I have had some issues with my 98 z since I put on new long tube headers. My first mistake was I should have waited to get a mail order tune, but I figured the car was down so why wait. At the same time I installed poly motor mounts and trans mount. Ever since I got the car going again I have been having a harmonic vibration when the car downshifts. And also a thump under the floor when I tap the gas when the car is in drive or overdrive. I dont hear it in 1st or second. I also do not hear it under hard acceleration. I raised the car back in the air and looked at the universal joints. They did not seem loose or worn out but there was movement in the shaft that goes into the tranny some side to side and up and down. There also seems to be a leake at this point. Could this be the issue? The vibration almost feels like torque converter shutter but seems to far back. I also have on order some bump stops for the exhaust but from what I read they should not be needed because of the poly mounts..?? Any Advice would be appreciated.

Thanks

 

The poly mounts are known to give "great vibrations". lol That is why the factory does not use them.

 

the 'knock' could be the header hitting the floor of the car, mine did this so i did a bit of bending

 

a leak of what type of fluid?

 

Tranny fluid. I took it to a mehanic and he said it felt like "launch shutter". From the angle of tail shaft being wrong. Not sure how but the motor diffinetly sits Higher in the bay from the new poly mounts. He will look at it on the rack tommorrow and let me know. I will keep you posted.

 

I have read that is more true of the poly trans mount... I would go for a stock trans-mount and see if that helps first.

 

The mechanic ordered me a factory trans mount today. He also told me that since the car is lowered and with the new poly mounts there is no pitch on the drive shaft. He showed me with a degree gauge that the drive shaft and trans tail was a straight shot to the rearend, and there should be at least a 1/2 of degree of pitch. I would have guessed the straighter the better?? He then told me this would wear out the u-joints?? Is he correct or feeding me some ****??

 

Liberty for all you gave good advice. Putting a factory transmount on made a huge improvement on most of the vibration I feel in the car. It no longer shutters under acceleration. Thanks

 

Back when I had a 96, I put a poly trans mount on and the vibrations were minimal. Your mechanic wasn't blowing smoke up your tailpipe about the driveshaft angle, or lack thereof. Your poly mount was just amplifying a problem that is there, don't ignore it now just because the rubber mount isn't transferring bad vibes to the body like the poly mount was.

 

the pinion angle on a stock 4th gen is 0 so i think your ok there.
Can you explain how to set and adjust pinion angle? :: Spohn Performance | Support Center