ok, so I'm getting a low vibration (I mainly feel it through the gas pedal and floor) when the car runs steadily at 3.5k rpms or higher. (so basically freeway driving) it happesn in any gear, when I'm NOT in boost, just cruising along. and it never happens when I'm below 3k rpms.

I did replace the flywheel with a lightweight one, did I mess up the engine balancing? what else could this be?

 

it also happens when the car is sitting still. any ideas?

 

We know nothing about your car, what year, engine,trans....


Light weight flywheels can and will cause more than normal drive line vibration. Removing the trans and not getting everything you put in balanced, together, can cause this. Clutch, pressure plate, flywheel can and should be balanced before installing. This is not overly necessary and was mentioned because it could be an issue.

What fly wheel do you have? Clutch? PP?

Sometimes the drive shaft needs to be put back in the order you found it, Diff flange bolts...

I read that it does it while sitting still. That tells you it's most likely in the clutch, fly, PP.

You could be the owner of an unbalanced flywheel.

 

I have a similar mistery vibration at 3600rpm. Not as severe, just enough that I hear what sounds like a heatshield vibration. Mine does it in all gears and when stopped too. This is on a 96 engine with nothing special, resurfaced flywheel and new clutch ~15k miles ago, but only recently developed the vibrating sound. Something is resonating at that speed. It will go away +/-50 rpm from 3600.

OP did you experience the vibration immediately after you made the last changes? Does the vibration go away above a certain RPM?

 

i also have a 96. and it isn't the tranny, because it does it even when sitting still.
no the vibration does not go away above any rpm. however, i replaced the spark plugs yesterday and set them to .03", and it got substantially better.

the last change i made was to put a turbo, lightweight flywheel, and new computers on it- and yep, it happened right after that change.

 

1 million dollars says your downpipe is hitting the transmission tunnel near the gas pedal. ask me how i know......

 

I don't believe that it is... But it's worth checking. Thanks for the suggestion.

 

any more ideas be chance? the down pipe was not hitting anything.