Idle Vibration from Hell

[SacSantorin]

So i've been hunting for the cause of a rough idle for over a year now. I have a B6 A4 1.8 with apr 1+ that is driving me completely crazy looking for the cause of this rough idle. I've replaced all the check valves, replaced most of the crankcase vent system, vacuum lines, coil packs, spark plugs, combi valve, pancake valve, and probably more that I'm forgetting. The rough idle isn't really that bad, more annoying then anything. Basically when the car is in idle the engine shakes the entire car, more of a vibration than a shake. The vacuum will sit around 16 in, but will drop to 23 in when at speed and not on the throttle. My B5 sits at 20 at idle and 23 at speed which makes me think there is a vacuum leak but I cant for the life of me find it. I've tried boost leak testing it with a air compressor, and sprayed brake cleaner when the engine is running to see if it revs up both of which returned zero results. Any thoughts here guys?

[diagnosticator]

Tip auto or manual trans?

[Sauci]

motor mounts?

[SacSantorin]

Its a TIP. I checked the motor mounts and they seem fine. I don't see how those could effect the idle vacuum though?

[surfer1037]

Hate to say it, but I was just in the same boat. After chasing a "vacuum" leak for about a year. Finally did a compression test. Had bent valves. Just dropped a new motor in last week with all my old vacuum lines and plumbing. Runs perfect. Smooth as butter finally.

[Charles.waite]

Does it vibrate in park or neutral too?

[Kevin C]

Possible causes, either the engine is running rough or the mounting system is transmitting normal 4 cyl vibrations to the chassis. Four cylinder engines have a natural second order vibration that and only be canceled using ballance shafts running 2X engine speed. They depend on having compliant mounts to keep the vibration from the chassis.

An easy start is to do a quick visual of your mounts to make sure there is nothing obviously bad ( leaking fluid, snub mount is centred). Still. it sounds like the engine is running rough. Two places to start, one is to see if there are any stored codes, the other is run a compression test to make sure the engine is basically sound. When the plugs are out, you can check to see if any have excess deposits. A vacuum leak should get the ECE to try and add more fuel. Trim values should be off. On my B5, a cracked vacuum line to the fuel pressure regulator did not cause a big air leak, but it sure did mess up the fuel trim and cause it to run rough.

[Charles.waite]

Someone correct me it I'm wrong but isn't 16inhg good for a Tiptronic?

[customa4]

Maybe the suction jet pump.

No, Charles, don't think it's normal even for a tiptronic.

[SacSantorin]

It vibrates anytime the rpms drop below 1k. If i give it a little gas the vibrations go away. The car drives perfectly other than that idle vibration. It vibrates in neutral and park as well, but not as noticeable as when its in drive.

[Charles.waite]

It vibrates anytime the rpms drop below 1k. If i give it a little gas the vibrations go away. The car drives perfectly other than that idle vibration. It vibrates in neutral and park as well, but not as noticeable as when its in drive.

My guess is motor mounts. Have you checked those?

[SacSantorin]

Ya i checked them by having someone else rev the engine and watched for excessive movement and saw barely any other suggestions for a better way to check them? I'm usually all for just throwing in some new mounts for good measure, but I have thrown so much money at this problem with zero results.

[MurkyRivers_A4]

Ya i checked them by having someone else rev the engine and watched for excessive movement and saw barely any other suggestions for a better way to check them? I'm usually all for just throwing in some new mounts for good measure, but I have thrown so much money at this problem with zero results.

Best way to check mounts, that I have done, is put the car in gear, place foot on brake as well as have ebrake engaged and rev the engine slightly. Youll definitely see movement then and an excessive amount if the mounts are bad.

[chad99]

What's the vacuum like in P or N with AC off and feet off brakes (should be same as a manual car 20, 21), 16 in D with AC on is fine

[SacSantorin]

What's the vacuum like in P or N with AC off and feet off brakes (should be same as a manual car 20, 21), 16 in D with AC on is fine

Its about 16. in drive with AC takes it to around 12.

[chad99]

Ok then yea your right you have a leak if you can only reach a best of 16 in P or D with AC off.

Have you checked the brake booster line right before it enters rain tray? It's only held there by some kind of glue at the joint where it goes from plastic to rubber hose. Mine worked loose awhile ago, I had to push it back in and put a hose clamp on it with some RTV for good measure.

- - - Updated - - -

Sorry meant P or N

[SacSantorin]

huh, Ill check that line out. I was actually thinking the brakes felt a bit spongy the other day, I wonder if that's the problem. I chalked it up to being the wifes car and not driving it very often. Just figured my B5's brakes had a different feel to them. Could just be wishful thinking as well.

[diagnosticator]

Adjustment of the front snub mount is critical on the tip trans A4s. I actually sanded down the diameter of my snub mount to increase the space around the snub in the pocket.

The position of the snub centered in the pocket must be adjusted with the engine idling and in Drive. This provides for a centered snub with the reaction torque of the stalled torque converter loading the engine mounts. If the snub is adjusted with the engine OFF, the snub will touch the pocket with the trans in Drive transmitting a lot of vibration into the body. On my Avant, with the stock engine mounts with 125K miles, when the mounts are fully warmed up, there is very little vibration in Drive. None in neutral.

Besides the engine mounts, and snub mount, the idle speed is very sensitive to minimizing idle vibration. With VCDS, adaptation channel 001, you can raise and lower idle speed in 10 RPM increments to find the sweet spot idle speed for your A4 in Drive. If the adjustment wont take using VCDS then you will need to use Unisettings for ME7 from Unitronics.

[surfer1037]

I'd suggest a compression test if you still can't find a vacuum leak


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