Overheating issue, need help

2002 f150 4.6
At first the heat stopped working then a few days later I notice the truck would over heat at stop lights or in slow moving traffic. I tested the fan clutch and the fan had plenty on resistance when the engine was cold so I replaced the thermostat and now the truck over heats while driving, very slowly though and some times won’t over heat at all and will cool off at idle or sitting at a stop light, idk what to do next. Could use some opinions?

Are you sure it’s actually overheating? Or could the gauge be bad? If it is truly overheating, it sounds to me like there may be air in the cooling system. Does the coolant level ever change (drop)?


If the heater still does not work, the cooling system is very low on coolant.


The coolant level has not dropped. How do you check to see if there’s air in the system or how would you I guess remove the air?

Idk if the gauge is bad or not I guess I’ll figure out how to test that out next.

I was also thanking of paying to get the block and heads tested but I’ve literally spent every penny on this truck so I’m trying to see what everyone thanks before I go spending any more money, this truck has a new rear end front end and transmission as well as a transfer case and all new suspension soooo I’m invested at this point :rofl::rofl:.

You could try bleeding the coolant system. The procedure varies a little with different engines. Some have a bleeder screw that makes it easier, but I don’t know if a 4.6 has a bleeder screw. Sounds lame, but I’d look at YouTube vids on how to bleed an F150 w/ 4.6 to see if there are any decent tips. I’ve never had a problem with air getting trapped, myself. A few hot and cold cycles just driving and letting it cool off purges the air out for me. Then top it off. Make sure the heat is on when bleeding it so coolant will flow through the heater core.

As for head gasket issues, I’m assuming you can’t see coolant in the oil? The oil would look kinda like chocolate milk. There’s a possibility the engine is putting exhaust gases into the coolant system. If it is, the pressure would be high and the coolant would bubble in the reservoir with the cap off, engine running. Or, they make a kit to test for exhaust gas in the coolant. It’s sold at parts stores.

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Thank you, I will see about the bleeder screw idea , ya that’s why I ruled out the head gasket because there is no bubbling nor oil and coolant mix no matter where I look and no coolant coming from the exhaust, it’s ridiculous I’ve always drove these ford’s but this is my first none obs I’m kinda regretting it at this point but if the pressure test comes back bad I guess that gives me a excuse to save up for an engine, thanks for the ideas.

theres no bleeder screw that I have seen. I have the same truck. mines a 2003

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Sooo… cold isn’t when it needs resistance. When the engine is hot, the viscous fan clutch goes to high resistance. If the fan clutch failed, the truck would tend to overheat in slow traffic or sitting stationary because the fan isn’t running hard enough.


Well sadly that was the first problem now I have a completely different problem my truck over heats while driving and when I say slow it over heats slowly, this morning headed to work the temp gauge went to 3/4 up and never went past that point and I have to drive a good 20 miles to work. Once you park the truck and put it in park the temp goes back to normal.

I know you mentioned that you have a new transmission, but I wonder if it’s overheating…If it was starting to overheat while moving but go back to normal in park. What’s the tranny fluid level/condition?


Transmission fluid is still full and looks brand new I don’t even have 5000 miles on it yet.

I might end up pulling the water pump because there is a slight rattle at the front of the engine and the water pump looks original at 200000 miles I thank its time anyways so Im hoping maybe this is the issue

Looks like no matter what I do I’ma end up dropping more money on this thing I just want it to last till Christmas I’ma be getting a Ford racing crate engine for it, that’s why I’m trying to just milk this old motor at this point

If it is the original pump, the vanes may be eroded and therefore not moving coolant effectively.


Ya this morning I burped the system and I’m going to also double check the thermostat again I got the box and receipt at home, going to make sure they gave me the oem one and then Ill continue with all the other ideas everyone on here have gave me. I will let everyone know what happened once I fix the issue.

Well, you seem to have plans for it if you have replaced the trans, front and rear ends and suspension. Or was it done to at least just drive it?

You know who supplies thermostats to Ford?




How much coolant did you add?