gaz928s4U.K

Please accept my apologies in advance, as this is my first post on this excellent site.

I have a problem with my 90s4 euro spec.
60.000 miles

I recently had the bearings replaced in my toque tube, now nice & quiet after 2 months ownership. Although the Porsche specialist who did it admitted that it was the first time he had changed the bearings!!!!!!!

The main problem I have is that whilst in Park(or neutral) I press the throttle & above 1000 rpm the car vibrates, through the throttle, wheel & seat, making what can only be described as a grumbling noise as opposed to a linear rev (if you know what I mean?)

Out driving I have the same problem, car vibrates above 1000 rpm.

Any ideas would be most appreciated,

Thanks in advance,

Gary

johnb

hi

nice to see someone else from the uk!

sounds like engine mounts to me. your engine would of been pulled around quite a bit while the torque tube was removed for its bearing change.

check that the engine's sump is clear of the front cross member. there should be a good clearance between it.

it is quite common for the engine mounts to collapse and loose their 'flex'. it may be that the grief they got while the torque tube was being sorted caused them to give up the ghost.

not much fun to change.

could be warped flex plate on the engine flywheel...connection point for the torque tube drive shaft. if your car is an auto then the inner drive shaft is continually driven..torque converter is at the back. if the flex plate was stressed while the torque tube was being removed, it would be possible to damage its alignment...this would cause some vibration.

also check that nothing was caught up when the car was reassembled, stuck between tube and car.

thats my four penny worth anyway.

good luck.

regards
johnb

gaz928s4U.K

John,

Ditto to the U.K bit,

Thanks for the advice.

I must admit it feels as though it's coming from the rear of the car, so hopefully mounts aren't the problem.

Another issue I have, which I'm hoping could be related is that i've got a lumpy idle, although it holds at 700-750 it constantly misses a beat.
Have checked vaccum hoses up to now, will be changing filter aswell, do you think this could be associated.

Thanks in advance

Gary

Nicole

Gary:

Your idle problem sounds exactly like mine: The engine apears to randomly skip firing a cylinder. It is still not clear what the problem could be in my case. My car has the following new parts:

- Spark plugs and wires
- Rotors and caps 10k miles old
- In-tank fuel pump
- Y-Connector
- Idle stabilizer, knock sensors, TPS
- Motor mounts

Air flow sensor and alternator have been tested; the injectors cleaned. The computer had no fault codes stored. A different LH computer did not bring any improvement either.

Vacuum testing so far only revealed an absolutely minimum, very slow leak in the vacuum line to the auto transmission.

The car drives and behaves totally normal as soon as I get above idle speed. No hssitation, and none of the vibrations you mention. If you figure out what causes your lumpy idle, please let me know!

dr bob

Sounds like you have an ignition miss. This would cause the idle vibration, as well as the torque tube/driveshaft vibration in the rear.

If your mechanic replaced the bearings in your torque tube, it's quite likely that the bearings are not in the correct place in the tube. I haven't done this, but I understand that there are supports in the tube, and it's easy to get a bearing moved out of that when you are pushing on the end of the shaft. My guess is that assembly is a little bit more than just sliding new bearings in.

Having the bearings in the wrong place makes it likely that the driveshaft can 'whip' a little as it spins in the tube. If you find that you still have the vibration sfter you cure the miss, take a look there at the shaft assembly again.


There was a note someplace, warning about a miss contributing to driveshaft failure. That's a fracture failure of the shaft just behind the splines at the drive end though, and not related to bearings.

If the ignition is fine and you still have what seems like a miss, start looking at mechanical stuff in the engine. A basic compresion check is usually a great place to start, after you inspect the plugs carefully of course.

Hope this helps!

WallyP

Your grumbling vibration is typical of that caused by failed engine mounts. The mounts are fluid-filled so as to damp both low- and high-frequency vibrations. When they fail, the fluid is lost, and the mounts collapse enough to allow metal-to-metal contact.

It isn't really obvious, but the entire 928 power train - engine, driveshaft/torque tube and transaxle - is supported on the two front engine mounts and the two rear transaxle mounts, so the mounts carry a LOT of weight.

Two checks for the engine mounts, one easy and one not-so-easy.
Easy - Raise the hood and watch the engine as it idles, and as you rev it. If it doesn't move around, the mounts are almost certainly bad. The top of the intake should be near-level with the cross-brace - if the intake is lower than the intake, the mounts are probably bad. You can actually move the engine by hand a bit if the mounts are good.

Not-so-easy - Get under the car with a good light and a small mirror. Examine the mounts carefully - if they are bad, you can see metal-to-metal on the upper edge of the lower part of the mount.

If the mounts are bad, there are few things that you can do to smooth the operation of the car as much as will new mounts.

If you put new mounts on, this is a good time to do several other things, such as steering rack mounts, rack boots, oil pan gasket, etc.

Incidentally, several years ago this job was the cause of my first major disappointment with the Factory Workshop Manuals - they don't even mention the mounts!

gaz928s4U.K

Thanks for the feedback everyone.

Nicole: if i can get to the root of the problem I will let you know, I have also fitted new plugs within the last week, took it over to a porscheshop & got their mechanic to check it over & test, he couldn't put his finger on it there & then!!

Dr Bob: Your theory could be correct, as mentioned it was the first time my Porsche mechanic had actually changed the bearings, they usually get them reconned elsewhere.

WallyP: Your theory could also be right, As engine doesn't seem to move, but I thought the engine wasn't supposed to anyway???

Thanks again,

Gary