jayk_S4

I've been trying to fix a misfire problem on my 2000 S4 for the last couple weeks, today it mysteriously started working again but I'm not sure what ended up being the real culprit. Here is the sequence of events :

1) Noticed lumpy idle, check engine light on
2) Took it to dealership, they said the diverter valves needed to be replaced for $400
3) Picked up car from dealer and replaced diverter valves myself
4) Problem still exists
5) Replaced all six spark plugs, cylinder 4 plug was darker than the others
6) Problem still exists
5) Bought vag-com and pulled codes, found misfires on multiple cylinders and bad b2s1 O2 codes
6) Replaced both pre-cat O2 sensors with vw o2 sensors
7) Problem still exists (getting really pissed off now)
8) Logged blocks 14, 15, and 16 using VAG and noticed that cylinder 4 was misfiring almost constantly
9) Switched coil-pack between cylinder 4 and 6
10) Problem still exists on cylinder 4 (about to give up)
11) Read some threads on here about misfires
12) Switched POSM chips
13) Misfire switched to cylinder 1
14) Started driving home (planning to buy replacement POSM chips) and the car started making this horrible rotten egg smell, I swear people were driving 2 lanes away from me and people walking were holding their noses
15) Fifteen miles and hundreds of misfires later the car mysteriously smoothed out, no misfires
16) Cleared codes, check-engine light went away
17) Test drive showed no misfires, no check-engine light, and the car felt better than before(no doubt due to the $400 in unneeded parts)


I thought it may have been a bad POSM, but everything seems fine now, so my admittedly completely uneducated theory is that some bad gas burned out or possibly my cat is going bad and was clogged but burned out once cylinder 4 started firing again.

So any ideas what could have been the real culprit? And should I be worried about it happening again? Thanks for any input...

silva

would come and go every so often. It would usually act up when the weather was very hot. You can try removing them and place some dielectric grease under them, so that they will have a good contact with the heat sink below. Good contact is essential so that they do not overheat.
Check the pins on either end of the POSMs and their respective plugs. If you see any traces of corrosion, clean it up and put some dielectric grease in those.

S4_Cinci (I S'edTFA)

Forgive my ignorance, but what is a POSM? TIA!

Kyle K: TampaS4

That's what I want to know.. They have abbreiviations for everything here

jayk_S4

Although, before yesterday I had no idea what a POSM was either so I have no idea what they do. The POSM's are the two chips bolted to the top of the airbox, each has a 4 pin and a 3pin connector. Apparently the output of each is connected to a set of plugs, I thought it was 1,2,3 and 4,5,6, but not sure anymore.

jayk_S4

I'm pretty sure at some point I heard a rotten egg smell is a sign of a dying catalytic converter.

Kyle K: TampaS4

yeah, and those are expensive.

jayk_S4

If anybody reads this, it turned out to be one of my power output stage modules (otherwise known as Ignition control modules I think).

I ordered a new one from rockparts.com for $193 and bought some thermal compound (or grease) from Compusa, installed the chip, and all the misfires are gone!