Mysterious vibration at 2500

[cgarr]

I just went thru my transmission with seals, carrier bearings and 1st and 2nd synchros, proper preload set, pinnion shaft set along with backlash, backlash is iffy, now I notice a slight humming or vibration, here is what I do, start from any gear, all is quite and smooth when accelerating say up to 4000 rpm, let off on the gas, let the car drag and when the rpm drops thru 2500 you feel the vibration, only at 2500 its only when you load the engine up and coast, its fine decelerating thur all other rpm ranges except 2500. Does this sound like an engine balance problem since its not related to the speed of the car only the rpm? I can accelerate thru 2500 with out any problem, One thing the backlash is not quite as tite as it should be, im about .008

Sleep on this one and let me know

Thanks

Craig

[Jim Richards]

Craig, could it be a resonance in the exhaust?

Jim
1995 Black 993 Coupe

[cgarr]

Come to think of it thats EXACTLY what it sounds/feels like, I may pull my tail pipe and try it without, I had to go slightly smaller on my tail pipe, 2" rather than 50mm Thanks Jim its a good place to start.

Craig

[912s]

In any gear only when you have the clutch engaged using the engine to slow the car?

[cgarr]

yes, only when engaged and slowing down, as soon as you push in the clutch or add a little gas it smooths out or if I am above 3000 or below 2000 rpm.

Craig

[912s]

How did your CV joints look when you had the tranny out? I had a similar problem with my '89 F150. At 55 MPH didn't matter if I was accelerating or slowing down as long as the clutch was engaged it vibrated the truck. It is an extended cab with a split drive shaft so there are multiple u-joints. It was the one right at the rear of the transmission. One side of the cross was dry while the other perfectly lubed. Replaced the u-joint and the problem went away....

[cgarr]

The CV's were taken apart when I had them out, They always had grease in them but I did clean them out and re-pack um and install new boots. There was no pitting and did now seem loose or any excessive wear that I could see. It doesnt matter what side they go on or what position the joints are placed back on the spline, does it, Now I know I do have a right side rear wheel bearing going bad, FYI

Thanks

Craig

[Jim Richards]

Wouldn't bad CV's or wheel bearings give you a speed-related vibration instead of an RPM-related vibration?

Jim
1995 Black 993 Coupe

[912s]

Jim,
Seems like it would be entirely speed related, but with my truck if you disengaged the clutch the vibration would go away immediately...

Craig,
I know you said it didn't matter what gear as long as the clutch was engaged but is the vibration the same when slowing down in 4th as it is in 2nd? Or does it differ somehow?

[cgarr]

I think I got it narrowed down to the pinion, too much backlash in the pinion, I know I had it set quite generous when I put it back togeather, I just drained the oil to check the plug for metal, it was clean after 500 miles. I refilled it with some mobil 1 SAE 75w-90 synthetic gear lubricant which really helped quiet it up (not perfect but better) and the shifting is improved (smooooother) since I will have it apart agian soon I ordered a new pinion and mainshaft bearing (just in case) then Ill reset the backlash and let you all know how it works.

Craig