Idle Issues - fine above ~2000rpm

I own a �99 C2 with 95,000 miles and have recently run into an issue that�s left me scratching my head. Any thoughts or insights would be appreciated. Earlier this summer I noticed a little bit of wandering in my car�s warm idle rpms. The plugs were coming up on 5.5 years of age and 30k miles so I decided that it was time to change them in accordance with Porsche�s service interval. I also changed the coils as I hadn�t changed them in my almost 6 years of ownership. Their part numbers indicated that they were likely original to the car. About two weeks after performing this work, while idling at a stoplight, the engine started to miss and she threw a CEL. The misfire then went away after about 25 seconds and my reader pulled the code as a misfire on cylinder 6. I went back in and inspected the plugs and coils and all seemed normal. I attributed it to a coil/plug that maybe wasn�t quite properly seated. Then few weeks ago (so about 3 to 4 months after the above situation) I attempted to start my car but she acted different than usual this time. I had been out running errands so the engine was already fully warmed up. My car always fires up after two or three cranks from the starter. Always. Except this time it took about 2 to 4 seconds to fire up and she rev�d somewhat high when she finally did fire (maybe up to about 2000rpm) before settling into idle. This didn�t occur again until now. Fast forward to this week and again I was out doing errands and the car was fully warmed up. I went to an appointment and she sat in the lot turned off for about an hour. I hopped in to leave and cranked the engine and she went BOOM. It was a loud backfire, the engine fired, she rev�d to about 2000 rpm and immediately died. I didn�t see any smoke or smell anything abnormal. No CEL either. I tried to start her again and she started normally but didn�t want to idle. I know what you�re thinking and generally I agree: don�t fire the engine again unless you know exactly what the issue was. Save the preaching please. I weighed the risk/reward and figured that my wallet can handle it. I was able to keep her running this time and she threw 3 CELs of P0300, P0302, and P0303. I only lived about 1/4 mile away and was able to make it home from there. I went to work on the car and immediately went for the coils and plugs again. All of the plugs on Bank 1 looked normal and similar in appearance. No soot or burn marks. No oil or anything. The coils all appeared fine as well. I went ahead and moved the misfire coils (cylinders 2 and 3) to cylinders 5 and 6. I figured this would let me know if it was a coil-related issue if she then started throwing codes for Bank 2. I also found that the rubber intake bellow that attaches to the drivers side of the throttle body had popped off. I attribute this to the backfire. I dug deep and found some vacuum lines that hadn�t been routed correctly (thanks to an old shop who I thought I could trust). I got them all resecured and properly routed. I also thoroughly cleaned the throttle body and the ICV. I reassembled everything and installed a new MAF. Mine was approaching 6 years in age so I figured it wouldn�t hurt. The car now fires up fine but she still isn�t happy at idle. She initially wants to die but she sounds normal and feels smooth if I rev her to about 2000rpm. If I let her go back to idle, she won�t die but she isn�t happy either. A fair amount of vibrations are occurring at idle but no CELs or codes are showing. VarioCam kicks in at 1500 rpm so I�m wondering if this is likely a timing issue? Maybe VarioCam�s engagement temporarily �fixes� my issue? Maybe my chain pads have finally deteriorated enough? The timing chain tensioners for bank 1 and 2 are new and were replaced about 6 months ago. The IMS tensioner is set to be replaced within the next few weeks at my next oil change. And before you ask, no I don�t have a Durametric or similar scanner and I don�t know if anyone nearby that owns one. I�ll have to drive or tow the car to a shop for diagnostic work beyond pulling CELs. Thoughts? (please be gentle)

Over 2000 rpm is not �idle�...

As you are well aware, this could be one or several of a myriad of things causing your issue. Start with the easiest, you've already done your plugs, coils and MAF, I would next look for a vacuum leak. Check that the plenum connections are all secure and that all your vacuum hoses are firmly attached, including the brake booster connection. When I put my engine back together it was running poorly and misfiring until I revved to higher RPMs, turned out I had a pretty big vacuum leak at the brake booster to plenum connection. Once I corrected the leak, engine purred like a kitten at all RPMs. Next, I would start looking at fuel filter/fuel pressure regulator/fuel pump. Be methodical and eliminate one possibility at a time and you'll find the problem. BTW, you stated that Bank 1 plugs looked normal and similar, but you didn't mention what Bank 2's plugs looked like. Best of luck.

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d, I�m not sure I follow ya here. I agree that 2000 rpm isn�t idle. Good call on the brake booster, and fuel. I�ll check that out today. The Bank 2 stuff all looked good to me when I checked the plugs and coils.

Ahhhhh...not sure I follow myself... I was probably doing too many things at the time, and misconstrued the meaning /intent of your title. At least we agree that 2000 rpm isn't idleing...

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I have very limited experience with these engines, but my experience with other engines raises a flag on the backfire. Most backfires I’ve experienced have been related to timing, particularly if it was back through the intake (as you noted it blew the boot off). That said, could a bad signal from a crank position sensor cause an ignition at incorrect timing?

