rreinoso

I have been battling a problem with my 2005 A6 for over a year. The car idle is rough and the engine does not run smooth at light load, like cruising at constant speed on the highway. Acceleration is just fine (no hesitation there). At idle, you can hear the throttle opening and closing (clicks and clacks under the hood) while the engine shakes. Also, every time I open the driver's door, I can hear the throttle plate running the adaptation procedure. During the winter, the problem disappears but it comes back when the weather warms up again. I fixed the vacuum hoses, clean the throttle body, ran adaptation procedure. Sometimes the car runs very smooth, and sometimes is just shaky at idle and when cruising at constant speed.

I have seen two or three posting of other people having the same problem but no one has posted a solution to it yet. Has anyone had this problem and was able to fix it ?

KevinGary

If it is the 3.2 I would try a carbon cleaning

ProtoFly

You don't mention many specifics on your car (what engine, etc). Normally, misfires will be caught by the on board diagnostics, and would help to show at least what is happening.

My 4.2 developed a miss on cylinders 1,2,3,4 very suddenly. The symptoms are similar to what you are describing, but what you don't mention is if the check engine light is on or not. Mine is flashing.

What I finally have diagnosed is that a timing chain tensioner/guide has failed, which is throwing cam timing off on one bank to cause the idle/small throttle miss. I'm in the process of pulling the engine now. I think that even if I didn't suspect the timing chain system, I'd be pulling the engine to reseal it, as it has been seeping oil from the valve covers and timing chain covers.

If you have VCDS or other code reader, can you post what you find? Also some details on your car?

-Darryl
2005 A6 4.2, 176k miles

rreinoso

================================================== ===========
Thanks all for replying to my posting. For now, it seems that I was finally able to fix the problem. The computer codes did not indicate anything bad. I guessed that the problem was with the throttle mechanism so in an act of desperation, I took the thing apart. I had cleaned it before so it was not dirty. I noticed though that it was sticky, particularly upon closing. The throttle plate's resting position is slightly open, and when pushed to close, it was sticky. So I lubricated the hinges using a Dupont lubricant I picked up at Home Depot, exercise it a little, pushed the sensor contacts down a little (very delicate stuff), and put it all back together. Then I ran an throttle adaptation procedure and the car has been smooth ever since (a few weeks now). My gas mileage also improved dramatically (it could've been helped by the change in gas formula from winter to summer), from 360 to 380 miles per tank, to 430 to 450 miles per tank.

I can't recommend this procedure since I don't know if it has any long term consequences and things can break. I was ready to buy a new throttle assembly just in case. This worked for me, for now.

Now on to the brake light and turn signal LEDs in the rear. The rear passenger stops working if the tail lights are on. I noticed that the plastic divider between on of the tail light bulbs and the LED circuit board has crumbled so the LED circuit board is exposed to the tail light bulb. With time, I guess that the light bulb cooked the LED circuit board. I tried covering the board to protect it, but too late. The board has been damaged. I tested the LEDs on the bench and it can't stay on for more than 30 seconds. So, I am replacing the whole lamp.

wtmmm

rreinoso, did cleaning your throttle body ultimately fix your problem with the "surging" at light load?

I had the same problem on my '05 A6 with 104k. I didn't own a VAG-COm at the time so I don't know what codes were showing, if any. The throttle body was first cleaned and that seemed to fix it but the problem came back within a week. Then I finally decided to pay the dealer $1,300 to de-carbon the valves. That seems to have finally fixed the problem.

Now I'm dealing with a separate issue with the engine misfiring and giving two codes: low pressure fuel sensor and misfire cyl 4. (see separate misnamed thread: "Transmission problem '05 A6"). I wonder if the valve cleaning might have caused this.

Wake96c4

Not sure if this helps you but my 4.2L had a similar problem to what you describe. The dealer took a couple days to find it and it ended up being the fuel pressure regulator. I was surprised to find out that the regulator was built into the fuel filter. They simply changed the fuel filter and I haven't had a problem in the last almost 1000 miles.

What they told me was that the fuel pressure was bleeding down within 10 mins of shutdown. When I'd start the car hours later it would buck and miss with the misfire code and the EPC light. It would stall but immediately restart then run smooth until I shut it down again and let it sit.

wtmmm

Interesting. I can't recall the last time we replaced the fuel filter so it is probably time to do that too.

rreinoso

Yes. My 2005 A6 3.2L continues running smooth. And I believe it was not only the cleaning that solved the problem. I had cleaned the throttle assembly before and after the adaptation, the car ran smooth for a few weeks and then the problem re-surfaced again. I cleaned it twice and same issue. What did the job was the lubrication of the throttle place hinges and the exercise to close and let it open until it did not stick anymore. The throttle place at rest is not fully closed, so when I pushed it closed, it was getting stuck in the closed position. So I lubricated and exercised it until it open freely after pushing it closed. My theory is that at idle or light load, when the throttle plate closed, it was getting stuck, then the computer had to put extra torque to get it open, then opened too much, then it closed it, got stuck, and the cycle repeated, creating an rough idle (or cruising at constant speed). I am good so far ...

wtmmm

Thanks for the update. All of the other posts point to the TC and some have even replaced/repaired the TC and solved the problem. I will take your suggestion and clean the throttle body again but also lubricate the hinges.

wtmmm

We thoroughly cleaned AND lubed the hinges but the problem has not gone away. The car still surges at low rpm and low load. The surging goes away completely during acceleration, idle and when coasting.

I sent some scans to Ross Tech and they pointed out that the duty cycle of the fuel lift pump is high so they think it may be one of the following:
-Restricted fuel line
-Restricted fuel filter
-Failed left pump
-Failed fuel sensor