Car (2008 A6 4.2 FSI BVJ) has been running rough for a while, exhaust sounds lopey (aftermarket) and it’s a little shaky while idling. Power doesn’t seem as strong as it used to be either. Was chasing misfire codes for the longest time, been in and out of the stealership a few times. Had a flashing CEL light under hard acceleration sometimes, got codes for random misfire and specific ones for cylinder 1 mostly, once and a while cylinder 2. Have replaced all 8 plugs, coils and even changed the injectors. After plugs and coils the light came on once in a while and after I changed the injectors I can’t get the light to come on, but the car still runs rough. Not getting any codes at the moment but after thumbing through some forums I see that the PCV or pressure relief valve or oil separator (whatever the preferred term is) can possibly cause misfires or misfire behavior. My question is how is it specifically tested? I see a lot of posts about turbocharged engines and a couple NA but I’m still a little fuzzy. I feel like mine may be bad per the videos, but there’s so many contradictory viewpoints out there.

With the car idling I hear the lope in the exhaust, then I unscrew the oil cap (turns hard) and lift. I gotta pull pretty hard to get it off and once it’s removed the car starts running really rough an stumbling bad until I put the cap back on, then it smooths out a bit, but not as smooth as these usually run. One post said something about placing a piece of paper or cardboard over the oil filler hole and if it flutters it’s ok, but if it blows it upward or sucks it down, something is wrong. When I place the piece of paper over the filler hole, it almost sucks it right into the cam cover. Wondering if I’m on the right track because when the cap is off, it just sounds like a more pronounced version of how the car already runs poorly. Anyone else had these types of symptoms? Or does anyone have a 4.2 or 3.2 FSI that can try the experiment I described and see if they’re getting similar or different results? The dealer just charges me a diagnostic fee and swaps around coil packs and plugs each time and tell me they can’t figure out what’s wrong. Been in multiple times and they have replaced the separator plate gasket (leaking), valve cover gaskets (leaking down by spark plugs), oil filter housing gasket (leaking) and an intake port/valve cleaning. Car still runs funny but they’ve ruled out timing chains and components. I myself have replaced the plugs, coils, injectors and at one time the HPFPs (misfire with EPC light and would not go over 3K RPM) and the battery, all OEM Audi parts. The dealership is not that good and that’s a common opinion shared around town. If anyone has any input I’d greatly appreciate it. I have VCDS and a pretty high mechanical skill set but mainly with slightly older domestic vehicles. Just trying to make my my buddy purr like she used to, but these cars can get complicated. Sorry fo the long winded posts, I just try to be thorough so hopefully you don’t have to ask too many questions. Thanks again.

Thanks Larry, I appreciate your input. No I’m not hearing any other noises coming from the bay that I can tell, such as the whistling that I hear in these YouTube videos. The only thing I really hear is an Audi engine that doesn’t run like a watch. With the exhaust I have all I hear is like a dead air pop or flutter coming from the tips. When I pull the oil cap, the sound gets much more pronounced and it sounds like an old muscle car with a huge cam installed. With the car idling normally with the cap on you can feel an intermittent stumble in the idle by putting a hand on the car, whereas you used to be able to set a glass of water on the engine and it wouldn’t have a ripple. As I said earlier I have no current or pending engine codes through VCDS so I don’t have a direction to point towards. I know there’s an indy shop about 30 miles away I’ve been recommended to, but again I’m afraid I’ll get the “no codes, no problems” response like the dealer has given me. I’m going to mess with it in a bit so I’ll see if I can take a quick video of how the car is behaving, plus go to the dealer and price out that part. From what I’ve seen it’s over $300 so I’d hate to order it and not be the culprit, but again don’t feel like I have a lot of options. Won’t be the first time I just threw money at the car with no result, but maybe my luck will change.

Thanks again, next time I get a chance to dig my hands in there I’ll try that out. Seems simple enough, just eliminate connected components to see if something gets affected by it. Stopped at the dealership parts department today to price up some of the components and get some schematics, luckily the smartest guy there is a good friend of mine and the parts manager, hooked me up with what I needed plus gave me a check valve that they stock because he says they’ve caused a lot of issues on various Audi / VW models. Simple change, one two wired plug and a vacuum line in and out with a pinch clamp. Changed it out but didn’t seem to do anything. It was right under the beauty cover near the throttle body. He told me $270 for the PRV as well and he’d have to order one in.