924S - Belt noise/resonance

[alxns7]

Greetings!

First post on the forum, so a small introduction : Alex, Switzerland, Black '86 924S of German origin. I decided to post here because I can't find the answer to my problem on a French forum I'm used to. The problem I have seems to be unusual.

I did a full belt change about 4000 km ago (both belts+pulleys+water pump+seals) and since then have been having a strange rattling noise at a certain RPM range (1000-1200 RPM). I re-tensioned the belt 1000 km ago and the noise is now more pronounced.

It only happens when the engine is warm.

Someone suggested it was the belt touching the water pump separation plate. After checking, it's not the plate, but I saw that the lower section of the timing belt vibrates. You can actually see the teeth from this section stopping while hearing the noise (stroboscopic effect).

Any thoughts would be much appreciated. I'm thinking of altering the belt's tension again and see what changes. If nothing changes I will try buying another tensioner roller.

The section vibrating:


A short video:
924S noise - YouTube

Another video without the timing cover. WARNING The sound is awful.
924S Timing belt resonance - YouTube

Note: This is a copy/paste of a thread already posted on the 944Online boards. If I find the solution on the other board I will of course post it here.

[MB968]

I'm not sure the stopping/stroboscopic effect you are seeing has anything to do with the noise. If you are using a light that is plugged into either 50 or 60 cycle AC electric to look at the belt, the stroboscopic effect is likely due to the light. If you did use a light that is plugged into an electric outlet, switch to a flashlight (as we call it in the US, or a torch as commonly referred to in parts of Europe). Since these are DC powered from a battery, there is no pulsation of the light from the AC current.

The noise to me sounds like a bearing on one of the idlers. You may want to check them for smoothness, or if there is a way to spin them without the belt (rubber wheel in a drill) you can check each of them to see which one might be the culprit. You can also get a mechanics stethoscope to help pinpoint the noise source. I think you have a bearing failing.

[v2rocket_aka944]

1 swiss franc says the balance belt idler is too close to the belt.

[Rasta Monsta]

Two francs on balance tensioner rotated in the wrong direction, causing balance belt to slap water pump nose.

AMHIK.

[John_AZ]

Thanks to TechnoDuck...

[Slam]

Take off PS belt and run engine. Any change?

Take off alt belt and run engine. Any change?

Take off BS belt and run engine. Any change?

Take off cam belt. Don't run engine. Spin all components by hand.

The culprit will reveal itself.

[Bradical]

Did you ever figure this out?

[alxns7]

Hi, just a little update. I could spend some time on the car the other day.

I loosened the timing belt tensioner roller and tried to remove it alone. Unfortunately it doesn't seem to be possible without taking the whole belt out.
Also, I thought the belt was a little too close to the engine, so I realigned it. But the noise is still here.

So I still suspect the tensioner roller, but I'm gonna need more time.

[Timmay!]

+1 for Rasta.
We find balance shaft belt tensioners swung the wrong way pretty frequently. This puts that belt too close to the water pump pulley.

Since you're re-tensioning, I assume you have a manually set belt tensioner (86 model). If the timing belt is loose between the timing gear and the tensioner- is it possible you forgot to turn the timing belt back one and a half teeth before checking the belt tension?
With the timing belt black idler and chrome-looking belt guard removed...
You must turn the crankshaft clockwise to get one full rotation of the cam gear to #1 cylinder TDC.
Then turn it counter clock wise 1.5 teeth- using the crank bolt (24mm).
Then back to #1 TDC and check your balance shaft belt gear positioning.
How about the T-belt tensioner it self? Wish I could see the situation in person.

[alxns7]

I did everything as you said (turned the crankshaft counter-clockwise before tensioning, swung the tensioner the right way...)

So yeah, I'm gonna have to pull out the tensioner and check it. Some people say it's possible without removing the belt. I probably haven't tried hard enough. It might be difficult but not impossible to get the lip to pass under the belt. Just like removing a bicycle tire.

[dlearl]

Funny that this should come to the top of the page today. I picked up my 968 with fresh belts, pulleys, water pump and tensioner yesterday and I have a rattle at precisely 2800rpm. I've only heard mine from inside the car on my way home but I'm going to look into it tomorrow.

Mine sounds more like one of the covers wasn't tightened properly, or the tin on the bottom end is rattling (don't know why after this job, unless it got bent against the engine by a lift or something.

But I'll keep this thread handy if it isn't easily found and I have to take it back to the shop for a re-do. Bought everything from pelican, with the exception of the tensioner, which we didn't know was bad until the mechanic got in there. It was frozen, and about a 1/4" off the belt. :cry

Glad it didn't jump a tooth!

[88Silver924S]

I watched your two videos and in the second one, the one with the cover off, it looks like the balance shaft belt is too high, it is difficult to tell on a video but it looks way off of the balance shaft idler (if I recall correctly it should be something like 0.5mm gap between the idler and the balance shaft belt. Therefore it does look like the balance shaft tensioner has been rotated the wrong way (an easy goof-up). If that is the case readjust the balance belt rotating the tensioner the correct way and see if that solves your problem.

To check the bearings I highly recommend a mechanics stethoscope; but when you are using it around moving belts be stone-cold sober, focused, deliberate, and steady handed. Touch the tip of the stethoscope to the stationary bolt on idlers and tensioners to listen for a bad bearing.

Do you see any belt wear spots on the plastic covers? What brand of belts are you running?

Be Careful! Good luck and have fun.

[Timmay!]

I didn't notice that there were video links in the first post!
Yeah, the second video shows the balance belt idler not even making contact with the belt. Make sure you have good clearance between the idler and the lower balance shaft gear
That noise may also be the water pump. The sound sure changed as you got closer to the pump. I've heard pumps rattle like this.
I got a stethoscope just for this kind of thing, but you can use a long screwdriver as a substitute.
Let's figure it out.

[Rasta Monsta]

Originally Posted by 88Silver924S

when you are using it. . .be stone-cold sober

Whoah, buddy, let's not get crazy. . .

[alxns7]

The idlers are easy to reach so I blocked them with a piece of rubber and a screwdriver. The noise was still there and did not change at all so I think I can eliminate the idler source. The tensioner roller is a bit harder to reach, especially with the engine running.

I'm 98% sure it's the timing belt and not the balance belt (that is just fine and well tensioned). As I said, the noise got louder when I retensioned the timing belt after 1000 km.

Although the part visually vibrating is between the tensioner and the crankshaft, it could be the water pump, but I don't want to think about it for the moment Other than that I didn't see any wear on the covers.

The belts are from Gates

[Timmay!]

Take the T belt tension off the water pump pulley and see if you can move the pulley up and down, left and right. If it's the pump and it's making that much noise, the looseness will be obvious. Just rule it out.
Keep going...we're almost there.

[alxns7]

Problem solved ! Just what I thought : I changed the tensioner. Which is an easy thing once you take the belt off the camshaft sprocket.
I did not know the brand of the one I bought. So this time I bought a INA one (official Porsche supplier as per Pelican)

The two tensioners are a bit different, but have the same dimensions. I can feel some axial backlashes on the faulty tensioner, not on the INA one. But I can't say if it's had lash from the beginning.

My advice : make sure you know the brand of your tensioners !