My wife's car, this week started idling the AC under heavy throttle. It's never done that before. Seems like lack of vacuum, so it idles vacuum accessories, right? This suggests vacuum leak....
When engine is warm, starts fine, and initially has vacuum at 15 inHg. Over the next 20 seconds or so, slowly rises to a hair over 18 where it stops no matter how long it runs. Needle is rock solid and steady. Which suggests not a vacuum leak....
I've never run a vacuum test on it until today, so I'm not sure if this is normal.
Any ideas?

 

I'm confused by this part:

"My wife's car, this week started idling the AC under heavy throttle. It's never done that before. Seems like lack of vacuum, so it idles vacuum accessories, right? "

I don't know what you mean by "idling the AC under heavy throttle" or "idles vacuum accessories".

Could it be the idle rpms increase a bit over the initial 20 seconds after startup?

I have to be honest, I've never watched a vacuum gauge on immediate start up, just when idling.

 

Ah, sorry. I assumed AC ran off engine vacuum, like old cars had everything vacuum powered. That was the only connection I could think of as to why it would change from blowing AC through the vents, to defrost setting while she gives it heavy throttle. I figured near full throttle = low vacuum, so if the engine doesn't have enough, it shuts down/alters settings of vacuum accessories to assist engine.
Not sure if that's the case though....
Yeah, vacuum might go up as idle sorts itself out after startup... Might not be a vacuum issue at all, but I can't think of anything else that would randomly start messing with the AC under heavy load.

 

I see, you problem is AC related...some cars stop voltage to the AC compressor at WOT, but I think in your case, your HVAC control system may be the issue..maybe old lines, iffy diaphram...my 89 does use vacuum to control the deflector (I think that is what it's called).

I'm not sure about Firebirds, as GM was using a motor control also in the late 80's on pickups with the push button type controls.

 

Jensen...try checking the vacuum line that runs from this check valve, around the back of the motor, that Tees into the Cruise Control and "the Vacuum ORB" (reservoir ball) located in the drivers fender. That line might be bad by the ball. Mine was hanging and I still had 18 HGs of vacuum like yours.

 

Vacuum reservoir is missing. Tube ends open to the street. Might be the problem....
Thanks.

 

Well.....ther're ya go!