92, 5.0, 4spd manual w/overdrive - Truck is overheating when A/C is on. After I turn the heat on, the thermostat moves back to normal. Yesterday, it got so hot it overflowed into the resevoir. Anybody know what could be causing this? It seems normal the rest of the time. Thanks. -al

 

Thermostat??
Clogged Radiator??
Leak in Radiator??

Pressure test the system and make sure you aren't getting a leak and allowing the temp to rise too high.... don't forget the radiator cap as well.
You should be able to hold 13 PSI indefinately on the cooling system and the cap seperately.

If they pass, change the thermostat.
When you change the thermostat flush the cooling system until it runs clean.

The added strain of the AC is raising the temperature. The problem exists with or without the AC on... the AC just makes it aparent.

 

Also check for a weak fan clutch, a very common problem.

Steve

 

I replaced the radiator, thermostat and heater core a couple of months ago and now i just pressure tested it. It's not leaking down. Anything else it could be?

 

what kind of water or antifreeze mixture are you running?

 

How's the fan clutch?

 

He seems to be ignoring that for the moment...




Steve

 

I know - that's why I wasted a post saying the same thing as you.

 

Was out of town for three days...the fan clutch seems to be okay. It doesn't have any play in it and it's constantly turning. I'm using the Advance Auto Premix Anti-freeze.

 

New cap radiator cap?? or did it pressure test ok?.. you didn't mention the cap condition or replacement.

 

Replaced the cap when I put in a new radiator a couple of months ago. It pressure tested fine. I think it's the thermostat. It stopped overheating after I put some radiator flush/cleaner in it. The jackass I bought the truck from dumped about 10 lbs. of stop leak in there rather than replace the radiator, and although I've flushed it about 10 times now, it's still got that brown rust sediment looking stuff in there, so I think it's clogging the thermostat from time to time.