2004 f-150 5.4 v8, just had a new motor put in 18k miles ago. Here recently with it being so hot in the afternoons (upper 90's) I notice my truck tempure gauge starts climbing and the when it gets to the higher end of norm it pegs hot and "check gauges" comes on. I pull over and shut it off and the reservoir is full, the hose coming out of the top of the radiator is hot but not hot enough I can't touch it and I can squeeze it. I can get back in my truck and turn the truck on and the temperature gauge needle is back at in the middle of normal operations temps. I've tried turning the heat on when it pegs out and I still have heat and it doesn't move the needle any. I'm stumped, anyone have any ideas? Or has anyone had the same problem?

It hasn't done it in the mornings any and only runs hot every once in a while but it did it 2 days in a row last week and has already started this.

 

Have you tested your coolant? Flushed your cooling system at all? Replaced anything?

What maintenance have you done to the cooling system?

 

Sounds like your thermostat could be sticking.
Flushing the coolant system is a good idea too. Especially if you never have, or don't know if it has been.

 

I'll double check with my mechanic to see if he changed the coolant when the motor was replaced. You would think he would have.

It just baffles me that it only does it in the afternoon!

 

Haha, I read the 18k miles as 180k miles. Big difference. 18k miles seems a bit too soon to need a coolant flush, unless subpar or the wrong coolant was used.
Is the motor brand new, rebuilt, or used? Was the radiator cleaned out during the engine swap?

 

It's a rebuilt motor- got all the papers on it. I believe the radiator was cleaned out during the engine swap, I know new coolant was used. I talked to the mechanic and he said he thought it could be a stopped up radiator or the clutch fan. I tested the clutch fan it's good. The only reason I'm hesitant about the radiator being stopped up is because when I cut the truck off and it sits for just a couple of minutes (less than 5) the gauge goes back to normal operating temp. I'm thinking it could be something electrical??

 

Could be the sensor if it "cools off" so quickly. Problem is, I think that sensor is under the intake manifold.

 

That's what I'm thinking it is, I took it to O'Reilly's and they pulled code P1285 and a list of possibilities that it could be. I'm hoping it's just the wiring. I've heard many places that it could be, I'm going to call the stealership and see if they can tell me exactly where it's at.

 

Talked to the stealership and they're telling me it's on the front of the motor near the valve cover- gonna check it out this afternoon and see if I see anything wrong with the wiring harness.

 

Possibly an air bubble? I've seen other reports on this engine developing a leaking head gasket and drawing air in. When it pockets around the sensor it gets hot quickly. Once cleared all goes back to normal.

Google it.