I bought my 2003 A4 Avant 1.8T last year. It's been running great until recently my wife said the engine light came on and it ran rough for a minute then cleared up. I checked the codes and found the random miss & cyl 2/4.
I started driving it to see if I could repeat the problem, as it's happened to her 3-4 times. I drive a long distance to work each day, and never experienced any problem until today, when I got stuck at a rail crossing for 5 minutes. When I started to pull away, "chug chug" and the engine light came on. It cleared in a minute and ran fine again.
I checked the engine today. Listened for vacuum leaks, looked for burnt wires, etc. don't see anything obvious.
It looks like the prev owner replaced the coil packs already with STI's. I pulled the spark plugs and they had oil around the threads and I can see a little oil down around the thread holes.
I replaced the spark plugs with new NGKs but I doubt that's the problem, heck it was cheap though. They nice old timer at Knechts said he suspected a sensor, since it only happened intermittently, and seemingly after idle for a while. I don't know what sensors to check. I saw someone mention a PCV valve, but I don't think this engine has one (or at least they don't call it that).
Any ideas? I hate to take it to the shop since the problem is so intermittent, they won't likely see the problem and will just replace stuff like I'm about to do! I'm hesitant to use Seafoam because the car has 80k miles, and I don't really want to create oil leaks in the seals (but maybe they already leak, in which case...)
Your Audi's ECU (engine control unit) records all fault conditions into various data logs, so even intermittent fault events can be reviewed and diagnosed after-the-fact. You need to find a properly-equipped independent service shop with a VAG diagnostic scanner system (or a helpful local Audi Forum member who owns one), which can be used to download the stored fault codes; these fault codes can be interpreted to determine the conditions that are producing your engine fault events.
I have a reader and it shows some of the freeze data info, however I don't really know how to interpret it.
I changed the sparkplugs this weekend and checked the cables, hoses, etc. I doubted the plugs were at fault, but it was cheap and they plugs were old anyways. My wife drove it yesterday and said the light came on and it shook and lost power, but I checked for codes, and none were recorded. I'm waiting for the next error and I'll post the results.
Well, I got lucky and the car started misfiring when I was moving it in the driveway! The misfire was cylinder 2 again, so I swapped the coil packs between 1&2 and the problem moved to #1. So I guess the coil packs really did go bad. Seems odd that they fail intermittently, maybe from heat?
Anyhow I replaced all 4 and hopefully that'll do it. If not, I'll post to let folks know.
Once the coil-packs become sufficiently aged, the failure of any single coil-pack seems to stress-out the remaining coil-packs, resulting in a cascade failure if you don't replace the coil-packs as a set. In the early phases of "coil-pack death", the faults can be intermittent in nature; however, once they start to "go bad", it's just a matter of time.
When looking for replacement coil-packs, please note that the brand of coil-packs can make a significant difference with regards to long-term coil-pack reliability and durability, so search the forum threads for advice on this matter. As a 3.0L owner, I've had excellent reliability with the OEM coil-packs; my car regularly gets 50K-70K miles from a set of OEM coil-packs, so it's only an infrequent pain-in-the-butt when I experience that first coil-pack misfire...
This condition is a know shortcoming of the Audi ignition system, but Audi has been very reluctant to take responsibility for addressing/correcting the situation, so most of us just keep a fresh set of coil-packs and the appropriate tools in the trunk... ;-)
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