BRB-83-911SC

84 Euro S 5-speed, 42k miles: I am troubleshooting a harmonic vibration that is RPM dependant. It occurs at approx. 1,200, 2,400, 3,600 RPMs, in a sine wave type pattern. It is most prominent under load during acceleration, in any gear – not vehicle speed dependant. It is less prominent under deceleration, and just perceptible when running the engine up through the RPM ranges while parked, both with the clutch engaged and disengaged, transmission in neutral.

This started after reassembling the car last week after a transmission drop for new 2nd and 3rd gear synchros. The rear suspension assembly was removed in tact, and the transmission/TT was dropped in one piece. The clutch was also removed. Much care was given when separating the TT from the transmission to avoid any stress on the input shaft.

I had the TT rebuilt with SuperBearings and had a specialist replace the synchros. The original driveshaft had some minor run out on the transmission end, so a straight driveshaft was installed. Nothing unusual was found inside the transmission, other than the worn synchros.

Reassembly was uneventful, mating the TT to the transmission before lifting into the car with special attention paid to not stressing the input shaft. Shifter linkage was inspected and a new ball cup bushing was installed (OE type). The rear suspension was installed and aligned to paint marks made before removing. All bolts are properly torqued, and new pinch bolts for the couplers were installed and properly torqued. The clutch PP/IP is indexed as it was when it came out, as is the clutch pack to flywheel. The transmission mounts looked fine and were left in place. Vibration was present during first drive after reassembly. Transmission shifts well and makes no funny noises.

During an engine out refresh last year to replace head gaskets, new Volvo motor mounts were installed, as well as new wires, caps, rotors, plugs, cleaned injectors, rebuilt MAF, clutch release bearing, release arm ball cup bushing, TB/WP, among other things. There were no vibrations present after that work last year.

Troubleshooting done so far:

• Clutch indexing checked and appears to be correct.
• Verified nothing is touching the body of the car – exhaust, heat shields, etc.
• Motor mounts look good, hooks are not touching, through bolts not touching rack
• Loosened rear pinch bolt on driveshaft to ensure no preload and bolt centered in shaft bore, re-torqued to spec
• Loosened transmission mounts to subframe and re-torqued to spec
• Noticed that driver’s side half shaft has a wobble when running in gear on stands (just the shaft – the flanges run true). There is no radial play in the shaft, just the usual axial play. The CV joints make no noise. Not sure what is up here. There is no vehicle speed dependent vibration. I think this is an issue, but unrelated to the RPM dependant vibration *OR* could it be that this condition just amplifies an inherent engine vibration when under load?
• Added grease caps to the half shaft flanges on the transmission ends – shouldn’t be an issue, right?

I’m working with Dave C. on this, and we are stumped. All logic points to an out of balance clutch since depressing the clutch while the car is not moving removes all rotational mass, except for the engine and the clutch PP/IP, with vibration still present, although much less pronounced than when driving and under load. But how does a once balanced clutch become unbalanced, assuming it went together the same way it came out and no parts were replaced?

Any thoughts on where we should look next? What could cause a driveshaft wobble, and could an issue with the CV joints amplify a previously unnoticed engine vibration under load?

Sorry for the long sob story, but wanted to touch on work done/thoughts to see if anyone has some other ideas. Thanks.

Black Sea RD

Hi Brian,

Just saw this post.

From our conversation I thought there was a prior vibration issue you were dealing with and the focus was on the drive shaft trans end being out of true. When we received it we confirmed your diagnosis and is why we replaced it. Apologies if I remember this wrong.

Since the vibrations seem more prominent while it's on the road, that would point to rotating assemblies used for rolling along such as tires, wheels and the CV half shafts. That there is a noticeable wobble of the CV shaft by eye would mean that it is more than a bit out of true, especially when it spins at higher RPMs. This would produce vibrations even if the joints themselves feel tight. These vibrations need to be accounted for somewhere and they will be felt. We would suggest to replace the bent CV shaft to take that known problem out of the equation.

