I need smart guys to help with my bird...

[N20LT4Bird]

I am hoping to catch some insight of some sharp guys on a particulary troublesome issue. I have checked the all of the obvious issues and I am not looking for the "did you check the u-joints?" answer.

Here it is. I have a high rpm vibration - out of the blue. I initially thought it was a drivetrain issue and even went so far as to replace all the bearings in the rear because I felt roughness in the pinion yoke. Upon further review, the vibration is not related to speed as much as rpm. I feel it in the shifter and the seat when I am at a higher speed AND higher rpm. Now, I am leaning more towards engine vibration as the car is not pulling as hard as it used to. Its not a huge power loss, but noticeable. The vibration only comes in under high speeds and high revs though. If your cruising at 75 in 6th - nothing. 75 in 4th - yes vibration. Im stumped. Could it be a valve sticking, causing an unbalanced engine situation? What else?

[ConElite]

It can be that a valve spring is bad and allowing the valve to flout. thats a tricky one.just hope its not something internal like a piston bolt. Seen it before. The guy ran like that for some time until it gave out.

[fastTa]

Valve flout Piston bolt Los Angales

Sean I'm assuming you are not *hearing* anything out of the ordinary from the motor?

If you are going 75 in 4th like you say, and then push in the clutch does the vibration stop?

[TraceZ]

Originally posted by fastTA

If you are going 75 in 4th like you say, and then push in the clutch does the vibration stop?

Here is another good question.... If you push in the clutch at 75 mph and vary the rpms can you feel the vibration change?

Make a note of what RPM the motor is at when the vibration is strongest. Next, stop the car and hold the engine at that same RPM. Can you feel the vibration?

[94 formula]

What about a bearing in the engine? The first thing that comes to mind though is the pushrods my brother has a 99 LS1 and bent two pushrods and you couldn't notice it when he got on it till 3rd or 4th but at high rpms it really was noticable. Also a slightly bent connecting rod could cause this but is very improbable.

[Joe 1320]

It could be either an in internal or external balance condition. Still have the external ring on your damper? If you had an automatic I would even suspect the flexplate, but with a stick doubtful there would be an instant change. Doubtful that you lost any crank weight.....

It could be an abnormal combustion event, anything show up on the scanner? Anything other than mechanical balance or funtion will show up here and mechanical function will affect scanner readings. For example if a lifer were hanging up, you would see evidence of the offending bank in 02 mv, injector pulse width, perhaps knock retard. Get it on a scanner and see what the computer is seeing. Are you absolutely sure that it is not an ignition related failure? The whole thing about higher rpms makes me wonder if some kind of spark scatter might be just enough to cause the instability.

I would also pull the vavle covers and check your pushrods too. A bent one or two will behave like what you describe. you weren't pushing it hard anytime recently were you?

[N20LT4Bird]

Originally posted by fastTA
Valve flout Piston bolt Los Angales

Sean I'm assuming you are not *hearing* anything out of the ordinary from the motor?

If you are going 75 in 4th like you say, and then push in the clutch does the vibration stop?

The vibration is not noticeable if the car is in neutral. If I put my foot on the clutch and maintain the rpm, the vibration goes away.

[Joe 1320]

Originally posted by N20LT4Bird
The vibration is not noticeable if the car is in neutral. If I put my foot on the clutch and maintain the rpm, the vibration goes away.

uh, oh..... might be input shaft bearing.

[N20LT4Bird]

Originally posted by Joe 1320
It could be either an in internal or external balance condition. Still have the external ring on your damper? If you had an automatic I would even suspect the flexplate, but with a stick doubtful there would be an instant change. Doubtful that you lost any crank weight.....

It could be an abnormal combustion event, anything show up on the scanner? Anything other than mechanical balance or funtion will show up here and mechanical function will affect scanner readings. For example if a lifer were hanging up, you would see evidence of the offending bank in 02 mv, injector pulse width, perhaps knock retard. Get it on a scanner and see what the computer is seeing. Are you absolutely sure that it is not an ignition related failure? The whole thing about higher rpms makes me wonder if some kind of spark scatter might be just enough to cause the instability.

I would also pull the vavle covers and check your pushrods too. A bent one or two will behave like what you describe. you weren't pushing it hard anytime recently were you?

Up until last night, I was searching driveline, thinking that my new rear end was the cause, but I think I have exhausted all possibilities there. When I took it out, last night, my friend and I noticed the slight power loss and the vibration is more sensitive to engine rpm than speed. I have not gotten it on the scanner. Unfortunately it isnt going to show misfires because the cps is disabled, but I will check pulse width and knock retard. Does the PCM isolate the cylinder that is causing the timing retard on our cars??

Ignition is a good possibility.....I did just get done sorting out an elec prob. The module wasnt getting power due to a broken wire. So, thats a maybe. I personally was leaning toward the bent pushrod or broken rocker or something. I will pull the valve covers today.

[Jeff_in_Atl]

I have seen this condition with a bad pilot bushing/bearing.

[Injuneer]

Might seem totally off the wall, but did you check the driveshaft/slider clearance on the tailshaft of the tranny?

[N20LT4Bird]

How much clearance should I have? The shaft definately goes in further than it used to......

Do you think this would vibrate only at high engine rpm's though? Wouldnt it be more of a speed related vibration?

[markd79ta]

A worn tailshaft bushing caused this in my car. The higher your speed & RPMs, the worse the vibration gets. It feels like it is coming right under the shifter and console. Although, I do get a little extra feedback when I am going down a hill and let off the gas. Otherwise it just depends on speed and rpm. I feel a slight bit of play in the slip-yoke... Another guy on this board had the exact same problem. I still need to get it fixed. This may or may not be the case with your car, but it sounds very similar. Both of us experienced this problem after the 4.10s were installed. The new gears and higher RPMs were just enough for this problem to rear its ugly head.

[Bond Hawk]

My guess is an unbalanced drive shaft. Are you still running the steel driveshaft? Does it still vibrate when you engage the clutch?

[N20LT4Bird]

Mark, this sounds interesting, how do you know that the bushing is the prob if you have not fixed it yet?

Bond, I have the aluminum shaft and the weight is still attached....