not open when cruising....but blipping the throttle or increasing the RPMs makes the thermostat open and wala, the engine temp goes way down. If I start cruising again, the needle creeps back up hotter until I increase the rpms, slow down...etc.... It's like the engine doesn't flow very well cruising...

What could cause this?? A bad thermostat out of the box??? Is the higher RPMs giving it more pressure to open?? More air in the radiator??

Any help would be great. Thanks

 

Since you just changed the thermostat I would dig out the sales slip and take it back, good enough place to start.

I would lean towards the thermostat only because it doesn't sound like there were any problems before changing it. I don't know what type of thermostat you picked up but I have always had really good luck with Stant's Superstat version.
If you still have problems after switching it out again I would check your fan cluch. Could be slipping.

 

ya, if you did not have the problem before, take it out, if you did have the problem before, fan clutch would be the first place to start

 

my 94 f150 has never gone past the o in normal.

it has elec fan and does run cooler at idle than at 45+ but in a year the fan has only kicked on half a dozen times if that.

i would replace the thermostat first.

 

Check your timing. If it is advanced to far it will cause the engine to heat up more while running hard, then if it is just idleing. It was a common problem on one of my previous 289 motors.

 

check the wiring on the temp sensor it may have been knocked when you were changing the thermostat.

 

My truck does the exact same thing , new thermostat and water pump too.