I have a 1996 LT1/M6 convertible, 88k miles, no real modifications. Ever since I've had the car (1yr, 72k miles) I have had some vibration from the engine, beginning at about 3800rpm and the strongest at about 4200rpm, and then fading away as you pull through to the redline. Car is dead smooth everywhere below that.

Now, this is not driveline related - this happens clutch in, clutch out, neutral, any gear, full load, no load - its not the clutch back. Now, with no load, its definitely more noticable, I assume under load, the stock (worn - I have a little drive line lash) motor and trans mounts are under tension and help absorb this vibration. It bothered me a little last year, I've had SBC cars before and never noticed vibration like this, but its not too severe, and everything works fine otherwise. Maybe its driving smooth running V6 cars all winter, but I seem to really be noticing it again now that I have the car back out of the garage. I would call the vibration moderate, you feel it mostly in the gear selector. The motor doesn't burn any oil, makes great power, and has good oil pressure.

So, am I worried about nothing? Is this just a common "harmonic" of vibration for this motor? I am planning poly mounts for the motor and trans at some point this year, I'm worried that'll heighten the vibration even more.

 

Not common. Has the clutch been replaced ever? A lot of cheap clutch kits don't balance the pressure plate and that can cause an annoying vibration that is rpm-related.

 

I have a similar vibration and it seems to following the torque curve.

The vibe, although very slight, starts at around 4.K, peaks around 4.K and then fizzles out after 5.K RPM or so. I did not recall it back when I had single exhaust but now I have dual exhaust and I notice it more. I am blaming some of it on exhaust pipes and resonance.

Also up until recently, I had slight misfires due to bad plug wires. Misfires can cause vibes especially at peak torque which is maximum VIE meaning the most air is being exploded in the cylinders. The vibe has been reduced some after fixing the wire problem.

I have pretty much ruled out the engine going out of balance cause it is silky smooth everywhere else and the short block is three years old with several thousand miles. If it were the engine balance, according to research on the boards, it would shake all many different RP M's across the board.

Regarding the clutch, its possible it could be a component there. It might also have been the severly worn pilot bearing I recently replaced just a couple days ago that cause a severe vibration, especially when coasting with the clutch pedal pressed. I have not gone out and blasted on it yet to see if that may have contributed to the vibe. Will do this in the next couple of days.

 

How does it start, and peak, at the same exact rpm?

 

It just sort of fades in then fades out at the RPMs above. Its not severe, nothing like a driveline angle vibe. Its just barely noticable.

Clutch was out a few days ago and i saw nothing unusual. Other than the severly worn pilot bushing.

 

Yea, last fall - cheap clutch too. The vibration was there before, but definitely more significant following the clutch job. And you know what, I think you're on the right track, because you can feel it in the clutch pedal on higher RPM gear shifts.

Dang - definitely didn't want to tear into that again - I guess I'll have to see what I can tolerate, or maybe I just need to get it over with. Its definitely not severe, just a little ragged feeling.

 

Have you replaced your motor mounts and trans mount with poly bushings, that may help?

 

Not yet, thats definitely going to happen in the near future. The car has an unrelated missfire at high load/low rpm (1700rpm, higher gears w/ 40%+ throttle) and an occasional hiccup in the 4500+rpm range - so I'm going to do plugs/wires/opti in the next couple of weeks, I may go ahead and toss poly bushing in there while I'm at it - the ignition and the bushings are all original, 14yr/88k mile old parts, so I think its time. I don't think the ignition issues are in any way related to this vibration, it was there before any of that cropped up and it was running 100%.

 

I've got one that starts around 1700 and ends around 1900 rpm. It's been there before my h/c, clutch, etc.... I only notice it when I'm on the express way at about 70mph to 74mph. It's non existant before and after that range. I can only assume it's in the motor, but it's never caused an issue.

 

OK I went out and blasted on my ride. I no longer have the vibe I described above. The stuff I can feel is all loud exhaust and above stock power.

I will consider the cure either fixing the two partial misfires in #1 and #7 and possibly replacing a bad pilot bearing. The misfires were diagnosed by OHMing the plug wires. #2 and #6 were 150 ohms which is good and the other two were 4.0K plus which resulted in partial misfires.

 

Thread from the dead here but just want to say thanks for posting a possible solution, I am experiencing the same problem exactly as you have described it.

 

No problem