kevin2012

My 1991 928 S4 recently developed some pulley like noise. louder when the engine is hot. Hardly can hear from the top of the engine but it can heard from below the bumper or from the left side wheel arch.

My timing belt and water pump were changed 5000km / 5 years ago. Everything runs fine no timing belt warning.

Video:

Any comment please. Thanks.

Lizard928

Use a mechanics stethoscope to locate the noise!

Leon Speed

Air pump bearing, alternator bearing, A/C clutch bearing?

mickster

Does it change as the RPM increases?

My 91's TB Roller was squeaky as a mouse in heat; Porken now in so it's out. We all feared it was my Waterpump. Does sound similar. As RPM increased the frequency in terms of number of times per second (not actual pitch) increased a bit (which is expected on any squeaky pulley).

Stromius

Try to loosen the airpump belt and see if you can turn it by hand. Mine seized so I cut the belt and eventually removed it.

yardpro

when my water pump went out, it was only 3 years and 5k miles old...

i heard a slight squeal reflecting off the building at a Mcdonalds drive through.

get a cheap mechanics stethiscope, they are only a few dollars at autozone.. it will positively Isolate the noise.

kevin2012

Thanks everyone. I really hope that it is not the water pump. I am ordering the mechanics stethoscope and see if I can ID the source of the noise.

Tails

If it is a WP then you should some water leakage!

There are numerous items of rotating parts in that area that can become a "growler" as mentioned above.

Another test is to use a mechanics listening device, a screw drive with the handle held against you ear and the end on the station part of the object that you suspect.

Safety hint, beware of the rotating parts catching clothing, hair and other appendages.

If you have ramps put the car on the ramps so that you can get underneath.

Tails 1990 928S4 Auto.

Schocki

Take off all accessory belts and let the engine run again. If the noise continues, don't start it up again until you fixed it!!!
If the noise without the belts is gone, put them back on one by one and isolate the problem. I think it is teither the alternator or AC clutch bearing.

kevin2012

Thanks for the advices. When I look at my pervious engine video that I took months ago; I think that the noise was there all the time but now I had fixed the vaccum leak and changed to a better grade 15w50 motor oil, the engine is much quiet than before and it makes the pully noise more obvious.
I will try to ID the noise with the drive belts off. If the noise is from the air pump, can I just simply remove the belt and leave the pump there? Anything else more to do to make the car runs normally with the air pump not functioning?

Schocki

Yes, you can drive the car without air pump. You don't need it to pass the smog inspection. Mine us gone since many years, no problem. Time to go for the Porkensioner without air pump bracket

Whitesands

If you are sure the noise is mechanical this wont cure it but try this simple test, spray WD40 over the belts while the engine is running.

I had an S2 that developed what I thought was a mechanical noise at the front of the engine and a friend said spray the belts/pulley with WD, I did and hey presto, the noise disappeared but came back when the WD dried out, I replaced all the belts.

I wouldn't of thought a belt would make a mechanical noise but in my case it did.

Good luck, hope it's something simple.

kevin2012

I had uploaded quite a few video on You Tube.

1. Checked the belt tension and roller. They are fine.

2. Checked the Air pump pulley, it is worn out. Making noise.

3. With the air pump and power streeing belt removed, the noise still there.

4. Removed the air pump belt. put back the power steering belt and , the noise still there.

I am waiting for the stethoscope and will take out the fan and check again.
Any advices? Thanks.

Schocki

What about the alternator?

kevin2012

I actually wanted to remove the alternator and the AC belts as well but the adjusting nut for the alternator is too tight and I will need the car in the new year day so I needed to put everything back first. Next time I will try to remove all accessories belts. I don't know about the alternator but the noise doesn't seem from there. As for the AC, the noise does not change a bit when I turn the AC on.

Seems to me that the noise goes away when I rev the engine a bit.