So I have a 1999 ford f150 5.4 Tristan motor recently rebuilt. While still doing the fine tuning we started noticing some oddities with it. While driving one day not under load it stated to overheat. So pulled over checked everything let it cool down, burped it the whole 9 cranked it back up and went on the way and all seemed ok. Then about 20 minutes or so stated running hot again so we cut the thermostat in half and put it back in ran fine for a bit then started running hot disconnected the heater core ran fine for a bit then back to hot. Got it home and then it ran fine for a few days still not under load and then randomly would start running hot. Ran hot both with and without the thermostat in it. Under load it runs hot every time sometimes within 5 minutes sometimes it takes longer but always. Water pump is good, radiator good, clutch fan good, proper amount of coolant, no air pockets detected, new thermostat…(. Keep in mind it does this with or without the thermostat) help us figure this out before we just say screw it 🤦🏽‍♀️

 

Welcome to the forum.

 

Are you trusting a 26yr old factory temp. sending unit to report the overheat condition? Have you replaced the pressure cap on the radiator with one with a proper PSI rating?

 

If replacing the cap and sending unit as mentioned above doesn't fix it and even though it's been rebuilt I would still suspicion a head gasket or cracked head allowing hot exhaust gases to get into the cooling system. Hopefully I'm wrong.

 

Rebuilt..is a term often thrown around ....encompasses everything from a simple repair like hone/rings on a single cylinder to a total remanufacture.

Sure sounds like a head gasket leak or cracked head letting exhaust gases in the coolant.....IF its real temp.


" Cut the thermostat in half".....wtf

What is " fine tuning"? There's no such thing. There's nothing you can adjust. You either did stuff right or you didn't.

Test the coolant for exhaust gases. A negative here is not a definitive negative but a strong positive is definitive that you got a leak.

No air pockets detected? I doubt it. You're coolant level should be Rock level all the time.... Very only maybe a quarter of an inch between hot and cold. If it changes anymore than that.... You've got gases in your system

 

Agree with Mbb. This sounds like a headgasket to me. I would probably even test for exhaust gas in the coolant prior to throwing any parts at it.

 

Replace the thermostat and reevaluate
Do a dye test on the coolant to check for hydrocarbons in the coolant
New thermostat is first

 

So the rebuild was a complete rebuild everything inside was new and heads sent out to a machine shop to be cleaned and tested. “Fine tuning” means the truck had been sitting before we got it and they just put a transmission in it and it hadn’t been run yet so after the motor was put in it still had some kinks to work out. There was a slight stutter and rattle in it. So all new sensors and new coil packs and new injectors . Now the stutter and rattling is gone but it’s the periodic overheating that is the issue. A new thermostat was put in and the cap is new as well. We will be doing a dye test next to see what that can tell us but hopefully a cracked head isn’t the case 🤞🏼

 

 

Stutter... Rattle.... Overheating....

What are your fuel trims? Is it possible you are running very lean?