One more question, are the vibrations felt in the steering wheel or at the seat? Steering wheel would point to it being more of an engine problem, seat of the pants feel would point to more of a chassis problem.

HTH,

BRB-83-911SC

Hi Constantine - thanks for the reply. You remember correctly about our conversation. I have always had a slight high speed vibration (very slight) that would come on as approaching 75 mph that wheel balancing hadn't been able to cure. This vibration was speed dependant. (the axle shaft wobble may explain this and it will be coming out for further investigation.) When we saw the driveshaft runout when we dorpped the TT, the thought was that it may be a contributing factor. I had no RPM dependant vibration before when driving the car.

This new RPM dependant vibration comes and goes (1200, 2400, 3600 RPMs) as you run up the rev range, and is strongest when accelerating (any gear) under load. It is also perceptable when the car is stopped, and revved up through the RPM range, clutch depressed, transmission in neutral. So, I think the source of the vibration is in the clutch or engine, since when clutch in, car not moving, nothing else is spinning. I am wondering if the driveshaft/CV issue might amplify this vibration when driving and under load.

The vibration is through the body of the car. It is *possible* that the slight vibration felt when running through the RPM range when parked, clutch in, was there before, but went undetected, and something upon reassembly has caused this to become aplified while driving.

I did not do anything to the inner CVs other than add some fresh grease and install the later car grease caps (which weren't there before). I did not disassemble the joints. Is it possible to have screwed something up in that inner CV joint so cause the driveshaft wobble? I never noticed this wobble before, but never really looked for it.

By the way, the TT is very nice and quiet. I don't suspect that to be a contributing factor here at all.

worf928

Just to be sure. Executive summary in bullet form:

- new vibration, not present before work
- nothing out of the ordinary until resonant frequencies reached.
- vibration has resonance at ~1250 rpm increments
- not road speed or gear dependent
- felt with clutch disengaged.
- Resonant vibration strength increases with drive train load (i.e. engage clutch strength increases, engage gear when on stands increases vibration strength, on road stronger etc.)

We do not suspect issues with the rebuilt TT or gearbox work. We suspect clutch or our own mis-assembly of the big bits. Have checked bit bits thoroughly. DD clutch system has many ways to put back together wrong, have check all PEBTAC (problem exists between tool and creeper) issues we could think of. Have plan of attack but looking for collective imagination here, before dropping clutch again.

Bill Ball

The axle shaft wobble could be relevant to the other vibration you reported - the road-speed related vibration over 75 MPH. I have had that for several years but still have not found the source in my case. Strangely, mine comes and goes and seems to have receded recently. Also I did not feel it during a dyno session, which would argue against rotating mass, nor do I feel it in the steering wheel, arguing against front suspension/wheel bearing/steering system issues.

Your current vibration that is RPM-related and at RPM intervals would lead me in the direction of the engine itself. I recall a similar report here that was ignition related. Sorry I have nothing more specific to offer. You've got the major brains already working on this. Can you run the engine w/o the clutch pack installed? If so, how does it feel then?

BRB-83-911SC

Thanks Bill. Yes, next step plan is to drop the clutch and run the engine without it. We won't be able to get back on it till after Easter, but will report back. I am hoping we can isolate it to the clutch pack. We had a heluva time with the centering pins. Not sure if I could have screwed something up driving them in/out. They appear to be seated properly, capturing the PP/IP and flywheel.

worf928

This is theory I have also. A very consistent, but low-frequency misfire could cause exactly the symptoms we are experiencing. The problem with the theory is that we didn't touch the engine and the vibration was not present before the T&TT pull. Of course it is not unheard of for something to break while sitting. With everything outside of, but bolted to, the short block new or almost new, there are no obvious places to start looking. Especially since this 928 feels quite strong. (Coils and amps come to mind, but it would be a bizarre failure mode. And, yesterday while Brian wasn't looking I checked all the plug and coil wire connections.)

