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Engine trouble - Not IMS, but AOS and more? Sound from steering, overf

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I'll start with a picture to make it more interesting to keep reading. ;)


It's a '98 996 C2 manual with M030 and M220 (LSD), with 98k miles on it. Pic related, it's me under breaking at a track-day the 1st of June. The friday was ok most of the day, passed a GT3, tried to hold a GT2, but lost about a second per (2,3 miles long) lap. Always fun to pass or hold cars that are much faster, just on late breaking and good-ish lines ;)

The CEL came on once on friday, but I calmed down my driving, pulled in and read out the codes: Multiple engine misfire (P03000), and three cylinder misfires, but I can't remember which cylinders. Did a quick check around the engine, everything looked ok, cleared the codes, started up, everything was ok, CEL didn't come back. Did a few more laps, still no CEL, the car pulled strong, no problems. Did the 20 minute drive back to the hotel, all was dandy.

On saturday I pulled out on the track, did a calm round to get some heat in the tires, but the CEL came up after just a few turns. Calmed down the driving even more, but had unfortunately just passed the pit entry and planned on just driving a very slow lap to get off the track. Got half way down the start/finish stretch when the CEL-lamp blinked then stayed on steady. It felt like it was running on just 4 or 5 cylinders. I shut it down and coasted in a u-turn and drove the wrong way in the pitlane exit. Read out the codes, cylinder misfires was the only ones logged, but I got four cylinders this time so I ruled out problems with just one bank.

Cleared the codes and, perhaps stupidly, fired it up again. No codes, no sounds, idle was fine, oil pressure normal, no warning lights, everything was like normal. Puzzled I shut it down and inspected some more. Found some fresh oil where the engine meets the gearbox, not a LOT, but enough to drip some drops to the ground. I figured that if this was IMS I'd hear the death rattle or have blown the engine by now so next on the list was RMS. Figured it couldn't destroy the engine and I didn't feel comfortable telling my insurance company that I needed a flatbed at a track, so I drove down to the main road. Car seemed fine, but lacking in power and shuddering above 3k rpm. Pulled out roadside and went back to have a final look around. If I blipped the throttle (to about 2,5k rpm) I could hear a faint rattle when the car came off the revs. Anways, luckily my insurance covers roadside assistance to the nearest _capable_ garage. We were currently in a small-ish town with the nearest Porsche Center in my home town, 400 miles away. I was convinced that no garage in that town would be dumb enough to start diagnosing a brand of car they'd never had in the shop, but flatbedding that kind of distance wasn't popular with the insurer. After two days of arguing they eventually towed it home. I got a rental the same day as the incedent and drove home.

My local Porsche Centre started up not even a year ago and currently have just one Porsche mechanic. He's an experienced mechanic, and have the Porsche training, but naturally doesn't have a lot of experience with these cars yet. In fact, my 996 was the first 911 he did a service on at the new centre. After a quick look around he found what you're probably screaming at the screen right now: The AOS bellows was disintegrated. They replaced the bellows and washed away the oil, ran it on idle and didn't get any more oil. In fact, they said that my 996 was the driest they'd seen so far, with no oil sweat at all. So, RMS/IMS is out of the question. Either way, he heard a screeching noise and shut it down. No faults were registered. By now I had done my research and read up on what was, to me, a new frequent fault: AOS. I figured that if the bellows was that destroyed the AOS-unit was probably toast as well. As they are known to sometimes screech when they're broken, that might explain what he heard. I told the guy in the service desk about that, and that they also had to check for oil in the coolant and coolant in the oil. They did, and both liquids were fine. The mechanic though, described the sound more like scratching metal with a screwdriver. I've never heard an AOS unit go bad so I can't say if that's the sound it makes or not. The mechanic said he also felt the clutch pedal felt weird, and he wanted to pull down the gearbox to inspect the flywheel. They haven't swapped the AOS yet. He also went in with a boroscope to check the cylinders and said it didn't look excellent, with some scratches. But as this was the first 996 cylinders he'd seen he couldn't tell if it was normal or not. All six cylinders were the same, which is a good sign. The car wasn't low on oil. Even though I lost some at the track, I was still above the minimum mark.

The problem now is that he's not sure what to do. Luckily, he's smarter than to do guesswork and throw parts at the car. They have a system where he can call other more experienced mechanics at other centres, but they are busy people and hard to get on the phone. The cars has been in the shop for more than three weeks now and I'm getting sick of driving around in a Legacy. Any good ideas here as to what might be going on? Are scratches on the cylinder walls normal for a 100k mile engine, and how much is too much?

