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Knocking Noise V Worried

M

Mark993

New member
My 96 Tip with 64k miles has started to knock from the engine. The car is well serviced and maintained.

It happens only in drive or reverse and sounds like a spinning cv joint or similar. Oil pressure is really good and sound is loudest when stood still on brakes. It is engine speed sensitive in that it speeds up with revs but above a certain rev it is not audible. There is no noticeable noise in neutral or park at any revs.

I will have to take it in to be checked out but searching web etc I cannot find any similar problems.

Any ideas, can feel a big bill coming on here.

Mark.
 
Yes that is what I am thinking but also thought engine might be under more strain when engaged in drive and that make the problem evident.

I was / am worried it could be a big end issue.

The car is a tiptronic so trying to figure if anything would be spinning other than the engine when in drive and brake applied. It definable speeds up with engine revs.
 
It's been a long time since I've heard big ends going and from memory it doesn't matter whether under load or not, when you rev the engine they get louder.
A 993 has hydraulic tappets but these are normally heard when initially starting the engine until the oil has time to circulate and get them working properly. It's usual to hear them after the engine hasn't been started for awhile (many days).
You can try using a broom handle or a long screwdriver as a stethoscope to try and locate where the sound is coming from.
 
Not sure how many miles the car has done or the type of driving, but it may be worth looking at the engine mountings and possibly the gearbox mounting.
 
Engine is on 64k only and has been serviced religiously through its life.

From what I can I have listened to engine mounts with stethoscope and not from there. I cannot pin point it even from under the car with the stethoscope place everywhere I can reach with the tray off.

Need to get it checked out as because it is only when drive or reverse is selected I think it could be a transmission issue rather than engine but I simply cannot fathom what would be spinning whilst in drive and stood on the brakes.
 
The fluid flywheel will be spinning but it's follower can't as the brake's on,so the oil slightly shears & falls out of the spiral circuit so could be a bearing -fluid couplings are however reasonably bombproof.
 
The clutch.
I'm not sure what system tiptronic uses, but there must be a clutch that engages when you select drive or reverse. You don't have a pedal but you still have a clutch which is handled by centrifuge or hydraulics and electronics. The plates are usually wet in that they are several plates that are running in gearbox fluid. The clutch is slipping when you select drive which is why your car will creep forwards or backwards and the reason why you need to keep your foot on the brake pedal when on a flat surface.
 

Mark,

In a conventional auto-transmission such as yours - as opposed to the latest dual-clutch (PDK-type) transmissions - the mechanical clutch and constant mesh gears of a conventional mechanical gearbox are replaced by an hydraulic torque converter and epicyclic geartrain, the clutches and brakes controlling gear selection on the epicyclic gearsets being handled by microprocessor controlled electro-hydraulics.

If, as seems possible, there's a problem with the transmission internals, I think it best to seek expert advice from an auto-transmission specialist ASAP to avoid inflicting any unnecessary damage. They will be able to get the car over a pit or on a ramp to investigate the issue thoroughly.

If you plan for the worst case, you may be pleasantly surprised to find that the solution is simpler - and cheaper - than you first anticipated.

Good luck.!

Jeff
 
My BMW has what BMW call Steptronic auto transmission-in my particular car it is a GM box but I was told that Getrag & ZF also supplied this system to BMW sometimes in the same model or/and across the range-mine is a 530D.

I have no reason to doubt that the Porsche Tiptronic unit is also of the same type & in my case this is an electronically controlled gearbox-with an dedicated addressable ECU & comprises :-
A Fluid Coupling or Torque Converter replacing a mechanical clutch between engine & gearbox-the torque converter is a refined unit which virtually "locks up" at a reasonable road speed thus eliminating fluid slip ,increasing efficiency,lowering engine speed & improving MPG.

This drives the input of the gearbox including a high efficiency gearbox oil pump & pressurises the hydraulic control system which usually comprises a flat module forming the bottom of the gearbox-this has all the ports/passageways/valves in it,which in turn are controlled by the ECU which itself is supplied with signals from the engine management system & the gear selector/stick.

This control system then changes the gears by operating the multiplate clutches which are an integral part of the gear shafts that look much like those in a conventional manual gearbox,there being a separate one for reverse gear-in a way the clutches replace the gearlever actuated selector forks in the manual box-operated by hydraulic servo cylinders.

Earlier auto boxes did use epicyclic geartrains with the gear changes/ratios obtained by braking the various different parts of the mechanism.

Tiptroncs are more like the current PDK twin clutch units .

The usual creep needing a touch of the brake is just the fluid coupling transmitting a slight amount of oil to it's follower in the torque converter housing just overcoming the rolling resistance of the car-worse if the engine tickover is higher -as on a cold engine.

One reason why I have trained my left foot for braking-but also for those self gratifying bursts from the lights when a hot rod merchant in what he thinks will leave you behind-switches lanes at the last minute to have a go-one can then anticipate the green light,keep left foot on the brake ,rev the engine & blow them away.

Sad-aren't I?

 
Thanks for filling in the details Colin, an excellent answer.

As for being a sad ol' b*****d well the jury's still out, though a recent telephone poll done by the Telegraph showed a swing of 99% to the Yes camp.
 

Sorry to pour cold water on this David/Colin but, as far as I'm aware, from the 964 series 911 Carrera 2 onwards, all Tiptronic transmissions have been conventional torque converter/epicyclic geartrain automatics with torque converter lock-up operating on all but 1st gear. The 4-speed Tiptronic unit used in the 993 was substantially unchanged from that in the 964, although some modifications were incorporated in the electronic programming.

With the exception of 996-series 911 Turbo Tiptronic S, which used a heavy duty DaimlerChrysler-sourced transmission with a high torque rating, Tiptronic 911s, Boxsters and Caymans have used transmissions developed jointly by ZF (the manufacturer), Bosch (electronic control unit supplier) and Porsche.

Let's hope that Performance Porsche can sort out Mark's problem but I suspect that unless they have in-house experience or there's a straightforward solution, they may well direct him to an auto-transmission specialist for an in-depth diagnosis.

Finger crossed it's something simple.

Jeff
 
Thanks Jeff-I bow to your superior knowledge-I did cover my tracks by the presumption that the ZF boxes could be the same as in Steptronic but admit hadn't researched that a lot-my understanding was based on having been inside Steptronic units which do go back to 964/993 times I think -anyway all grist to the mill.

Apart from controls,they are all basically mechanical boxes-the "tronic" is basically marketing stuff[:-] so plenty of shafts,thrust & radial bearings to possibly go wrong.

The BMW units were supposed to be filled for life-now even old models are recommended to have the fluid & filter changed at 80K miles!
 
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