The car is a 1982 GLT with the B23E engine and manual transmission. I recently had the car into my local Volvo garage for startup when cold problems. He determined that to be the warm up pressure control valve and he repaired it.

Now the car starts up properly but when it is warmed up it will drop its idle and threaten to stall after a deceleration and a lifting up of the foot on the accelerator pedal. Not all the time however and the warmer the car is the less the problem emerges. After an hour of driving it pretty well disappears. On occasion the car does stall.

I talked to the mechanic about the problem and he listened to the engine ticking over very comfortably and he guessed it might be a leak in one of the vacuum hoses. But he said he would have to do a smoke test to confirm his theory.

I couldn't leave the car but would like to hear an opinion on whether he is on the right track and is there some alternative to a smoke test to determine whether there is indeed a vacuum leak.