arthropraxis

I have a vibration that is RPM dependent. It is strongest around 3K RPM. The motor is a 2004 GTO that was pulled from a wreck. The car is a Porsche 944. The motor does have poly mounts and the transaxle has a poly filled mount.So far, I have had the injectors cleaned, PCM re-tuned, flywheel and PP checked for balance, added ground straps and I still have the vibration.

Is there anything else to look at before I change the crank pulley/vibration damper? Is the RPM specific vibration even a symptom of the damper going bad?

Thanks, for any help this is getting annoying.

03Sssnake

iTrader: (20)

The poly mounts on motor and transmission are the likely source of the vibration. I have had a bad crank pulley from ASP before and it caused random misfires at WOT, not bad vibration.

arthropraxis

Thanks, for the reply. I have read that the mounts can cause some vibration on F-bodies. Most of the 944 conversions use poly or solid mounts on the motor and transaxle without the vibration issues mine has. Could be my problem though.

HoLLo

iTrader: (17)

I've been chasing down some vibrations too, I'm getting some vibrations also around 3000 rpm. I've got a new rubber trans mount on the car, and stock 116k mile motor mounts, but I just ordered some poly motor mounts, we'll see how that goes.

arthropraxis

If you find out what the issue is, update the thread. I need to get someone to scan mine to make sure it isn't a miss. I can't see any wobble on the crank pulley/damper, doesn't mean it isn't there.

03Sssnake

iTrader: (20)

The miss was pretty obvious, the car felt it was laying down/cutting out at WOT. The flashing SES confirmed it. I am guessing your instrumentation will not display a misfire since this is a conversion...

arthropraxis

You are right, I don't have a warning light hooked up. It would be on all the time.

S10xGN

iTrader: (11)

Interesting! I'm not driving my car much because of this exact same problem...

Got poly mounts here too, but mine are stands with cylindrical pieces of tube and a poly sleeve about 1 1/2" OD x 2 1/2" long. What's the fix? Replace those with aluminum? Delrin? In my case, I don't have knock sensors...

arthropraxis

I may have found the cause of my vibration. The vibration damper/crank pulley inner hub is not flush with the outer ring. On one side the inner hub sits below the outer ring, 180 degrees on the other side the inner side sits above the outer ring.

bwkmaro

a dampener is made of rubber generally and they will dry rot if left sitting to long(aka junkyard). i vote dampener

arthropraxis

Thanks, for the reply. I am going to get it scanned this weekend for other issues. The damper is still high on my list for the vibration. The vibration happens in narrow bands all the way through to the range. Does that sound like the damper?

LilJayV10

iTrader: (39)

I have the same problem. Mine is worse between 2500-3000rpm. I have a SLP balancer, poly engine mounts and poly trans. It does it when the car is stopped with the clutch pushed in and when driving down the road. The trans is out of the car now and I'm going to try a stock rubber mount when it goes back together.

67goatman455

iTrader: (14)

wow. same here. just bought a car with it. I changed out the delco plugs to ngk's and some new wires and it helped a lot. i dont have a MIL light either.

arthropraxis

I have new plugs and wires. The car was run a few times really rich with open headers, maybe they fouled or fouled the O2 sensors. At any rate someone loaned me a basic scanner, I will check it today.

arthropraxis

Scanned and no codes. I am going to recheck the motor mounts, not the easiest to get to. If nothing is loose, then I will buy a vibration damper.

I8UR4RD

iTrader: (16)

i chased down a similar vibration

started with plugs and wires
then wheel bearings
replaced the seals on the TB intake etc...
it seemed real bad if i shifted into 3rd gear on the highway.
swapped the converter
poly mounts for the motor and trans
played with the exhaust and added things to stabilize it such as bumpstops etc..

it was still there.

bought the procharger kit and pulled off the balancer and put on the ATI one

had to drive too slow to tell if it worked because i did not want to get into boost with stock injectors. i had a good feeling about it though.

but if that was not it, i was damn near sure it was somethin with the driveshaft, or u joint. like it was out of balance while rotating. my point is...you can chase it till the cows come home. good luck.

arthropraxis

I found some new info. The Corvettes with a manual trans have the flywheel and vibration damper "hot balanced" essentially they add weight to smooth out the vibrations. If you put on a zero balanced flywheel then vibrations may be experienced but it is in tolerance for the motor. Since the 944 and the Corvette both use a transaxle that may be why there are still vibrations. I am still going to replace the damper just to make sure. I hope this helps someone else.
Here is the link http://forums.corvetteforum.com/c5-t...lywheel-2.html

S10xGN

iTrader: (11)

Well, now that is definitely interesting. My engine was originally an '03 manual shift C-5 that now has a neutral balanced flexplate and the original balancer (marked and reinstalled to original orientation). Guess I need to hurry my 427" build and go from there...

arthropraxis

Update. I tightened my header bolts, put clamps on the PCV hoses and put new gaskets on the intake manifold. It all made a big difference with the smoothness of the engine and did decrease the vibration, maybe 50%. I need to get it tuned maybe that will kill the rest of the vibration.

LilJayV10

iTrader: (39)

I thought I would update that I fixed my problem. It was my clutch. I had a monster 3.5 and it had not been right since I put it in. I ended up replacing it with a stage 2 and finished it up tonight. It's butter smooth from idle all the way up. It's like driving a different car.

I don't know if the assembly wasn't balanced correctly or what but my vibrations are gone.