Also, not sure if it's related, but figured I'd put it in here: The steering made some screeching noises when I made my way from the track to the main road to drive back to the hotel on friday. It was just the first 2-3 minutes after starting up and I haven't heard or felt anything different since then. I also felt some juddering in the brakes on-track, but that's probably just warped rotors from track-abuse and heat. Brakes felt normal at normal speeds. Also, I noticed some Pentosin had spilled out of the reservoir up front. As I said, not sure if it's related.

Give me your best guesswork, please.

The not so pretty sight on saturday:


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My advice, suck it up and have it sent to a Porsche Dealer. They are expensive but so far in my 15 years of driving 911's the fix everything right the first time. These cars are tricky, an inexperienced tech can change parts at will and you will go broke fast. It may cost you $500 to get it there if you're so far away or rent a U Haul and get it there your self.

I've read hundreds of posts of guys changing this and that with no luck. Take it to a Porsche Dealership.

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  • 1 month later...

To conclude this:

Porsche Center Tromsø (PCT) eventually came, with the help of the importer, to the conclusion that my cylinders are turning oval and I have cracked the lining on cylinder 6. As the cylinders are created with the Lokasil-process and doesn't have replaceable cylinder liners the block is basically scrap metal. A new-in-crate engine from Porsche was quoted at about 23.000 usd +installation, in total around 27.000 usd. Clearly not an option. There are various companies in Norway, Germany and the UK that specialize in boring out the failed cylinder and installing Nikasil liners. The cost is around 16,500 usd. But the process takes time, and even more so now due to the holidays. It was also a hassle getting the car to the shop as it's in a different part of the country and many freight-options are ruled out as the car can't be driven on and off various means of transport.

In the end I bought a salvaged engine from the UK and had it installed here. In fact I just got the car back yesterday. It runs fine but I still have a high idle-problem I have to figure out before everything is back to perfect. The engine cost me about 10k +installation.

The plan now is probably to sell it before it breaks down again and getting something else. Perhaps an E46 M3, or a 996 Turbo, though the turbo costs twice as much as the Carrera, and start at about 113.000 usd in Norway...

Will I own a 996 Carrera again? Hell no.

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  • 3 months later...

To finish this off:

Turned out that the ICV (idle control valve) had stopped moving as smoothly as it should. Cleaned it out and the idle got normal again. The car still gave worse mileage than usual and in the following MOT it failed on emissions. The dealer suspected a vacuum leak and left it up to me to decide what to do.

I picked it up, thinking that I'd have a check around for a leak when I had the time. But after 15-20 minutes of driving it started running real rough with low power, and a really rough idle, it felt like it was running on five cylinders, or less. Returned the car, but the dealer insisted that they didn't do anything that could have made the problem worse. Still, they requested the coils from the old engine, replaced them and the plugs. No improvement. I was annoyed as the car ran fine before I sent it in for MOT, except for the poor-ish mileage. After a few days of work, they concluded that it had to be a vacuum leak and wanted to drop the engine to check. That was a time consuming (=costly) process and I was quite thorough in checking all hoses and connections when the "new" engine got put in so I felt a major vacuum leak wasn't that likely. Even worse, the dealer didn't have time to take the car back in for another three weeks and seemed pretty reluctant to do work on it, probably because they didn't have a good understanding of what the problem was.

Went back with my laptop and a Durametric cable to check what was what. Found seven(!!) error stored, deleted them and fired it up. Car ran fine! Was rather puzzled and drove off. Kept the laptop on and connected so I could check for any error codes that might pop up as I drove home. The first two were to different error codes for the O2-sensors, then after about 15-20 minutes an O2-adaption error was thrown and the car started to run rough again. These were the only codes thrown, since the car isn't US spec it didn't light up the CEL. Reset codes, restarted the engine and it worked properly again. Turns out the engine does an O2-sensor adaption process if the battery is disconnected or ECU is reset. It has emergency programs for sensor failures, but apparently not for dual O2 failure. My 996 is an early ROW vehicle so it only has pre-cat sensors. These are things that the dealership should know...

Anyways, when installing the new engine, the exhaust was swapped over. As you would expect, the nuts and bolts connecting the cats to the headers was rusty and broke on disassembly. Quite a bit of heat and a rather large hammer was required to separate the parts and that heat and judders killed the O2 sensors. Installed new sensors, an easy job with no other disassembly required and the car was fixed! No error codes, correct emissions and proper mileage. Job done.

Still not to hot about the 996 Carrera though, but I'll probably keep it over the winter and look for a 996TT in the spring. Keeping my fingers crossed that the engine lasts that long...

Some photos to brighten up an otherwise sad thread, taken a few days after it finally got sorted:




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