i got a real problem my a/c blows ice cold in park but when i drive it it blows cold then after overdrive kicks in the a/c dies down and blows hot, but after u let of the gas pedal it blows cold but when u press on the gas pedal it blows hot again.

 

you have a vacuum leak somewhere. I have the same problem with my 88 mgm but it takes mine until I'm over 60mph or going up an incline to do that.

 

well i checked all the vaccum lines an didn't see any of them broke and i replaced the temp sensor the runs in on the heater hose. it has to vaccum lines for the a/c on it but it still does the same thing.

 

check the check valves and vacuum reservoirs. if the check valves let air through both ways, replace them. if the reservoirs won't hold vacuum, then replace them.

 

found one problem with my A/C. The check valve on the vacuum tree going to the two lines to the A/C was blown and wasn't holding vacuum. Changed that out. I'll see for sure on Sunday (next time I'll have in out on the highway to test the same conditions).

 

big vacuum leak that people dont see is the PCV valve. the rubber that it sits into that holds it can let air pass by causing vacuum leak. if it is old, the valve could even pop out of the rubber and spray oil and make vacuumleak worse. just thought i would mention this since it has happened to me before.

 

that thing is a bugger to get to on our 302s though. back side of the engine and all. best to do that replacement first thing in the morning while the engine is cold.

 

well i replaced the check valve on the heater core line still blows cold then hot. but i took the glove box down today an found a metal vacumm can could this be the problem? i took the line off an it did the samme thing as when i'm driving the car. but what is this part called? i'm stumped.

 

if it's the large can connected to the top of the ducting, that's one of the actuators (not real sure which). If you mean a tin can looking thing and you can see both ends and nothing but the vacuum line is connected to it, then it's a reservoir and yes, that could be the problem.

The check valve helped mine, but I still have the issue. so I have more leaks. and they're probably one of if not all of the actuators. BAH!

 

well just as i thought it is the [[{vacuum cheking valve}]]. it was a pain to find an to get a hold of cause all the part stores say it's not a high demand part but at $7.00 it was cheap.