So this is an odd problem. Coolant level is fine, thermostat is fine, dual fans seem to run normally and run fast (at idle) when the AC is on. The truck will start to overheat after driving about 25 miles with the AC on when the ambient temperature is over 80F and I'm going about 70mph.

Never overheats if the temp is 70F or below.

If I turn the AC off and turn the heater on high/hot, the gauge drops to normal within about a minute.

Or... if I slow down, such as in a 45mph zone.... the gauge drops to normal within about 30 seconds.

So do you think this points to weak fan clutch/es? High miles... 290,000+ they are surely the factory original fans.

 

I don't think electric fans have clutches. I would think at 70 mph you have plenty of airflow with or without the fans running. I wonder if the water pump impeller might be coming apart?

Is this an Ecoboost or a regular V6?

 

Sorry, should have said it was the 3.7L.

 

The 3.7 has known water pump issues. Not saying that's your issue but it's possible.

 

OK, wanted to give an update.

Still having this problem with overheating. It's not a problem all winter long, so it's been on the back burner but now that it's spring and the temps are getting in the 60s and 70s the problem is back. I really can't diagnose it if it isn't overheating so once its over 70F again I will be back at trying to figure it out.

Only overheats when going 60-70 mph and will overheat more quickly if AC is on or if I am towing something. Temp goes back to normal if I slow down and it helps lower the temp if I turn the heater on full blast.

I'm thinking it may be an air pocket, and have watched some videos on draining and vacuum filling the coolant, so I will probably try that. If anyone has any thoughts please chime in.

I've got a kit coming that will allow me to pressure test the coolant system, and to vacuum fill it.

 

I think you need to check the water pump.

 

Finally had some time today to work on it...

I pulled the old fan and radiator off... pressure testing to 16 lbs revealed a small leak on one of the plastic sides of the radiator so I figured I needed to replace the radiator anyway. And since the truck has 305,000 I figured I'd replace the fans too. The old ones worked but when you spin them by hand they no longer spin freely, so it was about time anyway.

I unhooked the radiator from the AC condensor rather than removing the lines and pulling it all out as a unit. So when I pulled the radiator out I found a pretty thick layer of caked debris between the radiator and the condensor. Kind of a dumb design... there's no way to clean it out unless you take them apart.


I pressure tested it before refilling it and realized I needed to put a new o-ring in the lower hose connector. After that new o-ring was in, it seemed to hold 16lbs for a good 5 minutes or so... I then pulled a vacuum on the coolant system and vacuum filled the system. First time doing that, it seemed to work well.

So I'm hopeful that with a new, clean radiator and a new fan... and now hopefully no air pockets in the system, that this will be the solution to my overheating problem.

 

I'm happy to report that a new clean radiator was indeed the solution to this problem. All the lint caught between the radiator and the AC condenser was causing it to overheat.