which vacum valve please help

Im having problems with my a/c,1991 OCC, works great until I accelerate and then switches to hot, and back to cold when I let off accelerator, any clue which vacum motor controls this????

It may be the vacuum check valve at the TBI. There is a small round piece about 2 inches from the TBI that is the check valve.

When you accelerate, the vacuum decreases, and if the check valve is defective, it will change the amount of vacuum in the line, and can allow the vacuum motors to change position if they are held in place by the vacuum.