93 Chevy Lumina APV A/C not working good.

[Bkpie1]

Our Lumina, APV has 202,000 miles on it and has never given us any major problems, just regular maintance. The problem we are having is that the A/C is blowing cold, but then when you step on the gas it goes to hot. When you take your foot off the gas it blows cold. We have charged the system and the compressor is good, (replaced at 190,000 miles) but the mechanics that we had it to said it was leaking freon somewhere and they could not see it, so it must be the evaporator core. (that is usually the last thing to go) We have never had water or anything leak into the passanger compartment that would associate it with the evaporator core, so this is why I think it is something else. There must be a relay or something electrical that we can replace, verses ripping out the dash and replacing the core. Any help on this would be appreciated.

[LMP]

well...this looks rather as a vacuum loss problem..that is, a lot of flap and diverters are vacuum operated ans when you depress throttle, you loose vacuum (manifold pressure is near atmosphere) and some valves will not keep their vacuum position and return back to normal only when you release throttle and manifold vacuum returns to high.
So before investing a lot in cores and things like that...which I believe are OK ...first check all hoses to the vacuum valves under dash near glove box.
I still have a little problem here with this explanation anyway because the diverter flap that directs air through or around the heater core is the only one that is electrically operated...so this one should be steady, vacuum or not..and there is something freak here....
..a low pressure in the a/c line that would develop at higher RPM can be the explanation also....but I understand the a/c works OK at all speeds and regimes except when the throttle is down... hence my doubts ..
Leaking freon shows.....hence my doubts about investing into that without further evidence.

[djamnicki]

Yep, sounds like a vacuum prob to me. See if it has a cannister and if it is ok.

[RobertWalter]

It's a vacuum problem. Check the vacuum manifold under the dash behind the glove compartment area. Remove the knee panel and you will see it. It could be supply vacuum to the unit also.