2007 Audi S6 - Low rpm misfire/vibration

A6 All/ S6 All
#1

Hi all,

Long time lurker, first time poster here.

After having it sit for almost 2 years, I started doing some work to my S6 to get it back on the road this summer. The vast amount of information available here helped me keep my sanity. Before I had parked it, it had an overheating issue and oil leak. Other than that, car ran well all throughout the rev range with no other issues.

When I first started it this summer to move it from storage into my garage, there was a horrible vibration. It was especially bad at idle and driving under 2500 RPM and seemed to almost disappear above that. I replaced every seal/gasket/o-ring I could get my hands on, which fixed the leak. Replacing all 10 fuel injectors eliminated some of the vibration and fixed the overheating issue (was not expecting that). After that, I replaced the oil separator, spark plugs, transmission fluid, serpentine belt, and tires and did a carbon clean. I also saw that the flap in the intake manifold for cylinder 10 had broken. All the others were fine and moved normally.

I checked it out with VAGCOM (don’t have the log handy at the moment) and I was getting intermittent misfires across all 10 cylinders, along with a random misfire code. I had a code come up for knock sensor 1, but after I cleared it, it never came back. On cold startup, cylinder 1, 5, 9 and 10 misfired about 50 times total in a minute. When driving under 2500 RPM, VAGCOM occasionally showed a misfire on a random cylinder, but the vibration was there the whole time.

I picked up another IM and just finished locking the flaps down with epoxy yesterday, planning to install it this evening. However, I haven’t see any mention of broken flaps causing any engine vibration in these cars, so not sure this will even make a difference. Just not sure what else to check at this point. Also thought it could be the engine mounts, but I didn’t think it would cause misfires. Anyone ever encountered a similar issue, or notice if I missed checking out or replacing something?

#2

Put back on the newly fixed intake. From there check to make sure there are no vac leaks. One of the easy ways to see this is to watch your fuel trim data. Also while you have the intake off. water test all of the PCV hoses and lines for cracks and leaks.

PCV cracks and leaks can wreak havoc on the car. For the most part a lot of low rpm missfires can be from vac leaks.

#3

That reminded me, I did have a vacuum leak when I first got the car. It was the line running from one side of the engine to the other, just above the crank. Car looked like an excited dog that couldn’t stop wagging its tail. Will check out the lines once I have the IM off tonight, thanks!

#4

I swapped the IM yesterday, definitely made a difference. All the PCV hoses checked out fine, too. Vibration has almost disappeared when stationary in drive, but I can occasionally hear and feel the light pops of a misfire. It’s worse in park and neutral, to the point where the car does shake, but overall a step forward.

While moving, in between 1900-3000 RPM, vibration is still bad and worst at 2500. Below 1900, it’s still there but not too intrusive. Above 3000, it’s almost unnoticeable (I only notice it because I’m focusing on it).

Gonna be checking it out with VCDS later today, I’ll check the short term fuel trim on both banks and see if there’s anything wacky there, along with the expected list of codes.

#5

When you say you can feel this between 2000-3000rpm can that be felt in N or just in D when driving?

#6

I have the exact same issue! Will be interested to see what the solution is. I have no codes in VCDS. I will check my fuel trims as well. I have smoke tested my system and found no vacuum leaks.

#7

I can feel the vibration in park, neutral, and drive while stationary. Much worse in neutral and park than it is in drive. When I’m in the 1900-3000 rev range, vibration has the same pattern no matter what gear I’m in. Builds up as I get to 2500 and then starts dying down as I approach 3000. From 1000-1900, it’s just a constant low vibration.

Today I also found that on a cold start, it’s especially bad when reversing. Feels like the car wants to lurch forward every half second. No issues reversing when it’s at operating temperature. No issues with the transmission shifting and no codes there either.

VCDS showed short term fuel trim while idling was mostly in the +0-5% range, but would randomly spike between +10-20% on both banks for a few seconds before going back down. Long term fuel trim was 2.7%. I’m also getting a code for random/multiple misfire and misfires across all cylinders. Just to be sure, I tested all the ignition coils (which I never replaced) in a friend’s 4.2 A6 and it ran perfectly fine. Gonna try getting it in for a vacuum line smoke test.

Also, is it normal that the engine won’t go past 6000 RPM while driving once the flaps have been locked? I did get a flashing CEL once I approached that range. I don’t recall anything mentioning that when I was reading up about locking the flaps, so maybe it’s related to whatever the issue here is?

Thanks for all your input so far, I appreciate it!

#8

Feels better knowing I’m not alone! Do you also have busted flaps in the IM or is yours all good?

#9

It is not normal to have the motor not go past 6000rpm. This would suggest you’re in some sort of limp mode. But that limp mode would be present if your car had to auto out of missfires. Meaning that you had so many missfires the counter reset.

