Mario H

Greetings all, I'm trying to fix a old problem. I have a 79 928 with 170k. The car runs great most of the time but if the weather warms up, I develope a strange problem. If I accelerate past 3000 rpm the engine starts to hum and run rough. The hum is coming from the intake plate. I thought this might be a vacuum problem so I ripped off the top intakes and replaced all vacuum lines and gaskets. Hum is still there, so I tried spraying starting fluid to find any other possible leaks, no leaks.

Any help or suggestions would be greatly appreciated or if someone know a great 928 mechanic in the Dever area please let me know.

AO

Check you vacuum check valve - The blue and black thingy next off of the brake booster. That can make some noise if it leaks. It could be that it only leaks at a particular vacuum level. Hard to tell with the information you've provided.
Does it vary in pitch?
Have you used a Mechanic's stethoscope to lacate/isolate the noise?
Have you always had it, or is this something new?

Mario H

Andrew, thank you for the reply. I have located the noise; it is coming from the intake air plate area. Once it starts it is very consistent. The only way I can get the noise to stop is to turn off the engine and restart it. If I accelerate passed 3k the noise starts all over again.

Mario

Fogey1

K-Jet, temp and RPM-related, a hum, near the throttle plate.

3000 rpm is easy to reach at a small throttle opening. Is there any chance the throttle plate is loose on its arm and vibrating in the throttle bore? If so, it would be good to fix it before the bolt falls into the engine.

Dennis Wilson

Check the plate to make sure it is centered and secured as Will suggested. Also, check the gasket/bolts/springs connecting the two halves of the lower plenum to make sure they aren't loose or leaking.

Dennis

Mario H

Gentelmen, thank you for the replies.

When I was checking for vacuum leaks I replaced the silicone boot and re-centered the intake plate.

Any other suggestions?

Mario.

Dennis Wilson

Mario,

The gasket I was referring to is not the silicone boot but rather the gasket that connects the two halves of the lower plenum. This is a paper gasket that could have been blown out by a backfire.

Dennis

Mario H

Dennis, I did not detect any leaks in that area when I sprayed starting fluid around the plenum.
Granted I could not spray that well in that area. I will try and check that area a ittle closer.

Thanks, Mario

Dennis Wilson

Mario,

You should be able to see if the gasket is protruding out of the plenum. IIRC the connecting bolts are spring loaded so this will open up during a backfire. Could some of the bolts be loose and causing the humming?

Dennis