So I have a 91 vert, 100k on odometer and i believe stock motor. When I got it I felt an abnormal vibration at about 6k but assumed engine mounts. Replaced engine/trans/differential mounts with delrin and now I know the vibration is not normal. I know there is going to be vibration with solid mounts, I put solid aluminum mounts in it and it was just a constant high frequency vibration and not a shake like I feel now. So It happens at around 3800-4200 and again from 5500-8k, at 3k and 4500 it is smooth. this is a shake the car like crazy vibration, cant see the mirrors clearly. I have pulled the fan clutch off and taken off a/c and p/s belts with no difference. Thinking it is something with internal balance and need some help. what should I check and is there any other known issues other that engine mounts for this.

 

Forgot to mention this is at a stop with clutch dissengaged

 

Not sure how to verify this offhand, but seems like the pressure plate would be suspect. Do the RPM ranges change when the clutch is engaged in first gear?

 

No its the same in all gears or in neutral. did a clutch in the car as soon as I got it and did not notice any difference, this was before replacing mounts so I wasnt as worried assuming it just needed mounts. Pressure plate turns at engine rpm anyway so gear wouldnt matter.

 

You're completely missing the point of this question.

 

Sorry wasnt dismissing you i think im just tired and not catching on, what are you getting at with the pressure plate?

 

This tidbit piques my interest since 3800 rpm is where your secondary injectors are kicking in.
I wonder if you might have an injector/fuel delivery issue.

 

I'm with Clocker. One of your secondaries might be opening up too slow maybe?? I know that an injector not kicking on would feel like a blown engine. The 5k+ vibration may be related to the vdi but that's just a wild guess and have no clue.

 

ive been thinking about that also, if i do a very slow roll up to 3800 i can feel the secondaries, almost like adding throttle. I may just unplug them and look for a change

 

where are the letter stamps on rotors? different letters are for weight right? would i beable to see it with a borscope through the spark plug holes?

 

stamps are on the sides of the rotors not the faces. youd have to tear it down to verify their weights