I bought an 77k mile 86E coupe a couple months ago. Since then I've replaced all fluids(all synthetic), and greased the chasis. Also to pass the MD inspecrion, I replaced the lower ball joints and installed a corsa exhaust.

My problem is I'm getting a plusating hum type vibration at 2200+ RPMS.

At first I thought it was driveline, but it wasn't wheels/tires as I've replaced them and while the bumpy/shakey ride at highway speed is gone, the loud pulsating humming type vibration continued.

Ive since determined that shifting down from 4th OD to 3rd OD(4+3) causes the pulsation to increase in speed dramatically at the same road speed.

I've been able to make it do it while parked in neutral as well by holding the RPMs up. So I'm 99% sure it's a vibration from the engine. But, it is much softer idling than when driving down the highway.

Any ideas of a likely cause?

Current, possibly unrelated problems:

I have a loud vibration from the smog pump when the A/C compressor is engaged. I've already got a cardone reman from autozone to replace it this weekend. I know the bolt for the rear bracket won't really tighten up due to the threads in the pump being damaged, so the new one should tighten up and fix the vibration. This noise is very loud at idle and sounds nothing like the pulsating hum I'm chasing down. I'm 99% sure the smog pump will fix this problem.

At idle it seems like it's trying to miss. With my laptop connected I can tell it's trying for 750, but it hunts around from 700-850 in a regular pattern. I previously developed a bad miss, and saw one plug with ash, one with oil. I've done the valve seals(and springs while I was at it) which fixed that problem, but not the idle problem. I've replaced plugs, wires, rotor, distributor cap, pcv valve, fuel filter, air filter. I've check vacuum, it pulls 20 lbs. I've ran propane around all locations where vacuum could be leaking and saw no difference. I've ohmed the injectors, all read 16.1-16.3. This one has me stumped, but doesn't seem like a critical problem.


Otherwise everything in the car works, and it drives nice, no problems with power... So this is really nagging me. If I hit around 75MPH it is a very noticable.

Any ideas? Sorry for the long post, but I wanted to include anything important.

 

Push in the clutch while sitting still see if it still makes it

 

The injectors are likely junk. Ohms testing is only half the inj diagnosis. Do a leakdown and post the results over several hrs.

BTW...the threads will strip in the alum brackets, so go find a bolt thats 1/4" longer and an aircraft nut. You don;t want to run it without ALL the bracket bolts snugged down and secure. Brackets break.

The vibration? clutch, or maybe the balancer if the vibration is rough enough and definately from the engine. if its internal, only way to check is to pull the pan...

 

It still makes the noise with the clutch in while sitting.

I have heard a faint grinding noise in the AIR pump with a mechanic’s stethoscope, which is why I decided to get a reman to kill two birds with one stone.

I attempted a leak down test a couple weeks ago, but the gauge failed, filled with gas, and sprayed it out making a mess. I've purchased a new one, different make, and am planning on testing this weekend. They are all black, bosch 1's I'm guessing, since it was pre-multec era. Otherwise I'd have already replaced them. I might just grab a set of remans Bosch 3's from FIC just to be sure. I'll post what the leak down test shows.

I have a suspicion of the balancer after reading forum posts. It's not rubbing anything, but it's original. I guess using a timing light and seeing if the mark is excessively off would tell me if it has slipped.

 

If your leakdown gauge is filling with gas you likely got an injector problem adress that issue first then go from there. A misfiring or dead cyl will certainly cause a vibration.
AIR pumps do make some noise but dont (I think) really cause what youre feeling. Taking the belt off and running it could solve that one.

 

..yrs ago I had a balance come apart. I did'nt figure it out for a couple days though...
There was no telltale sign, no vibration, no hints.
Just this weird dingging kinda noise that I'd hear when I shut down...

I'd jump out and run up there to open the hood and it would be gone by then.
I'd start the car and watch and could'nt hear anything becasue of the normal engine noise...this was a high pitch whine ding sound..

next day its doing it again. Can;t figure out what can possibly be doing this...
I finally get someone to help and shut it off while I have my face down there in the front of the engine....and see the damn balancer continue to spin for a good 20 seconds after the engine stoped !
Such a goofy thing that I had to laugh about it. But that noise nearly made me nutty..

Just looked at this one last week and see the edges of the rubber seal chipped and broken away. Looks like another Sat afternoon project coming up !

 

Ok, I installed the new smog pump and the bracket is tight. Seems to have taken care of the A/C noise.

I tried to check my timing, but I think my balancer is screwed. The only mark I saw would put it at at least 10 degree retarded, with the ESC wire disconnected. Also, I think it is back too far.

Here are two photos, let me know what you think.





Shouldn't it be in front of the timing scale?

 

Replaced the balancer. Still has the vibration. On the list still to do is the injectors, and I've since discovered the precats are making a lot of rattling noise. I'm going to go with a front Y with no precats and a new hi-flow main cat. Maybe one of the precats is clogged causing the high frequency vibration/noise.

Unfortunately the plugs with oil fouling, #2 and #4, have returned. I've put hotter plugs in for now, I'm going to replace the intake manifold gasket when I replace the injectors and hope for the best. If no improvement, it will be rings in my future.