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I tend to agree, especially because of how rev�ing her to above the VarioCam engagement rpm seems to induce a change in the engine�s behavior. They only other thing that comes to my mind is a leaky or stuck injector that temporarily pooled fuel in a cylinder which created the backfire. I�m mentally prepared for a worst case scenario at this point. I may have to give those kind folks at FSI a call depending on the analysis of my shop. An Arena Red Aerokit with LSD and an FSI motor sounds pretty sweet to me.

Did the backfire come through the intake, or exhaust? If through the intake, it probably popped off at least one vacuum connection, then created a vacuum leak.

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I assume (dangerous word, I know) that it was through the intake because the drivers side rubber intake bellow was popped off of the throttle body. I haven�t been able to spot any other disconnected vacuum lines other than some of the little ones connected to the three way and four way junction next to the starter. I�ve got an appointment with Cali Auto here on the south side of ATL in 10 days.

Update: The source of the problem is proving to be elusive. Coils, plugs and compression all check out but there are random misfires across the 6 cylinders. The misfires don�t reach the threshold to throw a CEL though. The tech isn�t able to pull any fuel trims nor is he able to get a cam deviation reading on bank 2. The suspect at the moment is the DME. We are now sending that out for inspection and possible repair/replacement.

Do yourself a favor and change the coils. I spent most of the summer chasing misfires because I assumed my coils were good... I changed them almost as a last resort (I had bought them and injectors but decided to do the coils first because they were easier) sure enough that was the problem. The car has been great ever since...

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The coils and plugs are brand new. And swapping them to various cylinders didn�t resolve the sporadic misfires.

I saw you have inspected vacuum lines but have you performed a smoke test?

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Did he check the wiring harness to the DME to make sure all is well before concluding that the DME is toast?

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Yes they did. The engine seems to check out on everything except for the wonky/absent readings when they use their PIWIS tester. 🤷🏻‍♂️ I�m not sure. That�s a good idea for me to check on, though.

So I finally have an update to this but am still in need of ideas as well. The DME was inspected and came back with a good bill of health. We finally decided to drop the engine as the shop suspected that there was some kind of vacuum leak occurring around cylinder #3. They sprayed some carb cleaner around that area with the engine running and the engine began to idle much better. What they found were that the vacuum line connections weren�t routed correctly and didn�t match the vacuum diagram on the underside of the lid. They mentioned that there was a 4-way tee in use instead of a 3-way/Y pictured on the vacuum diagram of the engine lid and so they corrected that. They also found that one of the intake plenum bolts at cylinder #3 was loose and backed out a little. They decided to remove the intake to look for any other issues and found a torn intake seal also at cylinder #3. Because the engine is now out, I�m likely going to have them proactively change the AOS (almost 6 years old), the oil fill tube (original) and both camshaft sensor (age unknown). What I�m looking for help on is that they�ve still not been able to get deviation readings off of both banks. None of us can seem to figure out why. If it were due to a bad cam sensor, would we also get an associated fault code for that? There aren�t any codes other than what�s showing for the misfires. I hate to have paid for the engine to be removed and for other issues to be fixed but still stuck with being unable to get cam deviation readings from both banks.

This is usually because of a mechanical fault that is picked up by the cam sensors which locks out the adjustment of a cam to protect the engine. In this case you have to find that fault using experience/ common sense, and understanding of how things work in the system. Once things are beyond a point that the DME doesn't like them to be, the adjustment is locked out, and communication goes to zero. In this case the Oscilloscope comes out, you throw away the OBDII (which has freaking worthless anyway) and you scope the sensors to determine what is happening in the real world. Paying attention to the down strokes of each waveform is the key to seeing what is happening. No, you won't "research" this one on Google. Other things can lock out adjustment, and kill communication, but this scenario is the one that I normally experience.

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Great that the shop was able to find a vacuum leak and fixed it. My vote would be to check and or replace the cam hall sensors too. They can cause a backfire if not working as designed. Also hard starting and wonky idle when hot. Shop can check continuity from X59/2 connector. I think pin 14 is for sensing and pin 22 is secondary voltage. Checking wiring harness would be done first.

So how do you connect an oscilloscope? Is it to the particular sensor or something the computer will output to you?

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Depends what kind of a sensor you are trying to check. Usually to start you would back probe the sensor but that's not always the case. The shop should know how to use it.

Back probing makes sense. Theres probably too many types of connectors to have adapters to test with. I�m always leery of insulation piercing unless absolutely necessary.

If it's a five chain, won't this be variocam pad wear?

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That is my uninformed suspicion.

