philpilot

928S MN

Still the original axles and CV joints? I've read more than a few times that changing out the axles, or rebuilding the CV joints, has eliminated some phantom vibrations.

Speedtoys

Cruise, or under load, coasting?


I didnt see caps 7 rotors on the list...

philpilot

philpilot

Speedtoys

Define ok.

I had the same, was caps.

FredR

Have you checked the front flexplate clamp for shaft creep or have you implemented any of the recognised mitigations to prevent it? Have you checked the rear clamp pinch bolt at all? Have you checked the crankshaft end float? Have you heard of the TBF phenomena that afflicts models such as yours?

When the clamp on my late 90 S4 was slipping some 18 years ago I could tell it had moved from a mystery vibration that appeared around 3000 rpm that I put down to compression of the drive shaft. Not that long ago I warned a new owner to the list that his symptom may be a pre-cursor to shaft failure - some thought I was being a bit fatalistic but after doing a boat load of refurb work the owner took his 928 out for a spin and bang- the drive shaft snapped. Back in 1999 my late 90 S4 had such a vibration when I acquired it and snapped its driveshaft not long into ownership. The brand new drive shaft I put in the car initially had no vibration but after a couple of weeks of driving it the vibration appeared. This scared the poo out of me and after finding this fine organ on our newly introduced internet over here, I learned of this issue. Took the car in and checked the front flexplate and voila, the flexplate clamp popped back about 2mm when the pinch bolt was released. This prompts the question "why did you find a need to fit a new drive shaft?".

The 90 S4 has the later tapered drive shaft that is noted for failure at the base of the taper and this failure can occur at either the front or the rear of the shaft. There are two schools of thought about this failure - one being that the shaft has an inherent weakness by design, personally I have feeling this may well be the case but I suspect it is compounded by flexplate shaft migration and the compressive force on the "elongated shaft" is focussed at the narrowest point of the shaft and eventually that duly fatigues. How many miles has your example covered? This failure mechanism typically seems to occur in the 60k mile range but driving style may also be a factor in this phenomena.

Part of the mitigation programme is to change out the front and rear pinch bolts every 5 years or so and when reinstating them to over torque to 110% of that specified. Beyond that sensible owners of these late model automatics take measures to prevent the front clamp from slipping. The best mechanical solution and the one that Porsche should have used from day 1 when they introduced the tapered drive shaft is known as the "Constantine clamp"- see his website Black Sea research and development- very informative. For ease of implementation there is the Ritech clamp- an easy to mount in-situ mod, the PKlamp and my solution of choice- the Loctite method.

To be clear- your problem may well be something else but if you cannot find what it is then this possibility needs to be eliminated. Given you are new to this list and possibly new to 928 ownership even if the above is not applicable to your symptom you need to be aware of such as a 1990 S4 owner.

philpilot

FredR

You had a drive shaft break?- why am I not surprised. Did anyone check for migration of the clamp after the shaft broke? The position of the clamp relative to the shaft is a partial clue but doubtless that one got thrown out with the bathwater.

Perhaps I can suggest you now mark the position of the flexplate clamp relative to the shaft with some white paint or whatever then release the pinch bolt and see if it jumps back 2mm or 3mm. If it does and the vibration disappears then chances are you will know what is causing your vibration problem.

If you do not know how long the rear pinch bolt has been in there just get another one, remove the current one, inspect the assembly to ensure the shaft detente is correctly aligned and then put the new bolt in the rear position with the additional torque. Note: do not touch the rear bolt until you have checked the front position for flexplate clamp creep.

If you have not implemented mitigation against flexplate clamp creep you might do well to do so now before we read about how your thrust bearing is history.

philpilot

FredR

What was the sequence of events? Did the vibration appear immediately after the new shaft install or did it appear after some running interval? I presume you checked the front clamp after the vibration started, found there was no slippage and tightened it back up again?

philpilot

The vibration was there but not that noticble after the new shaft installation. I thought new transmission and motor mounts would solve the problem. Wrong. They increased the vibration. That’s when I checked the shaft mitigation and retorqued the front bolt to 66’ lbs with no vibration change.

