I've got a problem. I've got a 4.6 and it overheats with the AC on. I changed the thermostat it overheated, then I had the radiator flushed, it overheated. I took all of the hoses back out and looked through them I flushed the block, it overheats. The fan is good, It overheats. When driving down the freeway at 75 and 85, it overheats. If you turn off the AC it cools down quickly, if you slow down below fifty it cools off. Driving stop and go with the AC on high in the city even at outside temps of 108 plus its OK. It sometimes starts overheating while still accelerating down the freeway on ramp. Turn off the AC its OK. Anyone got any Ideas?

 

Is it actually overheating, or is your gauge playing tricks on you??
Jimmy

 

I'm starting to wander that also as I haven't had a coolant blowout yet and its not pinging, I have definitely run out of other things to try.

 

HEY,check between condenser and radiator. can be full of stuff,ie nests,debri and such.good luck.

 

You might try a new temp sending unit, it may just be going bad. If it hasn't boiled over, it's probably not too hot. I read in the manual on my 2001 that if it actually gets too hot, the computer will go into a cool down mode and quit firing every other cylinder to cool the engine.
Jimmy

 

Yeah that space behind the condenser is the first thing I checked, It was overheating once and as I was going over it I found a Circle K plastic bag in there, it was sucked up between them and spread itself nearly perfectly over the radiator. Ever since then when I'm forced to drive over any plastic I look back to see if its still in the road and not in my radiator.

 

Replaced sender I still get temp warning and guage maxes out, this pig is starting to bother me.

 

You seem to indicate the the problem is prevalent at higher engine speeds. Is it possible that the water pump is either cavitating or that the impeller is slipping on the shaft? I had a problem once where my impeller was corroding, the pump was cavitating and I was not getting sufficent flow.

 

I haven't pulled the water pump yet, but I don't think thats it as I can hot rod it around stop and go with the AC on high and it doesn't overheat, but as soon as I hit the freeway and kick it up to 75 it starts to creep up until the light and guage indicate overtemp. And when I turn the AC off it cools off quick, and we are running about 110 to 113 depending on if you are in town or out in the country like us.
Now the electric cooling fan in my old truck isn't working because I've been too busy to replace the relay and If I drive through town without the air on and shut the engine off when I stop its okay but when I hit the freeway I step on it and turn on the air get up to 75 and I'm okay.

 

At highway sppeds, the fan doesn't do a whole lot, air being forced into the radiator is cooling the engine.
Check to see if you have a soft lower hose, it could be sucking shut restricting water flow to the pump and engine.
Jimmy