m20 vibration issue

[bmwtool]

Hi all. I recently did a bunch of overdue maintenance on my M20 including timing belt service, valve adjustment, new CPS, new coolant temp sensors, new plug wires, plugs, cap and rotor, and rebuilt 415 injectors. The car runs great and has a steady idle at 750rpm.

After that, I replaced the motor and tranny mounts with 635CSi and UUC black urethane parts. Immediately, I noticed a slight vibration just of idle at about 1k rpm. It disappears pretty quickly, then comes back stronger at 1750 rpm, then disappears again about 2500 rpm. The car idles fine, pulls to 6k rpm no problem, and the vibration occurs both with the car parked and in motion. I first thought it might be a bad harmonic balancer, but it seems odd that the vibration isn't more consistent. I also suspected the new injectors, but I pulled my plugs and they all have a nice toasty appearance with no wetness.

I searched the archives but I didn't see any problem quite like mine, and I'd appreciate any help anyone can offer.

[jeffnhiscars]

I have no experience with those mounts but if that's all you did to cause the vibration, I'd say they are hard enough to be causing NVH

[bmwtool]

I'll likely swap the tranny mounts for stock parts, but I'd still like to figure out what's causing the vibration. It's got me stumped.

[digger]

Tranny mounts will be the issue, the engine mounts are oem 6series ?

[bmwtool]

Tranny mounts will be the issue, the engine mounts are oem 6series ?

Yes, from the 635CSi.

I guess what has me puzzled is the nature of the vibration appearing only at certain rpm's. I've run stiffer mounts in other cars and any increase in vibration was noticeable mostly at idle and it smoothed out uniformly as the revs climbed.

I will try a set of stock mounts, either e30 or e21, and see if that fixes it.

[Seawolf]

Just before you tear the mounts out....small dimensional differences and moving the engine around may result in either of the following.

Check the clearance of the AC compressor hard lines to the body, particularly the frame rail. I've seen a couple of cars where this is close enough to touch the frame rail under load.

+, if your car is an iX check the oil cooler line isn't touching the cross member.

[ForcedFirebird]

I agree with the trans mounts. Seems as though they transmit vibration than engine mounts. Local here has an m30 swap with solid steel engine to sub frame mounts and softer poly engine mounts and it is far smoother than one with all UHMW/high-shore poly in both locations.

[bmwtool]

Thanks for the tips! I'm double checking the A/C compressor lines, but if that's not the issue, then I guess the tranny mounts have to go.

[bmwtool]

You guys were right, it was the mounts! I swapped out the UUC mounts for a set of Febi e21 mounts, and the vibration disappeared. Thanks for the tips!