Engine Trouble. Would love some help. AOS?
 

Pertinent Info:
2001 Boxster S
60,000 miles
Original AOS from what I know, had the car 37,000 miles and I haven't replaced it.

I was driving today, came through a roundabout in a spirited fashion then I had to stop a few hundred yards down the road for a car to turn. The Boxster started to stumble and act quite strange like there was a misfire. I drove another hundred or so yards and it threw a flashing CEL for no more than 10 seconds then it went away completely. Seemed fine when revved but idle is rough and unstable around 700 rpms.

Got the car the less than a mile back home and parked it. Slight smell of fuel or oil, wasn't sure what.

I let the car cool some and restarted. It fired back up no problem, no smoke. Gave it revs in neutral in the garage and all seemed fine. I opened the oil filler cap and it was easy to take off but I heard a vacuum type sound from under the car right when I took it off and the car started to run worse. Then I let it sit back at idle and it started to stumble again and eventually the CEL came back on but not flashing.

I've had a similar experience a few months ago that the car stumbled, but no CEL and it went away that day and hadn't come back. I ran Techron in the fuel for two tanks and it seemed fine. One random day in between I noticed a small but not unusual plume of smoke on startup, but nothing ever while driving.

So here's what I did today, after a search through the posts here I went and pulled the engine cover and looked around. I notice one pipe has a fair bit of oil on it near the AOS, but not sure that's the problem. I pulled the vent tube that connects the AOS to the passenger side intake and there's a good bit of oil in there. It didn't run out but it was more than expected.

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Any thought on what's going on? Since it's Sunday I haven't had a change to get any codes read, but I may try to get to the dealer or at least get the codes pulled tomorrow and see what's what.

I'm thinking the AOS is toast and needs to be replaced, but would love opinions and help.

Step #1 - Read the codes and post them up! :)

I'm not sure about AOS because usually an AOS failure looks like a 007 smoke screen and you didn't report that.

OBD Codes
 

Took the car to Autozone and had them read the codes. I know it's not a Porsche scanner, but here's what was there.

PO300 - Random Misfire Detected
PO303 - Cylinder 3 misfire condition
PO305 - Cylinder 5 misfire condition

They cleared the codes and I drove the 5 miles back home without the CEL coming back, but it still sounds like it's misfiring with the engine at idle. I avoided coasting and kept the revs up when stopped. I'll do some more searching to see what's already been posted on these 3 codes, but please chime in if you can help.

Driving a car with a misfire can wreck your expensive pre-cats. Boxster pre-cats are $1,700 EACH.

Normally P030x codes are misfires caused by ignition problems, and are detected by the crank angle sensor. In other cars I've had them from a dirty MAF sensor wire too. Do you have an oiled air cleaner element?

Thanks for the note on potential damage to the pre-cats.

Everything with this engine is 100% stock. Genuine Porsche air cleaner that was changed 5000 miles ago by me.

This can often be a spark plug or coil issue.

Swap the coil packs from cyls 3 and 5 with the coil packs on the other side (4,6). Then see if you get a CEL for misfires on cyls 4 and 6. If so, replace those two coil packs. If the misfire stays with cyls 3 and 5, replace the spark plugs.

The reason that misfires are damaging to the cats is that when a misfire occurs, there is incomplete burning of the fuel and there is only so much fuel (or hydrocarbon) that a cat can take before it is permanently damaged. You should be fine running the engine to diagnose and repair the misfire but don't ignore the problem and continue to drive it while misfiring for a long period.

Thanks, I'll give that a try and see what happens. I may just go ahead and invest in new coils and plugs all the way around since the car literally turned over 60,000 miles as I drove away from the Autozone after having the codes checked.

Had those codes and all that associated Harley Davidson idle thing as well.... and done this on my 986 at 50k (this spring). Mind you mine is an old car and see the road once a month only (if lucky!).

1. New battery
2. Coil packs
3. Sparkplugs
4. MAF (genuine, not the ebay test-failed unit)
5. ultrasonic cleaning of injectors
6. TB cleaned ultrasonic also
7. AOS
8. Mobil 1 Oil (every 2,000km!) 1,500miles?
9. All filters
10. Coolant change
11. ATF and filter change
10. New car freshener (lavender)

I liked it soooo bloody much I took off for a 700km round trip in the mountains of Zhejiang province. Non stop. No music. Alone (don't tell)

Buy "lavender". You'll see!

I'm going to try plugs and coils this week and see what happens.

I was all ready to do an oil change and swap sway bar end links yesterday before this happened, so I'll get in there and do everything at once now.

Thanks for the tip on the lavender. I'll have to be sure to explain that to the wife though. Coming home smelling like lavender might make a misfire on the Boxster the least of my worries.