Odd sequence of events that I'm having trouble sorting out. I have a 2003 B6 3.0 quattro, bone stock, 115,000 miles. This is a commuter car, so no hard driving in it either.

A couple weeks ago on the commute home, my engine started acting like it misfired. I got the blinking CEL, rough engine etc, so I pulled over (I was running about 75mph when the symptoms started). By the time I got to the side of the road, it was running fine again...drove home, no issue. The same thing happens sporadically...no real pattern to it that I can see (various conditions, speeds, etc). I assumed a failing coil, so I replaced all 6, but the issue still happens...but each time goes away and clears the code before very long.

A couple nights ago, I'm idling at a drive through and the same problem comes back...same thing, gone again after a minute or so. Today, the issue pops up right when I start the car...but again clears. I drive a couple miles to a gas station and when I pop the fuel cap, I hear air escaping...tons of back pressure in the tank. I literally had to hold the flap open for 5 minutes before enough air had escaped to accept fuel. Strange...but again the car starts fine. It runs with no problems for the 85 mile drive home...then 2 miles from home, again the misfire and gone.

I fired up the car this morning and everything seemed fine, so I attempted to drive it. Made it 35 miles (long commute) and the misfire started happening again. I pulled over immediately, checked the gas tank (note: full tank this time, last time was near empty), no air pressure issue. I popped the hood and could hear a hissing as though air pressure was being released. it sound like it was coming from the upper left side of the engine bay...just left of the battery. The hissing slowed and stopped after a minute or so. I fired the car up again and it ran just fine for the whole 35 mile trip back home.

I've searched around a bit without much luck on a possible solution. I don't have a VAG Com and the nearest dealer is 60 miles away. Any ideas?

 

sOunds like it could be a vac leak if u hear a hissing in the engine. Might want to check those lines.

 

I'll take a look at those lines. Any idea which lines run in that area of the engine bay?

 

hmm not to sure as i have a 1.8t but if u hear a hissing noise its gotta be a vac leak. Maybe spray some cleaner on it and see if u can pinpoint it.

 

if the sound is related to a vacuum leak you can sometimes find the leaky hose/ clamp / check valve by taking a short piece (2 feet) of small hose or plastic pipe to your ear and passing the other end around the area you suspect. you can somtimes locate the source of the noise.

BE CAREFUL!

 

If it is vacuum leak, might as well go and replace as many as you can. The rubbers go out overtime/use so if one is going, the others will follow too.
Also Valve cover gasket leaks will act the same, but they don't hiss on idle with little pressure. Your spark plugs maybe fouled up from rich conditions. Should replace them as well.