KYNZ

Anyway, regarding my ongoing quest to isolate an annoying low rpm (2000-2200) vibration that i get in 4th and 5th gear at 60 and 72mph, and a rough idle...ive tried about everything there is to fix it...plugs, checked CPs, new Power output stages (fixed my nasty misfires) new rear motor mounts, new tranny mounts, new DRIVESHAFT!!!, NOTHING fixed it. Well, i was surfing the Tyresmoke.net A8 forum last night and came across a dude that had the same prob at low rpms and he ultimately found that it was a bad ignition lead to cylinder 3! We (my mechanic, my dad and I) havent checked into that possibility yet. That being said, and hoping that it IS the problem...how would one go about checking something like that? Do i need VAG-COM? would it throw a code...cus currently i have none pending and the car runs well other than that annoying vibrating (similar feeling to light rumble strips). HELP.

jfrahm

a little power braking might help to load the motor. You can also disconnect the HT leads one at a time (engine off) then restart the engine and see if there is one that has less of an effect on the rough idle.

If the HT leads are old you might just replace them, unless you are worried you have a broken valvespring or something. Have you checked the plugs and done a compression test?

-Joel.

A8CT

I know you're correlating the vibration to a specific RPM but it might just be the speed that you are feeling it kick in.

Easy test, drive it somewhere (down a hill) where you can coast at that speed/RPM and throw it into Neutral. If you still feel the vibration than you'll know it's not engine/RPM related.

I've been chasing my own vibration and finally found the root cause as a wheel bearing. Remove the wheel, caliper and rotor and then check the hub for play.

I changed my bearing last week which eliminated the play but my hub is slightly bent so I'm going to replace that next week. That's due to hitting a curb so your hub is probably fine. Another thing to rule out if you're running out of things to check.

Bob

marcodant

the car and then align it again? I have seen special tools to remove the bearings without taken everything off so no need to align after words. Are you located in CT? thanks

A8CT

and get it pressed in/out. The tool works but it doesn't look like it would be much more work to take the arm off and have the bearing work done at a shop. You'll still save the bulk of the labor.

I'll post some pics so you guys have an idea what's involved.

Bob

yes, I'm in CT

marcodant

I would love to see the pics.. Maybe when I am ready you could help and bring your tool. I am on the NY CT border.

KYNZ

Jfrahm - where do i look? Also, would this "bad ignition lead" cause a vibration ALL THE TIME or just at the above mentioned rpms. i WAS having misfires left and right but the power output stages were replaced and i havent thrown a code since then. i tried pulling the cables off the CPs and they all seemed to make the engine run slightly rougher...as it should. The plugs are maybe 2months old. would a bad lead cause a code? I may have to go the wheel bearing route for the 2K rpm vibration, although it is very specific to the 2K rpm range (under acceleration ONLY...not coasting). thoughts?

KMU

If before, double check plugs for fouling/carbon.

jfrahm

If you are getting arcing, just look under the hood in the dark. The 928 guys use this method to detect bad HT leads all the time.

I had some questionable leads on my 928 and it did idle rough but seemed to clear up under load. I never looked for arcing in the dark, the age of the leads (18 years, 100K miles) was enough for me to want to replace them.

-Joel.

sportQ

I have the exact same vibration you describe. You put the gear selector in into drive and the rpms drop a bit and the car shutters. Otherwise, the car idles fine. I think it was Tozo that said the ignition changes when the selector is placed into drive. I'm going to throw some plugs at it just in case.

Also, when driving along at certain speeds the car will shake and vibrate, almost like it is in a gear to high. And, it takes a lot of throttle input to get a gear kickdown.

I am not experiencing any hunting, but I do get the occasional hard or lazy shift.

So, when I put all of this together, I'm beginning to think sensor/TC issue. No codes, though.