Drive train vibration at 3300 RPM

[Lyle O]

Everyone, I am having an engine vibration issue and am running out of ideas on how to address it. I have searched here extensively and nothing seems to apply. My car is an ’87 Carrera cab. I am getting a pronounced vibration in the drive train (proceeding up through to the shifter) right at 3300 RPM or so, both accelerating and decelerating, and also in neutral. For background, I just completed a rebuild on the engine about 1500 miles ago (3.4, MM pistons @9.8cr; heads redone and flowed by Walt Watson; WEB 20/21 cams; M&K headers w/heater boxes & 2/2 muffler; custom SW chip; Centerforce DF clutch; Patrick Racing lightweight flywheel; etc., etc.). The thing is, the vibration was not there (at least I did not notice it, and I’m sure I would have) until about 300 miles ago. I had the wrench who did the rebuild take a look when the valves were adjusted (at 1300 miles on the new engine), and he could not find a cause. It does seem to not be as bad when the engine is warming up, though. I have 2 year old Rebel Racing blue mounts on the engine, but the trans mounts are original (115K miles). I had planned to send the trans mount beam to WEVO this winter to have them re-do the mounts with the black pillows anyhow. I also have the JWest shifter and new coupler, both done about 3 years ago. Never an issue with anything before 300 miles ago. Also, I do not have the AC belt running.
I had planned to try swapping out the Rebel engine mounts back to originals (I still have them) just to see if it affects anything, but other than that I am looking for ideas. I have scrupulously looked for any drive train parts touching the body or chassis, but see nothing unusual. Because the car is a cab, the engine mounting cross-bar has rubber isolation from the motor, so the “loosening and re-tightening procedure” (from a service bulletin) to relieve any inherent stress does not apply.
I would really appreciate your input. The motor and car run really great otherwise (car was recently corner balanced and aligned also). Thanks in advance for your ideas.
Lyle.

[kyngfish]

Edit: if you're getting it with the clutch engaged, or in neutral, you can cross off the transmission mounts, and it's probably something in your engine.

[Lyle O]

Vibration occurs in neutral, also with the clutch in, whether in gear or out; it doesn't seem to matter. Once it gets to 3300, vibration is there. If I rev to 3500 or so, or go below 3200, it disappears completely.

[T77911S]

i would look at a balance issue with the flywheel/clutch.
pilot bearing
could also be a cylinder not firing properly at 3300. ignition/injector/ECU
i would look at AFR at 3300 vs 3k and 3500

when my 930 was not running right i could feel a very slight vibration from the engine, but only as the RPMs increased, not a single rpm. to me it was very noticeable but to others they could not tell. but then i drove it all the time

[78-911SC]

Just a thought. Possibly tran or engine mounts???

[ganun]

Could something with the fan or alternator be going

[Lyle O]

Thanks for the input so far.
PJ, the engine mounts are pretty new, but they are a stiffer version from rebel Racing. Nonetheless, I will be swapping them out for the original sport mounts (it's a cab, so sport mounts were standard) just to see if makes a difference (hopefully tonight).
T77911s, the flywheel and clutch are new, as well as the TO bearing, etc. Also, there was no issue for the first 1000 miles or more. What could have worn/changed in the that time on these parts? I will drop the engine and look here if other easier fixes don't work. Also should have mentioned that the plug wires are new (Beru) and the injectors were cleaned and flowed (InjectorRx).
George, when we had the engine out for the rebuild, the alternator and fan were obviously removed and examined and all looked good. I do know that the alternator is not original (10years old).
Thanks for the ideas; keep them coming!
Lyle.

[zippy_gg]

Check engine and transmission mounts for tightness...
Something may have come loose after engine/transmission were re-installed after your rebuild.

[Lyle O]

Thanks for all the suggestions so far. Here is what I have checked/done lately:
Replaced the Rebel mounts with the original stock ones: no change
Removed and re-installed the transmission mounting bolts and washers: no change
Relaxed and re-torqued all header mounting nuts to heads, and muffler to headers bolts/nuts: no change.
When I ran the engine and observed from the engine compartment (as opposed to being inside the car) I have to say I could not tell there was any issue, though. What I mean is that when i hit 3300 using my hand on the throttle body and doing an acceleration sweep, I expected to observe a shake or something at that point, but I could not notice any difference at all (3300 rpm) coming from the engine area. But inside the car, it in very pronounced there, especially coming up through the shifter.
When I can get my son to help this weekend, I will try to check for vibration from under the car. Anything else more I can try, before I actually drop the engine?
Thanks again everyone.
Lyle

[Lyle O]

Ok, so I have to give an update...
This has been driving me crazy. I contacted one of the most knowledgeable NVH experts I know in the industry and discussed this with him. He explained that a resonance at 3300 rpm equates to a frequency of approximately 53hz (he used a formula to calculate this based on engine type, etc.). That is a pretty low frequency, which indicates that there must be something loose forward in the car attached to the body/chassis that is going into resonance. I thought about it overnight, and then it hit me: spare tire? This AM, I went into the frunk and checked and, sure enough, spare was loose and even the mounting hardware was rattling around. I tightened everything up, started the car and...NO VIBRATION!!!. The guy was a genius, I feel like an idiot, but, in the end PROBLEM SOLVED. I hope this helps someone else here. Thanks, all, for your inputs on this. CASE CLOSED.
Lyle.

[78-911SC]

Nice follow through. Thanks for the update.

[brighton911]

Fascinating solution. Who woulda thought the spare was the demon.

[sugarwood]

Brilliant teamwork.

1 Revolution per minute is equal to 1/60 Hertz.

How did low frequency translate to something loose in the front of the car??

[Lyle O]

That low of a frequency means something fairly massive must be vibrating. The rear of the car was eliminated because there was no evidence of noise coming from the engine compartment, only inside the car, and forward. Therefore, it had to be something large or heavy up front. There are other possibilities (battery, a blower motor, etc), but this's was it. The expert I know explained that it 's all about resonance: if the resonance frequency of an item is reached, it will shake (unless it is bolted down, in which case it then becomes part of a larger mass, with a differenent resonance frequency).

[T77911S]

the engine can have a "miss" or vibration due to it not running smooth at a certain RPM that you may not hear but you will feel.

my 930 has had that problem.
i can feel the engine vibration thru the gas pedal and my seat. others dont really notice it but i do.

[Brorag]

I've had the SAME issue, same set of circumstances...........could not find the culprit. Just ordered motor/tranny mounts.
After reading this went outside to check; found spare hold down clamp VERY loose--tightened. Scared to drive it now.

[jlex]

Wow.... good thing you didn't drop the engine to look for it

[Brorag]

yup--time for a run to ck it out.

[Brorag]

jubilation and hosannah!
That fixed it.

I spoke to soon, but I think I found the problem on this car.
Same issue, seemed temp related; went back to rebuilder after I replaced motor and tranny mounts; vibration at ~3K RPM. He shimmed athe mounts, next day, same issue--back to mechanic and we were finally able to replicate the vibration by turning A/C on/off. ck, and compressor/clutch greasy with residue under deck lid. Able to replicate vibration while watching compressor--belt vibrating also. I swapped whole system out for 134a several years ago, so am looking for the most current/efficient compressor/clutch available--suggestions?

[Lyle O]

So glad to hear this! Isn't it goofy how that silly thing makes such a difference? I spend time every week now checking for loose items...congrats on the fix.