Weird engine vibration. HELP!
 

to the point, when my engine is cold and running at oil temps under 140 degrees, its fine, nice and smooth, I've run it up to 5k to test it and its buttery smooth. once it warms up and is running 170 and above, it has a vibration that goes up and down with rpms, it does it free revving and under load. its not really noticeable until 3k and above. no loss of power, no weird sounds, just a strange vibration. it started doing this on a Sunday drive when I had a rocker adjustment screw come loose, backing out the elephants foot and barely lifting the intake valve on #4 open. I tightened up said screw and nursed her home. it was on the way home I noticed the weird vibration. I figured I had adjusted the rocker too tight since the engine was still hot. But I've since replaced the rocker and set it correctly and its still doing it. any ideas??? i have to get her ready for the funf gruppen track-day in two weeks at PBIR and only have a couple free days this week to work on her.

Outside of checking your valve adjustments again you might check your ignition. Breaking down over 3000 can be a bad coil, failing rotor resistor or CDI.

the cap and rotor are brand new, CDI is good as well. all was checked/replaced roughly 4 months ago when I chased down an ignition problem. its not missing or anything, it just has a vibration when its warmed up. it does it below 3k but it gets more noticable the higher it revs.

Did you look at the cam lobe for any damage? Maybe do a leak down and compression test to see if everything checks out.

just a WAG, but if there is no noticeable difference in idle or power, maybe a trans or motor mount is taking a dump on you?

Does the clutch work properly?
Has there been an episode where it did something strange?

Is the clutch spring-center or rubber-center?

Best,
Grady

motor mounts were new 15k miles ago. trans mounts aren't, but the car only has 78k miles on it. I'll recheck the cam lobes when I re-adjust the valves tomorrow, but they looked fine less then 1k miles ago when I had to replace 2 rocker arms due to a clogged spray bar, I know what that noise sounds like when the rockers are wearing on the cam lobes, but there is no noise. at that time I cleaned both spray bars, adjusted my valves, and buttoned her up.
I took the upper valve covers off a couple days ago just to have a look and everything looked normal. the engine was reringed 1k miles ago as well, due to cracked rings from running 1.2 bar

the clutch works perfect. has less then 1k on it. spring center kennedy puck clutch. nice and grippy, but not 934/935 jerky

Have you checked for a family of racoons?


one never knows!

Although I'm somewhat sure this isn't your case, but I once discovered my AC compressor mounting bracket had a few stripped threads and had worked a few of the bolts loose...very pronounced vibrations (of course, with the AC on...)..,..above 3K RPMS......

Other than that, not sure. An imbalance somewhere in flywheel, clutch maybe?

Best,

Doyle

I don't have A/C. whats throwing me for a loop is that when the engine is somewhat cold, everythings normal. when its warm the vibration comes back. I'm doing the valves again tomorrow, we'll see what that turns up.

Subscribing for information purpose.

well from experience i would say double check your plug wires are on tight as sometimes they will not get clipped on and cause this problem. also check resistance on the wires my 79 930 had a vibration or miss in it and it ended up being a set of faulty wires that were not to old. we can almost rule out a bad valve adjust as normally the miss will lesson on higher RPMs and make noise at idle if loose . im thinking look at the wires for connection and for resistance.

good luck im sure its simple.

cheers ed

i must have missed this, now im really leaning towards somthing ignition I.E wires as thermo breakdown plays a large part in faulty wires or coil.

ed

Pete, after talking with you, and thinking about it some more, would have to agree with Ed. On my SC with 250Kmiles, i had a similar problem that drove me nuts for a while and turned out to be the #4 spark plug hole with worn threads was expanding when the car warmed up and would start to loose power because of a compression leak. Something related to heat expansion or increasing electrical resistance is occuring once the car warms up. R ur plug wires new? Any way you could get someone (Greg?) to rev the motor when it's warm while you listen and look, or check the timing with a dwell meter? Give me a shout later today.

If the clutch only has 1k on it , could be a developing issue with the installation of said clutch. Could this have anything to do with the turbo?

Lets try and separate two different possibilities:
This is due to the cold/warm difference.

When cold, it is (at the beginning) running on the warm-up regulator (WUR).
When cold, the oil is more viscous and dampens vibrations.

The question is: Is the ‘vibration’ a misfire, possibly from lean running when off the WUR?
Is a mechanical ‘vibration’ there all the time and masked by cold oil (or other)?

I agree, it is useful to make sure you don’t have a misfire that appears when warm.

Your ‘seat-of-pants’ should be able to tell the difference between a mechanical vibration and a misfire. Measuring the exhaust gas is another way.

Best,
Grady

Getting ready to head into the bat cave to start on it. I'll do the valves and change the plugs just for good measure, I have the whole proceedure down to a 2hr operation. My plug wires are oem that I bought new at the first rebuild 15k/7 yrs ago. the coil is a msd unit 2k/7-8 months old.

Grady, I'm gonna owe you a lot of money, especially if you're right! love the signature.

Good luck, but since you're driving a Turbo I'd go back to basics and perform a leak down. Your new rings could be cracked. and what is happening to your oil pressure?