Hello all.

Sorry to post a new thread, there's 110+ pages and I've got limited search time.

I own a 2000 F150 4.2L (I think off the top of my head) v6 XL model. For the past couple of years I've had an issue with overheating. So far I've had the following replaced:

Radiator
Thermostat
Radiator Cap
Water Pump

The intake manifold has been repaired, that was the first item to have some work done to it. The shop I took it to said the coolant hose that runs into the manifold was corroded at the entrance to the manifold. They stated there was no way to actually replace the hose, but they gave me two options: 1. replace the entire manifold for $750 or 2. cap the existing entrance port, drill a hole into the manifold next to the port, and run a new hose $280. I chose the cheaper option which I suspect is part of the problem.

Now to the problem.

I can drive around town all day long no problems even in 100+ degree Oklahoma summer heat. I left it idling on the side of a street for 2 hours during summer and no problems. The moment I get the truck to 55mph it starts to overheat. Not just overheat, but peg completely out in just over a mile after hitting 55mph.

Once I drop the truck down to about 40mph the temp starts to lower, but will fluctuate between normal and 3/4 to max.

Anyone have any idea what I need to look at? I've been told head gaskets, but there's no leaks and no water in the oil. I've been told to replace the engine, which if I could afford that I could afford a down payment on a new truck. It's a bit frustrating at best.

 

I'm not familiar with the 4.2 engine but just throwing some ideas out there. Is it actually overheating or is the temp gauge just pegging because of a defective temp sensor? Has the fan clutch ever been changed?

 

Another perspective is the coolant system can't move enough coolant to take away the extra heat made at higher power from the motor.
Is the modification repair causing a restriction?
Might seem funny but is the fan on backwards?
Is the radiator shroud being used?
Fan clutch?
Has air been urges out of the system?
Has a system pressure test been done?
Does the coolant ever get to the boiling point?
Is the cap pressure high enough?
There is basically not many parts to the system.
.
The suggestions arise from your statements about running cool enough at idle and low speeds in very hot ambient temperatures then rising when the motor is producing more heat from combustion than the system can get rid of.
I have saw this happen in cars with restricted engine bays and high output engines (hoped up) where the airflow can't move fast enough through the radiator because it can't get out under the car fast enough after the engine changes were made.
Good luck.

 

Does it ever puke any coolant out of the cap on the plastic tank? Smell the coolant in the plastic tank ...does it have an odd or burnt smell? AS ALWAYS.....DON'T REMOVE THE CAP ON A HOT ENGINE.....LET IT COOL AND REMOVE SLOWLY.

 

Thanks for all the replies!

@Lime1GT: The fan clutch is still engaging, but I'm not sure if the sensor gauge is working or not, I'll see if I can get that checked.

@Bluegrass7: I have no idea if the mod is restricting flow or not, but that has been something I've been concerned with seeing as when I've said that to another mechanic he gave me a wtf look. The fan is on correctly, fan clutch engages (I tested that with a small metal tube, slowly placed in the fan to see if it stopped it, took the tube out of my hand), haven't done a system check that I'm aware of, I'll see if I can get that done, the cap pressure builds correctly, but not sure if the coolant has gotten to the boiling point, I'll do a test to see.

@Spotty: the coolant smells like coolant, chemical sweet smell. I've had enough water pumps go out on me in mid drive to know exactly what coolant is suppose to smell like never pleasant.

I've had my truck for 10 years now and it has the best feature of any truck, it's paid off. I'd hate to lose it considering it drives nice and has low miles on it considering the age (just hit 97k actual, bought it with 54k on it). I don't drive much, but would like to get back on the interstate with it as my wife's PT cruiser sucks when it comes to leg room.

I'll do some tests and post an update, thanks again for all the suggestions!

 

how is the coolant level? Any loss? Checking directly at radiator and not overflow? Possible defective t-stat but I would think it to act up more in slow moving traffic rather than fast.
Pressure test the system if you do find level dropping. Many can be rented through auto parts stores. Good luck with everything