WallyP

Before starting the car, disconnect the small black/blue vacuum check valve at the vacuum brake booster and cap the line from the booster. Start the car and see if the buzz is gone.

If there is no buzz with the check valve disconnected, and the buzz reappears when it is reconnected, your problem is almost certainly a leaking HVAC actuator in the dash.

If the buzz doesn't change with the valve disconnected, it may well be the temp sensor fan behind the grille button on the dash.

If the buzz is vacuum related, get a MityVac - a very useful tool. Remove the side panel from the driver's side (LHD) of the center console. Look at the forward end of the opening, and find a bundle of colored plastic tubes.

You can follow these down to the vacuum solenoid rack and pull it out to test for actuator leaks as below. It can be difficult to access the attaching screws.

If you want to take an easier route (not always a popular one), get a few inches of small rubber line that fits snugly over the plastic lines.

Using a sharp razor knife, cut the orange line. You now have an upper end that disappears up into the dash (to the vacuum actuators), and a lower end that runs forward into the console (from the vacuum solenoids). Use the MityVac to apply vacuum to the upper end, and see if the vacuum actuator holds vacuum. If it leaks, plug the BOTTOM end (that goes to the vacuum solenoid) using the small rubber line and a piece of a nail or something similar. You can just leave the upper end open. If the actuator holds vacuum, use the small line to splice the two ends back together.

Repeat the process on each of the lines except the black one, which is the source line.

The white line operates the heater valve - if it shows a leak, fix the leak or replace the heater valve - you need for it to be functional.

After you have disconnected any leakers, the rest of the actuators will now work, provided that you have a good strong vacuum on the black source line. If you don't have a good vacuum here, check the rubber cross and the black/blue check valve near the brake vacuum booster, and check the line running to the cruise control in the front fender for leaks.

markpaul

I agree. My friends either love or hate the color - luckily I'm the former! I'm thinking of replacing the front & side indicators with clear (or smoked) units. Orange and purple just don't go in my book & I think it would update the look somewhat. Similar to the pic attached. I'm sure there'll be plenty of people who don't agree but I'll always keep the stock lenses so it can be put back to stock. Trouble is I can't find any images of a 928 with the smoked lenses (sold by Klearz), which could look great, so best go for clear I guess. Will need to get amber bulbs probably.

markpaul

Thanks Wally - this is really helpful.

markpaul

The tires that came with the car are rubbish so will be replacing these at some stage (Hankook Ventus K102's). Not many options out there for 16's these days - especially for the rears (245/45's). I notice the GTS also had 9" rears (albeit they're 17's) but had 255/40 tires. I guess I could put 255/45's on the D90's if they're available.

Bertrand Daoust

You're welcome.

Sorry but I don't have any pic right now.
I can take some later this week if you want.
BTW, these spacers are "plug and play" on a '91.

My car came new from Porsche with the D90's. If I remember well, the GT came stock with the D90 but not the S4. But Porsche sent some S4 with the GT wheels as an option.
And yes, I still have them.

markpaul

Looks like mine came with the D90's as stock too. Good to know I don't need to replace the studs. If you are able to upload a pic sometime of how yours look with the spacers that would be great. Could you have gone wider than 15mm or is this really the limit on the studs?

Bertrand Daoust

Not sure if I could go with more than 15 mm. Maybe a little bit.
But 15 mm is just perfect for me.

polecat702

I'm partial to black, but this one is stunning!

oups59

Bertrand,

It looks better with the spacers

markpaul

Thanks for the pics Bertrand. 15mm it is.

Bertrand Daoust

You're welcome markpaul.

Merci Luc.

Vilhuer

Cheapest possible pump seems to be GMB (not same as GEBA) from Amazon. New pump for under $100 but casting is rough and impeller is metal version. Friend has bought few just to see what they look like. I wouldn't use them in any expensive engine. In daily driver winter beater maybe.