[drumvudu]

Whenever I drive down the road, if I have the heater on and the fan to the heater on middle to high settings, if I push the gas pedal down half throttle or more, the fan stops blowing air out the vents. I can still hear the fan running, it's just that no more air comes out till I back off of the throttle. It's not just that HEAT isn't coming out either, it's actually the MOVING AIR that stops coming out. Is that really weird or what? I've never heard of or seen this before. Anyone have any ideas?

[gsalisbury]

On your car the heater function and selection controls are vacuum operated. When you step into the throttle the engine loses vacuum and the selector control moves to a "closed" position during that time which allows the fan motor to go but effectively closes the heater plenum. When you back out of it, it restores vacuum and the plenum opens to the setting you had selected (vents, defrost, etc.). Look for a broken vacuum line between the heater controls and plenum then work on vacuum leaks in the engine compartment. Your problem should lie there somewhere.

[drumvudu]

Originally Posted by gsalisbury

On your car the heater function and selection controls are vacuum operated. When you step into the throttle the engine loses vacuum and the selector control moves to a "closed" position during that time which allows the fan motor to go but effectively closes the heater plenum. When you back out of it, it restores vacuum and the plenum opens to the setting you had selected (vents, defrost, etc.). Look for a broken vacuum line between the heater controls and plenum then work on vacuum leaks in the engine compartment. Your problem should lie there somewhere.

Wow, thanks a ton for all that info. Ya know, it never dawned on me for a second that those controls were vacuum actuated. So, I guess you are telling me that this is not normal operation. I already went through the engine compartment when i got the car and fixed all of the vacuum leaks that I was able to locate. Do you happen to know where the line that controls the HVAC enters the passenger compartment? Also, are the two hard vacuum lines that go to the EGR valve, the only two hard lines on the car? They can be so hard to locate leaks on as they develop those hairline cracks that you can't see with a naked eye, or even one that's partially dressed. wa ah ah.... ANYWAY, any kind of idea where I might locate that line to the HVAC would be really helpful. Is there any thing else that could cause this besides an actual leak? I am going to replace the EGR valve next week as It keeps giving a code and I can find absolutely nothing else wrong so I assume that it must be the EGR itself. Could a bad EGR do this?

[gsalisbury]

OK, so here's what I got. My car is a '89 GT and the vacuum lines lead to a tree mounted on the firewall next to the master cylinder. One line marked AC runs behind the motor to a connection at the right rear of the motor. One line goes to smog, one, to heater valve and a few others. That bliock splits and goes under the left fender where I lose the lines. The HVAC control have a 4 plug line that feeds to the plenum on the left side as you are looking through the glove box. I would suspect that your problem lies between the HVAC controls and the plenum but that is only a guess. I would doubt the EGR valve is causing this problem but if it is sticking I would replace it as it would be the easiest thing to get to.