hatched91

okay so this has been an issue since i bought my car. When idling around 1500 rpm or so (obviously giving it some gas), the car vibrates a decent amount, and you can see the steering wheel vibrate back and forth. Now, when you rev up the car, then allow it to rev down (approximately 2000 rpm and less), you can see the tachometer sort of "bump" down, rather than having a steady smooth decline. So what could be causing this? there is no doubt in my mind that the engine has been pulled apart before. could the harmonic balancer (crank pulley) be misaligned? or is there something internal wrong? i have good oil pressure, and the car runs fine, aside from the excessive engine vibration.

Please let me know if you have any suggestions

orangeapeel

iTrader: (18)

Did you just recently buy the car? Or have you been living with this for a while? You say that there is no doubt that the engine was apart before. Any idea why? Is the car cammed or headed? Does it do it under a load? Free revving the motor isnt going to tell you a whole lot cause its moving around and you are sitting still. You have bad motor mounts?

Give us a little more history/background on the car.

LS1-450

iTrader: (33)

+1 on needing more info.. You'll get a low RPM vibration depending on cam & as noted, motor mounts?

If it is actually a balance issue, start w/ the clutch/flywheel assembly before assuming that it's the engine. Always balance the clutch pressure plate assembled to the flywheel prior to installing. If the balance is off enough your teeth will chatter. If she's an auto, then ignore the statement. Mods are not listed in your signature, so I don't know.

orangeapeel

iTrader: (18)

Yeah, if I had to guess I would say he was an M6 just reading about vibrations. The crank pulley could have just not been seated correctly, but it would probably be eating up belts like a dick sandwich at a gay parade.

hatched91

the car is not eating up belts. i've only had the car for 2 months, so i've been living with it. the engine has been apart, because it is both clean, and the gaskets look a lot newer than 10yrs old. I have no idea why the motor may have been taken apart. The car vibrates under a load, but i think that may be related to bad tires (those come next week). The tachometer does not "bump" down when decelerating (under a load). as for more info, it is an m6 with some sort of aftermarket clutch. and it has had this vibration issue with factory manifolds and with the longtubes. i do intend to change the clutch to an oem one, since this clutch is very difficult to push in, and i don't have a lot of mods, thus not really needing the heavy duty clutch. anyways, my mods inlcude:

aftermarket clutch (possibly ls7 ?)
long tube headers
ORY
hooker cb
slp lid
smooth bellow
predator programmer

i do not have an aftermarket cam, and the motor mounts looked fine when i installed headers (a few weeks ago). also, when revving and watching the motor, it doesn't move around any more than other ls1's. After considering that the car does have an aftermarket clutch, it may have an aftermarket flywheel too. one or both of these would definitely affect the way the engine revs.

orangeapeel

iTrader: (18)

1. Dump that Predator and get a dyno tune.
2. A stock Z28 will rape the stock clutch. Don't replace with one. Atleast not an LS1 oem replacement.

You are not going to have any easier of a time with a stock clutch. A hydraulic clutch will pretty much have the same pressure. I ran a stock clutch, a ram power grip, a centerforce, Spec 3+, and a ram twin disc. All had the same pedal pressure. Its not a foxbody mustang with a cable clutch. It may just have a poorly resurfaced flywheel or completely unbalanced one at that. I wouldnt blame the clutch as much as I would the TO bearing or the flywheel. Because if you are free revving the motor the clutch isnt even engaging the surface of the flywheel. The flywheel will be spinning no matter what.

Then that isn't "idle".

hatched91

good point. the reason i would say to go with a factory clutch is because you have to rev the crap out of engine to get this one to grab smoothly. i'll keep my eye out for a new/used flywheel. how do I balance the clutch assembly to the flywheel? i've never heard of doing that.

also, as for tuning, i will get a dyno tune when i install a cam. as for now, the predator is a cheap tune seeing as i only have $50 in it.

orangeapeel

iTrader: (18)

Usually they come balanced if you get a clutch w/ a flywheel. You have me lost on the "rev the crap out of the engine to get this one to grab smoothly" portion. Makes me think you have a fried stock clutch that is having problems engaging at high speeds. Do you have a burning clutch smell when it does this? You know it has an aftermarket clutch right? Do you have a stock master as well?

hatched91

i was told it had an aftermarket clutch in the car when i bought it, i didn't really care whether it did or not at the time, so i cannot verify it unless i pull the trans out. however, the trans has been rebuilt, so i assume the clutch is aftermarket. I do not smell clutch, i've smelled it plenty of times before and i know that distinct smell it does have the factory master cylinder in it.