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?

This does not align with your rocker adjustment, but check your fan pulley. I had similar symptoms (mechanical vibration) and it turned out to be a broken fan pulley. The material between the smaller holes, surrounding the large mounting hole had sheared. This allowed the pulley to be off-center.

Regards,

Al

I agree.

Since you have the plugs out and are rotating the engine to TDC compression on each cylinder for the valve adjust, it is easy to screw in the cylinder leak test adapter and take a measurement. It can’t take three minutes extra per cylinder.

Al has a good idea.
If you dissasemble the pulley, inspect the flats on the pulley half and the fan hub.
The 'cup washer' cannot be concave.
Remember - six shims total.

Best,
Grady

hey there

i just noticed you have no mention of pulling the #4 plug out and checking it. this would be where i would start if that cylinder wasn't firing due to valve not opening this plug could be fouled and causing your miss.

just on a side note, if the problem didn't start until this rocker problem look at this area before you go off looking else where. a vibration that is caused from oil viscosity or such will end up being a sound before you can feel it. just my experience with these motors. a clutch coming loose could be the culprit, but what you have here i believe is a miss fire. turbos have lower compression and the misses aren't as harsh feeling at idle as a motor with 8-9 to 1 compression.

#4 plug first, then wires, i would go as far as to loosen all the wires on the cap so they pop off easy, then when at idle remove one at a time till you find the one that makes no difference, then dig in.

cheers ed

Ok, first things first, thank you all for your quick responses and help! I dug into her today and changed the plugs, adjusted the valves, and took her for a spin. shes running smooth again. 175F oil temp and I spent some time from 5-7k. she was smooth again like she used to be. the intake valves were a little tight on 5 and 6 and the exhaust valves were tight(almost no play) on 1, 2, and 3. the plugs were richer then usual, mainly because Justin and I tweaked her a little bit to provide the excellent flamage videos during Okturbofest. but 4 was so rich it may have been fowled, and I think I have failing injectors on 2 and 5, as they were the lightest of the 6. The cams don't look great with some odd wear patterns, I'm not sure what to make of it.
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Well the vibrations still there. its not as bad, but its still there. I'm wondering if its a sign that the cams and rockers are on their way out??? I cleaned the spray bars out again, we'll see how it runs this weekend.

Plug wires

ed