I just purchased an old 1999 Audi A6 Quattro sedan. Before I can get it plated, it needs to pass an inspection. However it can't pass inspection because the engine light keeps coming on.

The reasons for the engine light are varied, and seemingly random. Sometimes it's "P0300 random misfires" or "P0302 misfires on cylinder 2". Other times it's "P0422 catalyst efficiency down (random)". Once it was "P0411 secondary air injection flow down". Any one of those can come up at any time, and they return in completely random order after I've cleared the MIL.

When the engine is cold and idling (coolant temp below 60c), misfires are clearly evident. The VAG-COM registers a slew of misfires from cylinder 2, and occasional ones in cylinder 4. They can be clearly felt, the engine runs rough. Once the engine is warmed up (80c+), the misfires go away and the engine runs smooth as silk. Very occasionally, I've experienced misfires while driving with the engine warmed up. This has happened twice: I felt a string of misfires while accelerating gently, stepped off the accelerator, stepped back on it, and the misfires were gone.

I've gone through the procedure to test every emissions-related sensor and system with the VAG-COM, and all tests came back as OK. Logging with the VAG-COM while the engine is warm reveals no oddities in the data, everything appears to be exactly to spec. Logging while the engine is cold shows that the fueling on bank 1 is fairly high, and air through the MAF is a bit high too. But this settles out perfectly once the engine warms up.

An injection problem was initially suspected since the misfires are concentrated on cylinder 2 and the fueling was showing high on that side. That injector was swapped with another on the other bank, but the problem remained in cylinder 2.

Then the ignition was suspected, so the plug for cylinder 2 was swapped with another, and the problem remained at cylinder 2. The resistance of the wires and coil packs was checked, and everything came back within spec. Out of desperation, a whole new coil pack was put in, but to no avail.

Yesterday, while the engine was idling warm, I listened very closely near the combi valve and could just barely discern an intermittent hissing sound. It's very irregular, comes and goes for short bursts at a time. To my ear, it sounds like an intermittent vacuum leak, but I'm not very familiar with Audi engines so I don't know if the sound is normal. When I was puttering around back there with a stethoscope, it seemed like I could induce the hissing by poking in a general area. But it's really tight back there and I can't see anything, even with the intake bellows removed. However all lines to and from the combi valve appear to be in perfect shape.

I checked every vacuum line I could reach with a vacuum gauge, but nothing seemed out of order. I'm getting -15 at idle and -20 at higher throttle. However, every vacuum line I can reach appears to be in front of a check valve, so it's possible I'm getting false readings.

After that ultra-long spiel, here are the actual questions:

- is that intermittent hissing sound normal?

- where is the best place to check for vacuum leaks? AllData has very poor documentation on testing the vacuum lines for this engine

- is a vacuum leak a likely culprit for these intermittent and random issues?

- is there anything else I should be looking for with the VAG-COM which will point me in the right direction?

 

I think I may have found the problem after all. I had a heck of a time diagnosing it. The vacuum line that goes to the fuel pressure regulator appears to be badly worn. It was leaking vacuum right where it hooks onto the FPR, because the seal wasn't tight. It was also frequently collapsing altogether from the vacuum. This was causing insufficient vacuum at the FPR, thereby causing the fuel pressure to be at or near maximum at all times, even at idle and part throttle. It was also the cause of the intermittent hissing sound. I temporarily "fixed" it by cutting off the loose end (I had just enough length to spare). Now the engine runs smooth at all times and pulls MUCH stronger at part throttle.

I guess I'll replace all of those old flexible vacuum lines. It's dirt cheap maintenance and very easy to do. I wish I could get rid of most of them like it's possible to do on newer 1.8T's... I plan to do that on my A4.