Need help with a low frequency vibe.

[3rd Q]

I have a very low frequency vibe on my '90 Q that is noticeable only at a stop with trans in gear, otherwise never hear or feel it, if I shift to park or neutral it goes away. Feels like the exhaust at rear is amplifying the vibration. Don't feel it at idle in park, don't feel it traveling up through the gear shift lever when it is making the noise, but do feel it coming from the exhaust in rear of the car. It sounds just like being next to a diesel rig at a stop light or next to a car with big sub woofers going- a very low frequency throb.

Just came from the trans shop and looked at trans mount and motor mounts which seem OK and I had just replaced exhaust mounts and hangers. Can't see any part of the exhaust touching anything it shouldn't.

Did a search with no results . Any ideas?

Thanks for the help,Paul

[elwesso]

They had a TSB on this... Do you notice the vibration similar at 50MPH in 3rd gear?

[DrewQ45]

A transmission mount can look good till you pull it.

[3rd Q]

Wes,No, I don't feel it or hear it at any speed once underway, only at a stop when there is a load on the drivetrain as it is in gear. My original exhaust had lost that heavy rubber vibe isolator on the rear of the exhaust and even then did not have this vibration. That exhaust system was replaced since it was leaking under a heat shield up front- put the exhaust from my parts car on and guess what- leaks in the same place. I'll weld in a section as a temp fix, maybe I'll be in on Jeff's group buy for exhausts .

Drew,Well, I was thinking of replacing the trans mount, so I think I will just in case.

Weird that it started all of a sudden, and does it pretty consistently now but with varying degrees of vibration. Starts about 10 seconds after I come to a stop with it in gear. Haven't had this on either of the other 2 Qs even though they had 30-70k more miles. Perhaps as Q45tech has said rubber has a life expectancy- may have exceeded its shelf life.

Thanks, guys!Any other ideas welcome, also.

Paul

[Q45tech]

Before changing rubber things make sure engine is smooth by power balance test. The idle speed progressively declines as the ecu has a harder and harder time raising IAC duty cycle and advance.BG44k, and FREQUENT air path cleaning, spark plugs, etc.

The oem recommendation for 60k plug life ONLY applies to the first change at 60k...............as the engine ages it is expected that owners would change more frequently to maintain as new low vibrations.

Old cars are very finicky and require lots of attention and parts replacement why I keep on about a $3,000 [prorated] annual repair and maintenance budget at $75 per shop hour.............Sure is a lot better than $750 per month for a M45 lease [plus maintenance tires/fluids/brakes].

[3rd Q]

Q45tech,I agree- I've put almost $10k in parts (not labor) in this one since I bought it 2 1/2 years ago-

*11 injectors *new transmission*guides*under plenum job*knock sensors*oem spark plugs*water pump*tstat*fuel pump*alternator*AC compressor*upper links*tie rod ends*rack boots*tension bars*FSTB*rear sway bar*new rotors all around*Z32 brakes*battery*Bose rebuilds*NICO ECU*1st gear start TCU*O2 sensors

- and at least a dozen things I can't remember now. Probably need injector number 12 soon. I think I'm over the hump on the bigger ticket items, hopefully this vibration is nothing too expensive- but, yes there is always something to spend money on. All this on a car that had 58,000 legit miles when purchased and was in cherry condition.

I've cleaned the plenum/TB/IAC completely now twice- have had it off 4 times for KSs and all those injector replacements . BG 44k every oil change. So I think the intake path is in pretty good shape. When in park/neutral it idles just about as smoothly as it has at its best, just when it's in gear at a stop with the brake on that the vibe starts. I'm thinking that the drive train under load is moving/compressing a mount enough to transfer vibration to the exhaust system or undercarraige and then the harmonics set in. As the exhaust vibrates, the low frequency vibe grows louder, exhaust note stays the same, just feels like constructive interference with the frequency causes the vibe to grow.

Haven't done a PB test on this one yet, so seems like it might be worthwhile. Have a trans mount on the way too.

Has anyone had anything similar?

Paul

[Q45tech]

Until I found T3 and started using ebay and buying wrecked cars for parts after 1999, I was spoiled by warranty and extended warranty.

That year with $7,000 in work followed by $6,000 taught me to get active and try to cut my costs while actually getting more work done.

For sure my Q feels and drives better now than it did 10 years ago with 60,000 miles.