A few months ago I replaced my thermostat and it has ran fine since then. Now, since its gotten hotter out cause of summer, I have noticed if I drive it for about 45 minutes or more it will creep its way towards running hot on my temperature gauge, almost to where I have to pull over and let it cool down. Especially if I try to run the air conditioner it definitely shoots right up there to hot. I don't think its the thermostat because when it went bad it wouldn't run more than 10 minutes without getting hot but now it will go about 45 minutes to an hour as long as I don't run the air conditioner. Any ideas? (2001 F150 V6) My anti-freeze/water is up to the cold fill level as well so its not that either.

 

Is it boiling over? If not, it could be the temp sensor or indicator.

Has the coolant been changed when needed? The green stuff needs changing every couple of years. May be a plugged radiator if it is boiling over.

Retarded ignition timing can cause overheating but with today's computers that probably is not the reason.

If it ran hot before changing the thermostat, you could have a leaking head gasket.

I seem to remember something about the early V6s having some intake manifold problem that caused a loss of coolant or overheating or something. I don't remember the details. Do some research on that issue.

 

did you use the correct thermostat???

 

and is it installed the correct way????

 

Use a infrared temperature gun to scan the cooling system for temperature differences which may be caused by a blockage. A blocked radiator can cause the problem but also a radiator hose that is collapsing while driving can restrict flow. A 50/50 mixture of coolant and water is important as this mixture is much more efficient than just water or just coolant. Check the radiator fins for debris blocking them. Not sure of the 4.2 water pump design but if it has fins like the 4.6 and 5.4 then a loss of one or more fins can slow coolant flow as well. Make sure you haven't overfilled the oil pan with too much oil which can cause oil aeration and higher friction within the engine and higher temps follow. Of course a blown head gasket can cause elevated coolant temps as well and a hydrocarbon test will indicate if that is happening.