On the other hand, there is something not quite right about the #7 plug, but it has been consistent since Brian's owned the car.

BRB-83-911SC

Just for grins and another look, here is a picture of a transmission mount, which we thought looked fine. Agree or disagree? I can fit a finger or two between the crossmember and transmission case.

worf928

Weekend Bump

Brian's wacking on the clutch pins as I type.

Any thoughts?

tilac999

I hope you guys solve this mystery. I feel bad for Brian, he and Dave have done so much good work, this is a bitch.
Good luck.

Mrmerlin

A few thoughts ....
Did the front bell housing to block have the alignment collars installed? SB 2 of them
Did the TT to both bell housings have the alignment pins installed?

Is the stub shaft properly spaced away from the crank.
Important...
IE it can be pushed past the pilot bearing and possibly drag on the rear of the crank,
NOTE as the crank is being pulled rearwards with the pedal pushed down, this is a feel item to figure out.
push the stub shaft as far forwad as it will go till you feel it bottom out then move it rearwards about 3 MM,
this should set the bearing surface of the stub shaft into the pilot bearing.

NOTE the clutch discs are mounted so the alignment dots are to be 180 degrees from each other,
also make sure the orientation of the discs is correct , follow the WSM for further details

NOTE Instead of pushing the dowels into the flywheel.....
loosen the bell housing bolts about 5 turns then remove the trans mount bolts and slide the TT/trans back.
Then install the 3 U clips to the PP pins and then drop the clutch pack .

NOTE this will reduce the possibility of damage to the thrust bearing inside the engine, hammering forces will be directed to the thrust bearing.

Mke sure that the trans cross member bolts to the chassis are tight,
their are 4 bolts on the cross member,
2 to hold the trans ,
and 2 to attach it to the chassis.

worf928

Yes.
Yes.
Yes.

We've have quintupple-checked the alignment of the various bits.

There are six bolts. Four for the x-member and two for the transaxle. All torqued to spec and the x-member is within 0.5mm of where it was before we removed it.

All good things to check, think-about, or double-check. So far, we've covered everything on this list.

BRB-83-911SC

Thanks Stan for the thoughts. I was just typing up a response when Dave's much more concise response came through. Thanks Dave.

Keep the ideas coming, we're running out.

Mrmerlin

sorry actually there are 8 bolts for the rear cross member 2 on either side =4
2 for chassis to X member,
and 2 for the trans to X member. Its not uncommon for the trans and chassis bolts to not all be tightened especially if your working on poor lighting conditions.
That said I think we are on the same page here.

Is there any chance that the TT has gotten bent?
IE from an accident or possibly by jacking the engine up at the bell housing and then letting the TT hold it as a balance .
OR was the TT swapped out for a rebuilt unit that may have been damaged?

Black Sea RD

The TT was rebuilt with our Super Bearings and another drive shaft was placed into it since the original drive shaft had a slight wobble to it at the transmission end. That TT shouldn't be the cause of any vibration issues.

This situation doesn't sound like a bad TT since the vibrations as explained (sine wave pattern vibrations at certain RPMs) do not sound like a classic vibration problem of a bad TT. Also further confirmed with their statement that the car exhibited the sine wave vibration with the clutch in (disconnecting the TT) while running up the RPM range.

This sounds more like a rough running/missing motor due to some problem. A bit of a clue came in Dave's statement about #7 spark plug not looking right. I would focus the attention on that clue to include replacing just that one spark plug wire and spark plug and see what happens. I would do that before touching anything else on the engine.

When dong a TT R&R the engine is usually pivoted at the back to help R&R the TT. This pivoting does jostle the wire harness of the engine and, IIRC, puts strain on the coil wires too since they are mounted to the chassis.

Good luck with the search and I hope I gave you some areas to double check.