Open vag com and log to see if the vibration is from missfires. I would take that on as the next step

#10

Well i’ve searched a fair amount of time after a similar issue…
Stationairy misfires over all cilinders except for one… car ran great otherwise…
CEL once every two weeks for returning misfires… fuel trims in VCDS where good, no vacuum leaks visible when smoke testing… new injectors and coils and coil packs, you get the idea…

For me after all it was a cracked line more specific the line under the PCV back to the engine… after i changed this the car ran great again.

#11

Yes I can’t stress enough that the tricky PCV system can have hair line cracks in it and thats not something that is going to show up on most smoke tests.

One thing to add to the tests is unplug the MAF and see if that helps. But I would check for missfires that would most likely be the vibration.

Also the crank pulley can start to fail and fall apart. That too might be worth checking.

#12

Wouldn’t vacuum leaks that don’t show up through a smoke test still affect the fuel trims?

And for check the MAFs, do you just unplug both of them, go for a ride, and log misfires again? Or what are you specifically looking for with the MAFs unplugged?

#13

i can’t explain why but the only fault i had was intemittend missfires at idle… perfect fuel trims nothing unusual…

#14

What is the best way to check for these hairline cracks? I have looked over the hoses and not found anything previously by just looking at them, but is there a better way?

#15

Just to expand on this issue:
Before the IM swap, it would go past 6000 without a problem. Using the IM with locked flaps, as the engine reaches around 5700 RPM and climbs to 6000, it’s similar to gradually bending a hose to stop the flow of water. The climb from 5700 to 6000 is slow and then it just runs out of breath. I may try swapping the old IM back on to see if that changes it.

First, we checked misfires with RPM and took it for a 15 minute drive. While idling at around 600 RPM, occasionally will get 1 misfire. Same thing staying under 2200 RPM. Normally accelerating from 2000-3000 RPM, I saw around 25-35 misfires. Beyond 3000 RPM, counter barely goes up. The party is in the 2500-2600 range. When staying in that range, the counter climbs by hundreds in a couple of seconds, every time. It’s also where the vibration is the worst.

We then went for another drive and checked misfires according to cylinders. Not much under 2200 and above 3000 RPM on any of the cylinders. Going from 2200 to 3000, cylinders 2, 6, 7, 8, 9, and 10 showed 1 or 2 misfires on the counters. Cylinders 1, 3, 4, and 5 showed between 3-10. The number of misfires on 1, 3, 4 and 5 rapidly grew between the 2500-2600 range.

The CEL did not flash at any point during the drive. Didn’t take it beyond 5000 RPM, but that’s the only spot where the CEL does flash. I might just pick up a cigar or something to do a vacuum leak test today, I’m becoming more and more convinced that’s where the issue is.

#16

Best way is changing them :flushed::man_facepalming:

#17

@dzow I dont like changing parts just for the sake of it, so will look into some other ways to test for leaks. That main PCV hose isnt cheap!

@GreetingsWench It is somewhat comforting to find someone who describes the exact same issues I am dealing with. Hopefully we can get to the bottom of this. I have a gutted intake manifold, so mine is in the short runner position. I can say that I have no issues with revving past 6K RPM. Mine takes off around there as that is around where the intake would go to the short runner and pulls all the way to redline. The one thing in common is that we both have IMs that have been messed around with.

One thing another member had mentioned that I havent got around to yet is looking at adapting the throttle bodies through VCDS? I have just received my cable to will look into how this is done this weekend. Apparently they might need this adaptation if they have been roughed up a bit in the IM removal/install. I am not getting my hopes up that that is the issue, but worth checking.

#18

I disconnected the MAFs and car ran pretty much exactly the same. I also did a throttle body adaptation and it made no difference (@spencerprince car on, engine off, in VCDS go to Engine > Basic Settings > Group 060 > click ON and wait about a minute, then click OFF).

I did hear a loud hiss coming from the bank 1 side. I pinched the hose circled in the photo and the noise stopped. Did not change the idle or sound of the engine. No idea if the sound is normal, can’t recall if it’s always made that noise. I took the line coming from the brake vacuum pump off to check if it was dirty or broken but it looked fine.

IMG_20210715_181404~2
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I then took the oil cap off. There was a massive amount of suction keeping it down and engine began to stutter once it was removed. I did the same on a Q5 with a 3.2L V6 (no codes, car is in perfect running order). Did not feel any suction and there was no stutter with the cap removed. I put my old oil separator on and it was exactly the same. I’ve seen people on both sides of this, saying there should be lots of suction or there should be very little suction. I assumed there should be suction, but then I’ve got no idea why the other car has no suction.

#19

Lots going on so here we go.

Best way to check PCV parts is to water test. submerge the part into water. Usually you can use an oil drain tray or pan. Submerge the part cap off one end and feed air into the other end (low pressure) don’t go crazy. You should see bubbles if there is a crack or leak.

Check the drain line going to the oil pan as well. Also make sure the oil separator diaphragm is good. If you feel really manly you can take it apart and see the diaphragm itself.

Always check fuel trims when this is happening.

#20

The drain line is the one that was cracked on mine… cheap part that costed me a lot just to find it… if i would do it again i would immediately change it with the PCV…