FredR

Interesting that the problem was amplified when you fitted new mounts - for sure most unexpected and it will be interesting to see what if anything others can make of that given it seems rather relevant to the problem. One the classic signs of failed engine mounts is a pronounced vibration at 1400 rpm and also at 2800 rpm. Perhaps next time you are in there you can check the clearance to ensure that one of the new engine mounts has not failed prematurely due to a defect. I have never heard of anyone accidentally puncturing a mount accidentally but I suppose in theory it is possible.

Presumably when you checked the front flex plate you released the front pinch bolt and no movement was observed?

dr bob

From the symptom that it's RPM related rather than road-speed, appears under load and disappears under no load, I think you are looking for an engine miss. Possible causes: Ignition, injection, compression, in that order of likelihood.

-- Start off by inspecting (and replacing) the spark plugs, looking for one or two that are different colors than the others. Best to run the engine under the same load that causes the symptom right up to the point that you turn the key off, if that possible, so the color on the plugs might relate to the same conditions under which the symptom shows up. For me that's sometimes included a romp up a mountain road to establish some load, then key off, out of gear, coast to a safe place to pop the hood and look at plug condition.

-- If you can do the plug inspection at home, plan on checking the compression while all the plugs are out. Remove the EZK (ignition) fuse so no spark or fuel while you test. Connect your battery charger while you test so the cranking speed stays consistent through testing all the cylinders. Prop the throttle open with a screwdriver at the intake manifold bellcrank relay linkage quadrant. You can usually free-rent a compression gauge at a local parts place. It doesn't need to be super-accurate, but it does need to be repeatable. You are looking for the cylinder that has a noticeably lower number.

-- Sometimes a break in injector wiring will cause one to not get a signal to open. An inexpensive "noid" light fits between each injector harness connector and the injector body, LED flashes with pulses from the injection controller. Do this with everything buttoned up again, and only if you didn't identify a problem already. The wiring in the injection harness tends to be fragile at this age, so use care when disconnecting and reconnecting the old wiring connectors. Inspect the wiring at the connector and in the few inches of pigtail folded up under the cover over each injector. Repair/replace those connectors as needed.

-- '89+ cars come from the factory with a protection relay fitted to turn off injection to half the cylinders when it detects an ignition fault in one of the systems. The car seems to drive OK on the four remaining cylinders, but it's down on power so needs a little extra pedal to keep up. The relay is mounted on the frame/bracket that holds the controllers in the passenger footwell. There's a snap-fitted cover there that comes off to expose the controllers and the relay. Go drive the car as you normally do, then look on the relay for a red or green LED indication that one or the other sets of cylinders are receiving "protection". Report back if you see one or the other of these LED's and we'll dig deeper into possible causes and solutions. Note that Porsche specifically calls out the potential for snapped driveshafts if the car is driven extensively on only four cylinders. The constant dynamic twisting of the shaft fatigues the metal in the shaft until it fails.

-- Have you performed an "intake refresh" yet? The intake comes off and the hoses and vacuum stuff, idle air control valve, flappy actuator, knock sensors and Hall sensor, crank position sensor, throttle position switch, and any damaged wiring underneath all gets replaced. Injectors come out for full professional cleaning or replacement. This project is often done in conjunction with refinishing and resealing the cam covers The key component to this exercise for chasing the miss under load is the injector cleaning or replacement. Our 928's use "batch-fired" injection, where all the nozzles open and close at the same time. There's one oxygen sensor in the exhaust, and any correction applied for CO drift is applied to all cylinders the same. One or two cylinders running lean enough to misfire may not drop total exhaust CO enough to compensate for one or two "weak" nozzles. My car ran great before injector replacement, but still was noticeably smoother and cleaner (much less NOx) with fresh clean injectors installed.

Adamant1971

Were the motor mounts Porsche or the Volvo mounts? If Volvo did you bend the motor mount safety hooks back?

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