Unfortunately I’m still dealing with these issues. My wallet is $5k lighter after dealing with a local “trusted” shop in ATL and yet I have no resolution. Thanks to @Porschetech3 for giving me the tip that my missing Bank 2 cam deviation is due to the car having a RoW DME flash. The Indy couldn’t figure it out until Skip messaged me about it. The car will have random misfires across any of the 6 cylinders and also throws a general misfire code. This behavior only seems to occur after the car is fully up to temp and has been driven for about 45 minutes or more. 1. Fuel trims and pressures appear to be fine. 2. Cylinder compression is good. 3. Coils and plugs are all still less than a year old. 4. The motor was dropped and found to have loose intake bolt on cylinder #3 as well as a torn gasket. All intake gaskets were replaced as a result. 5. Someone in the past had misrouted the engine vacuum lines so those were corrected too. 6. The AOS appears to be fine. 7. The DME was sent off for analysis and is supposedly ok. 8. Bank 1 cam deviation is 5.0. 9. The MAF is new. 10. The idle air valve has been cleaned and appears to operate correctly. All of the above is to be taken with a grain of salt because I subsequently lost complete faith in the shop due to numerous missteps on their part. Additionally, I’ve done one oil change during this period and had a Fuel by GC test done on the sample. The % of fuel in the oil was 1.9%. I’m really loosing my mind over this car especially after such a terrible experience with a shop that I knew for many years and thought I could trust. It’s not reasonable to expect any particular shop to have all of the answers, but I do expect them to stand by their word.

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You can�t assume that the AOS is ok without proper evaluation. Cracked intake manifolds, cracked emissions system conduits, and etc are all realities today. cam deviations may be allowable by the factory at -6, but that doesn�t mean that -6 is adequate. Sounds like one I�d like to use as a challenge.

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if it were me and i got this invite to the party, i'd already be on my way over.

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She�s all yours if you want her. I�m about two hours south of you.

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Start at ticket at www.flat6innovations.com and list everything that has been done so far. I have a pretty good idea from this thread, but need to see the whole picture before trying to figure it out. Has the throttle body been replaced?

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Thanks for offering a helping hand. I just got home from a last minute work trip so I�ll fill out the form in the morning. The throttle body hasn�t been replaced, only cleaned a few times throughout this ordeal.

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Replacing the TB should be done next. Cleaning them just doesn�t work. The idle air control motor can sometimes be found separate from a new TB for the cable driven 99 cars. I look at it this way: the TB is 22 years old by this point. If it hasn�t failed completely from an idle air perspective, it will soon. Considering this I don�t really think that you are chasing the issue by changing parts. It�s damn near impossible to properly �diagnose� one of these TBs. This should have been done along your way to spending 5k... You should do this before you consider bringing the car to me. It�s a simple fitment, just make sure that you perform a hard reset of the DME before starting the car up with the new TB fitted.

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Sorry if this is a really dumb question, but what can go wrong with a 99 throttle cable throttle body other than the idle air valve dying or maybe the spring breaking? Isn't it a pretty simple mechanism overall?

Simple things can fail too. The 99 C2 (I assume the car is a C2) is the only 996 that uses the IAC for idle positioning. The 99 C4 is in the group with all the other 996s newer than 99, that have �E-gas� or an electronic drive by wire throttle plate. These do not have a traditional IAC, since the throttle plate position is manipulated by the DME to provide idle air volume. The 99 C2 throttle bodies give us issues all the time, and have for years. Cleaning will only help lessen the symptoms in some cases, or delay the inevitable. Cleaning generally does nothing to substantially improve the situation, since wear on the stepper motor components is the issue. This should have been explored early in your quest for sorting this issue.

Ticket submitted. It's long as I tried to provide as much info as possible. Thanks again.

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Can anyone give guidance on how to do this? I think I remember reading somewhere that you disconnect the positive and negative battery cables and then touch them to each other for two minutes? It�s a throttle cable car (not E-Gas)�.not sure if that makes a difference.

Well I might finally be getting some closure on this. In a last ditch effort, I decided to take the car to a local trusted indy (Kelly�s Automotive in Peachtree City, GA) who mostly works on BMWs. The owner occasionally works on Porsches but will be the first to tell you that they aren�t his specialty. I�ve known him for a few years and he always struck me as a salt of the earth air-spark-fuel type of mechanic and so I thought I�d see what he had to say about my continuing CELs and misfires. Sometimes a fresh, independent perspective is just what the doctor ordered. Keep in mind that this is after I dropped many thousands of dollars and about 6 months with a big name Porsche indy and basically got nowhere. In less than a day he had zeroed in on the coil pack on cylinder #2. All six coils were less than a year old but he said that the #2 coil seemed to start to misfire once the engine got hot. This would explain why my misfires and CEL always seemed occur when idling at a stoplight after driving for about 45 minutes. So he replaced that coil and spark plug and we haven�t had another CEL in about a month now. The idle was still occasionally just slightly rough, especially without the AC compressor engaged so I decided to go ahead and replace the fuel injectors with the later 2001-2005 style too. I finished that task yesterday and will keep my eye on the car�s behavior.