ElRicardo

On my '87 944S, 58K miles, no known mods, recently put back on road after an unknown hiatus of being defunct due to valve chain snapping. Engine runs beautifully, smoothly across all rpms quite powerfully, in fact an the car is a pleasure to drive until... at 3100-3700 rpm I have a vibration in the stick, a non-audible "buzzing" at a fairly high hertz. Quite disconcerting, but not enough to throw the car out of gear. Some details:

-The vibration grows starting at 3100 rpm, peaks at 3400 rpm and fades away at 3700 rpm.

-The vibration happens ONLY when the car is in gear, and happens in EVERY gear when the tach passes through the 3100-3700 rpm range.

-When the clutch is depressed, on stock in neutral, the engine freely revs at all rpm ranges without the vibration.

-At highway speeds, if I depress the the clutch (to let the car "coast") but leave the stick in gear, or move the stick to neutral, the vibration immediately goes away, and if I rev the engine while "coasting" there is no vibration.

-i ruled out the balance shafts, they are aligned, no 180 degree reversal.

Any thoughts?

931guru

Dd

Steven Kemp

Check your Engine mounts and Transmission mount - its up top transmission and hard to inspect. Is it hard to get out of gear sometimes when parked, especially on incline? sign of Mount problems.

ElRicardo

Thanks for the reply Steve- I had the engine mounts changed when reviving the car from the dead (hadn't run since 1998, the old mounts were flatter than a mouse in a waffle iron) . Have not checked transmission mounts yet- that is on the list, and as long as I am checking I will be a-changin'.
I live out in Iowa so finding a hill to test on is s bit of a tall order, but I never noticed any difficulty. Hmmm- if so what are you thinking?

968workaholic

Check your torque tube. Disconnect the coupler at the transmission and see if the vibration is stil there with the clutch engaged. That would point to one of the bearings going bad inside of the tube

Steven Kemp

Ricardo,
Transmission mounts are expensive. If you need one, message me. Have exact parts car.

ryan@usaimage.com

Do you still have the original rubber center clutch?

Constantine

Can you affect the vibration with the clutch pedal?

Modulate the clutch pedal and see if that affects the vibrations. Could be something in your clutch system, especially if you have a rubber centered clutch as Ryan suggested.

Doesn't sound like a bad bearing in your torque tube.

Good luck with the diagnosis!

EliteThink

was it the clutch?

ElRicardo

Well, daughters are graduated, parties held, vacations taken- back to the real world and my vibration issue. Since typing the original, I have noticed one change to what I originally described- when the car is standing still and clutch is depressed, I can get a vibration in the range i originally described. Much fainter than when the car is under power and under way, but still there. Not in the clutch pedal though, still just the shifter.

Does this change anyone's suspicions?

PaulD_944S2

Possible balance shaft timing?
Wouldn't hurt to double check it.

ElRicardo

Thanks PaulD -that was one of the first suggestions I checked. The shafts are in correct position with the proper markings.

PaulD_944S2

Just a thought, if it's not too hard, try running the engine without the balance shaft belt installed and see if the vibration changes any.

ElRicardo

Now that is an interesting proposition! I need to tighten a new timing belt at 5k miles- that would be a perfect time!

Therosser77

Did you have any luck finding the issue?