Nicole

Bill and I have been trying to diagnose a strange rattle that comes from the front of my engine ONLY under the following conditions:

1. Engine cold (noise disappears after about 3 minutes)
2. Transmission in gear

Particularly the fact that this completely disappears after about three minutes is puzzling.

The location of the noise seems to be in the vincity of the power steering pump or alternator. The alternator is relatively new (Bosch rebuilt, about a year old).

The character of the noise - in my opinion - is as if some aluminum pieces rattle (vibrate) against each other.

I do not believe it comes from the timing belt - if it was from under the covers, I would expect it to be at least a bit muted. But it sounds rather loud and clear.

The only thing we found that the two belts on this side are borderline lose, but not critically. I have a hard time believing that the belts could make such a harsh rattling noise. We will tighten them on the lift in about two weeks, though.

Anyway, has someone experienced this before?

Any thoughts on what else could be lose in that area?

heinrich

Guess 1: Water Pump
Guess 2: Alternator
Guess 3: Air Pump

worf928

Did the noise develop spontaneously? Or did it appear shortly after some maintenance/work was done? If the latter what was done?

Guesses:
Three bolts bolt the alt/ps console to the block. Are they tight?
Is the power steering fluid reservoir clamp tight?
Is the plastic alternator shroud tight against the alternator?
Are the three bolts for the left-side belt cover tight?

Mrmerlin

check the power steering line clamps there is onenext to the left manifold and one under the left coil and this screw can back out and the line can rattle. check that all of the pulleys are straight IE not slightly bent so the belt changes its bite in the pulley. Check the bolts that hold the PS pump pulley onto the shaft mount and if you dont find anything, try removing the timing cover and checking the tightness of the oil pump shaft nut and also the crankshaft pulley nut

Bill Ball

The noise disappears within a few minutes, so we didn't have chance to do much tracking today. It is not the alternator - I had time to hold a hose against that and listen after I decided the noise was coming from the front of the car. It didn't appear to track to the PS pump either, but by then it was fading. I did check the three bolts on the console. I thought it could be the left or center cover as well - the bolts are tight, but the left cover did seem to move a little. We had just refit the right cover after checking TB tension, so that's not it.

I was having a hard time coming up with something on the front of the motor that would be drivetrain load dependent (the PS pump and alternator should not) and would heat up quickly. That would lead to something in the cam belt/crank track. It's a light, irregular rata-tat-tat. It really strikes me like a loose cover, but why it disappears after 3 minutes has me puzzled.

We drove the car pretty hard and nothing fell off.

Nicole

I can't say that for sure - I can only tell you that it got louder and "harsher" in the last months, since I first noticed it.

But it has always been only when the car is in gear at idle, and it still goes away after running the engine for about 3 minutes.

Those are great suggestions!


One more thought When I put the car into gear (versus idle), the engine speed should not change, right? However, the engine sounds different. And maybe this different sound/vibration is what triggers the rattle. Like it hits some resonance frequency. And then, as the engine warms a bit, idle changes and it no longer rattles.... would this make sense?

Bill Ball

Yes, that's possible. You should be able to test this by slightly revving the car with the tranny in gear.

Nicole

Did that already. If I rev it with the cold engine, the noise goes away. I have never heard it with warm engine, no matter what the engine speed.

Tails

When the auto transmission is engaged the engine rpms should drop by approximately 60rpm, as the engine comes under load from the transmission. The rpm drop is is limited to 60 via the LH ECU.

At the engine comes under load from the transmission via friction, TC and the actual operation of the A/T and when it is cold there will be some vibration until the engine starts to expand as it come up to operating temperature. The system that experience the most rapid expansion is the exhaust system, so there maybe someting around the forward part of the engine that maybe loose or touching another part, like the exhaust shrouds, the exhaust mainifold, the air injection pipes, the exhaust temperature senors or the actual exhaust pipes.

As previously stated the engine will vibrate more when it is cold and as the noise only last approximatley 3 minutes, I would suggest your fault finding procedure needs to proceed along the lines as undertaken by Bill. This will take time, as when the noise dissapears each time you will have to allow for the engine to cool right down before moving on to the next items in your attempt at locating the source of the noise.

Tails 1990 928S4 Auto

Bill Ball

Thanks, Tails. It seems far too far forward for exhaust. Sure it could be transmitted sound, but I started out under the car, thinking it might be exhaust or driveline rattle. As soon as Nicole started the car, I could here it forward but not under the car. It appears to be at teh front of teh motor, but it could along the sides. We'll get it on the rack and smack things until somehting rattles.

If it is a low RPM resonance, I'd sure like to be able to hold it at that RPM so we have more than 3 minutes to find it, but there's no obvious way with LH, is there?

ErnestSw

You MIGHT buy some rattle time by disabling one bank of injectors and running in limp home mode.

heinrich

Have you guys checked the flex plate bolts/rivets?

SteveG

Check heat shields at manifold, near rack ?? IIRC mine has 3, rather small I thought, bolts. Had to remove to get access to exhaust bolts when checking flex plate. One twisted off.

AO

Nicole,
The fact that this goes away after things get "warmed up" is a bit puzzling. If all the bolts for the power steering check out to be snug (pulley bolts, bracket bolts, tensioner bolt), then it's proabbly not it. The onlt other items in that area that I can think of that could rattle are the alternator (but you said you eliminated this), and the oil pump.

Did you get a chance to listen to this from under the car? I think that would be very telling. I like the idea of the heat shilds. There is one ont he driver side that protects the steering lines. This would be obvious from under the car.

BTW, do you have the belly pan on?

Hi Ho Silver

Ive got this problem on my 83s, Ive had the fan off, removed the air pump, it takes a little longer than 3 minutes to stop, it has happened since my air pump seized and I had to remove the power steering belt to get it off, Im thinking that the power sterring pump is not tightned properly, but when it gets hot it expands and does not vibrate so much. It is getting on my nerves though.

Peter