I've looked at the usual suspects with no fix. About a month ago I had an issue where, after the car was hot, it would drop vacuum from 15 to 10-11 whenever it was at idle, turning on the A/C seemed to resolve this. Now it is the opposite. When the A/C is in use the idle and vacuum drops during idle. Any ideas?
ISC is within spec and clean(actually only a couple years old), voltages are good on the TPS. Looking for ideas.

Thanks

 

go to the Sensor S/W check screen and make sure the A/C sensor is coming on when you turn the A/C on at all 4 speed settings. The PFC is very picky with the resistance of the A/C switch, and sometimes i will not come on, and therefore the Idle won't rise to compensate, regardless of how clean or in spec your ISC is. The excess load on the motor will cause your idle to drop, and therefore your vacuum to lower.