Rod Thomson

I know this topic has been done to death but I may have learned something to add to the knowledge base, so I float this balloon. Shoot it down if you can.
The problem: Intermittent vibration at ~3500rpm, evident in the shifter and the clutch peddle.
The car: 1987 951 - stock with an unidentified chip. New motor mounts, balance shafts checked and double checked bang on correct, no reversed cogs, no missing woodruff key, perfect.
The theoretical cause: misfire

The long story:
I bought a cheep wreck(don't do it) and have been hammering away at it for about a year. Lots has been done, more still to do. What i have been experiencing is that the car is like Jekyll and Hyde, sometime it runs silky smooth, sometimes the vibration makes the shifter rattle around at about 3500rpm. Idle is usually ok. I did the research and all indications were "balance shafts" with a few dissenting votes going to "clutch" or "TTube". After the third time having the flashing off and tweaking the belts but finding nothing, I gave up. The day before yesterday, while checking the oil I pushed down on the plug leads and found them all not properly seated. Once all were snapped in, I drove Dr. Jekyll home; butter smooth, gobs of power. Next day I start up Mr. Hyde and the vibration is back big. I seat the wires and lo, Dr. Jekyll smooth. Did the same again today. Each day the plug leads are popped off the plugs. My theory is that the seal on the plug lead is too good and as the head heats the plug lead is popped off the plug as the air pressure increases due to temperature. The vibration is caused by the balance shaft as the vibration pattern of the engine is changed due to misfire but the balance shaft counter vibration is static.

Reasonable?

John_AZ

Short answer:
Has the "cheep wreck" had the rubber clutch disc replaced?

3200-3500rpm vibration is the normal area of the rubber disc vibrating with a few chunks missing.

I do not know why your plug ends are popping off.

???

John

Rod Thomson

No rubber on a 951 clutch.

John_AZ

Sorry, not enough knowledge on 951 clutch disc.

But I'll add a couple more ideas.

Do you get the vibration when standing still, clutch pedal pushed in, and rev to 3500rpm?

If you get the vibration, I would guess the problem is in the clutch.
FW & PP not balanced.
Pilot bearing failure.
Clutch disc going bad. This diagnostic guide is usually good for when the clutch has been removed.
http://media.centerforce.com/DiagGui...cGuide04DL.pdf

If you do not get the vibration standing still, it could be the torque tube bearings going bad that is giving you the vibration at 3500rpm when you shift.


Sometimes the torque tube chatter or vibration comes from the transaxle.

The only way to separate the two is to disconnect the drive shaft coupler at the transaxle and see if you get the vibration with out being hooked up to the transaxle.

Out of ideas

GL
John

F18Rep

One way or another you're have to fix this... maybe spray a little wd-40 into the boot - although I have never had to do this on any of my cars and they have had all sorts of aftermarket wires. Bruce

thirdgenbird

my plug wires were coming off too. the last time i installed them i put a little water on the seal (it was all i had) and they pushed on further than before. so far they have stayed put quite nicely.

Mark944na86

After putting my exhaust system back together after my clutch job I had a vibration/rattle at about 3200rpm. It was my exhaust system rattling on the heat shield; I needed to adjust and recenter it so that it had more clearance from the heat shield (not much room for error -- fairly tight at that point.) Just in case you exhaust all other possibilities (so to speak.)

Rod Thomson

Problem solved: spark plug lead boots were popping off because the plugs were in too loose. They were in only hand tight and all plugs had a fair oil ring on the outside of the plug base. Blow by or shaking would cause the boots to pop. New plugs (NKG BPR7ES) in tight and no boot pop, no vibration. Love a 12$ fix. So... I conclude: misfire can show up as a vibration at higher rpm.
Thanks all for the replies.

Rod Thomson

Thought I might resurrect this thread as I have finally solved the riddle. It was the TPS. I thought I had soled the problem but it kept coming back. It seemed to be heat dependent. Most often I would get vibration at 3.5K on a cold start, but if I would stop and restart the engine after it was warm the vibration would be gone, sometimes. After much research I came across a quote on another board that went something like "The symptoms of a bad TPS can be the same as if your balance shafts are out of sync." So... I found the procedure to test the TPS at Arnnworks and low and behold my TPS was none functional. Replaced it and reset the throttle body using the proc from Lindsey Racing and joy. Smooth starts, smooth power all the way up to the redline. Nice:]https://rennlist.com/forums/graemlins/burnout.gif

John_AZ

Thanks for posting. It becomes an obsession to find the problem when the obvious fails.

John

odurandina

the vibrations are caused by physics. 4 cylinders is a buzzing, vibrator on steroids. the engine starts destroying your motor mounts exhaust and accessory components in the first seconds of operation.


the exhaust on my 968 weighed half a ton when i took it off. small wonder why.

EJZero1

I can relate... Got a similar issue in my '89 N/A 944. Tinny vibration at 3500 that seems to be resonating from/through the exhaust. Through the painful process of deductive troubleshooting, I think I may have it narrowed down to a shelled out cat converter. Got an appointment at a local exhaust shop tomorrow to verify.

Isn't it great when you go to a shop, fingers crossed, hoping they find something wrong just so you can finally have that "Ah Hah!!!" moment?

Dwad

Really .... Can a shelled out cat convertor can cause vibration

Wisconsin Joe

Not so much 'cause' as 'show up in'.

As pointed out a few posts (and a few years) ago, 4 cyl motors are inherently 'vibratory'. The balance shafts help dampen that to a large degree, but nothing makes it go away completely.