When I have the blower switch on and driving normally (out of turbo range) the blower motor operates fine... When I step on the gas the blower motor decreases in output and at times completely stops until I let up off of the gas pedal... When I let up off of the gas pedal I hear a 'popping' sound like a vacuum switch is closing (or opening)...

I'd appreciate any guidance on determining what specifically is wrong and how to repair this...

Thanks,

Frank

I experienced the same problem on our 1988 760T. On our car, the vacuum leaks were at the engine where the vacuum check valves are. I installed worm drive clamps to seal the old hoses better. Problem solved. See the clamps in the picture.

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john

Neat trick, John. Just this month I've found two of our non-turbos have defective check valves, made evident by the wheezing sound from inside. Couldn't find them on-line, so I visited the junkyard. Interestingly, to me, the three I pulled there were all fine. Something else to dissect, I guess.

Looking at all those clamps, I wonder if you've pressurized the intake as a method for weeding out small leaks? I was thinking of a ~15 pound setting on a regulator from a large tank.
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Art Benstein near Baltimore

hi frank
i had the same problem and it turned out to be a faultly vacuum pump motor
when i hit the gas fan motor would stop and no heat foot of the gas it will work again
check your vacuum lines as well for leaks
good luck lindon

This does not sound like an actuator problem to me. It sounds like you can't maintain the vacuum for the system when under boost. This is most likely caused by either a leak in the in-car vacuum system or a failed vacuum check valve in the line from the manifold to the firewall. Look for a white and black valve (ITE makes it). It it's bad, try to find one in a boneyard. Nobody sells them but the dealer and I paid over $20 from Borton. They either work or they don't, so grab several from the boneyard. The by operators never know that the little valve is worth that much.

HTH,

someguyfromMaryland

How to you check it ? Is it simply a one-way valve ?

Noel

Sounds like one of the vacuum actuators that operate the various dampers in the heater distribution box have failed. There are three in all, but the dual acting actuator is the most famous for failue. When the output is decreased, check the airflow out of the defrost vents. In the normal failure mode, the air is directed to these vents.

The bad news: not an easy fix. It requires quite a bit of disassembly of the dash and beyound to get at it - much like replacing the heater core. Check the FAQs and archives. As I remember, you can remove and plug one of the vacuum hoses and get by. You do loose partial function of this damper, but as I recall, not a catastrophic loss.

Good Luck,

Dan