1999 Boxster with Idle Issue

Greetings, I just picked up a nice 99 Boxster. The car had a CEL that turned out to be an issue with the ECU not sending a signal to one of the vario camshaft solenoid. I had the ECU fixed and no more CEL, however there is a persisting idle issue that I can�t seem to solve. When the car is at idle the RPM is continually fluctuating. Idle ROM seems to be around 790, and varies up to 40-50 RPM. I had my mechanic look at the car and he is telling me it is �normal.� It seems to be more variation than I would call normal. I have had the oil changed, cleaned the MAF, ran injector cleaner and nothing has made a difference. The car runs amazingly and I have put 5,000 miles on it with this annoying issue. Please advise if there is anything else I can clean before take it to my nearest Porsche shop (not close). I was considering cleaning out the TB. The car just went through 45,000mi, one owner. I am hoping this is not a major problem; my last Boxster had a massive IMS failure within 200 miles of me buying it. -Will

Welcome. You may want to clean your idle stabilizer and throttle body. Try the Boxster forum for more suggestions

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Is that the small black electrical box on the side or the throttle body? I took that out and cleaned it with MAF cleaner a while back, that didn’t seem to help. What sort of cleaner should I be using. I noticed it has a paper gasket that has a rip in it; maybe a tiny air leak there is causing my problems? I may try to make a replacement gasket, or see if I can pick one up on Amazon or something.

Yes, that's the part. MAF or Carb cleaner should be fine. Air leak is definitely a common issue but it's kinda hard to track down by yourself especially if the leak is small, which seems to be your case IF air leak is the issue. Yes, any leak after the MAF sensor will give you problems. Spec rpm is 700 +/- 40 so 790 is definitely not in spec.

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Yeah, I didn�t think it should idle so high. It occasionally (once a week) manages to get itself to a idle around 700 and it sounds great... Then at the next stop it�s back to somewhere between 790 or as high as 1100. I will pull off the TB and clean it and the stabilizer tonight. Thanks for all the help. -Liam

Could be lipstick or eyeliner pencil rolling around under the gas petal from the previous owner. Please forgive my bad jokes

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OP, welcome. Here's the link to that forum in case you missed it when you were searching https://rennlist.com/forums/boxster-...-986-forum-67/

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Oops... I thought I had posted in the Boxster forum... No worries, I removed the eyeliner and lipstick.

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Ha ha, don't take me too seriously , boxster is a beautiful car

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It is a fun car. For the $10,000 I paid, I can't really complain. I installed the rear speaker kit last night instead of doing the TB.

Pull the idle air controller off again, apply some power to it so it opens (9V battery) and then use some carb cleaner and a dental pick and clean off all the carbon from the plunger. I'm in the middle of fixing my PCM because one of the vario cam solenoids aren't being triggered by it. How much did they charge you to fix yours (the PCM only)?

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I pulled the TB and cleaned it (it was really dirty). Cleaned the idle air controller, reinstall and..... Still the same.... Maybe slightly smoother, but idle is still just a hair below 800, sometimes right on 800. I think they charged me $800 to fix the PCM, my mechanic sent it to Florida to be done. There is a place in New York that looked at it first but sent it back as "no repair needed." The place in Florida also required the immobilizer to be sent in with the PCM. If you want I can get the contact info for the place in Florida, they were quick and professional.

So I am looking at the idle controller. How big of a gap should there be where the little metal actuator is? It seems rather large, maybe about half open. Maybe I need to submerge it in cleaner for a while to get some of the crap out that I cant see. Seems it could be slightly stuck open. I cant seem to find a picture of what it should look like when "closed." Anyone have a picture of a good one?

Thanks for the info but I'll be fixing it myself. Just wanted to know how much someone is willing to charge to change an $8 part.

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Yeah, I wish I had known you back then... Just took out my IS again and it is soaking. It already has closed a little more than before... or maybe that is wishful thinking... Look like it stays open about 1/8th of an inch.

Well I noticed a little smoke after re-installing. Maybe it is my AOS going south?

Are you sure it's smoke like burning engine oil and it's not just normal steam at cold start? Can you remove your oil filler cap with ease and with just a little bit of suction? Idle speed and sound should also change once the cap is removed.

I don't know if it was cleaner burning off, oil, or steam. I will be taking the car out on a drive today and will see how she behaves after a few miles. Is the idle RPM adjustable on the Boxster, or computer controlled. Is it possible the previous owner had it set to high?

start simple. look for vacc leak or the bellow hose on the AOS, Run some drygas thru it, some injector cleaner, Injector cleaner made a HUGE differanjce in my fuel mileage FYI. I get another 50 miles out of a tank and smoothed out my idle

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Not adjustable AFAIK. May want to check when the gas pedal is not pressed, the throttle springs back fully and you have just a little bit of slack in the throttle cable where it connects to the throttle body. You should also check for air leak like Shark suggested. If you can read fuel trims, the numbers may give us more insight.

Took her for a spin today, power seems up, idle is smoother and idle speed was close to 700 most of the time. It seems like I am getting closer to smooth every time I do something. I had put techron through it about a month ago. When I took off the J-pipe today I noticed it already has a slight oily residue in it near the AOS, is this normal? I wish I knew someone in the Seattle area with a "normal" Boxser that I could compare to, I don't really have anything to compare the smoothness to other than a V8 Jag (smooth as silk), ford focus (utter crap) and an Austin Sprite (rough as sandpaper currently). I noticed there is an oil leak where the oil filler connects. Could this be caused by anything particular, or is it just a leak? I guess I need to replace the connecting part, anyone have a link to a good how-to on that?

Oil filler tube needs to be air-tight, any leak you will get unmetered air and rough idle.

I had some similar weird rpm issues on my 2004 986S, though mine typically occurred at low throttle levels under relatively high loads (3th gear going up a slow hill, etc); cleaned the TB with no joy. It ended up being a loose vacuum fitting. AOS usually gives pretty clear symptoms when it fails (godawful whine and/or massive clouds of smoke all the time).

The throttle cable has a little slack in it so I dont think that is the problem. No smoke again today so I am less suspicious of an AOS. Can somenoe send me a picture of where the RPM should be? My needle has the jop of the needle just touching the bottom of the 800RPM line, I think this would be considered high, correct? My mechanic wants to charge $800+ to change out the oil filler neck, does this seem reasonable.... Its like $100 of parts and $700 labour.... Is this a hard part to swap out?

Anyone have feedback on what it should cost to have the bottom hose replaced on the oil filler? When I take the oil cap or dipstick out there is a definite RPM change, so I doubt there is much of a vacuum issue from the leak.

New update. I took the J tube off today to see if there was any oil and it was very oily, like it looks like i just used it as a funnel to fill my car. I am wondering if there is anything other than an AOS failure that could cause this? If I had a vacuum leak in the oil filler would that suck excess air/oil through the AOS? I think I'll just go ahead and replace the AOS unless there is a way I can test it/clean it/inspect it? Thanks guys.

There's going to be oily vapor that will condense (hence why emissions requirements drive things like the AOS to exist), it just shouldn't be actual oil flowing through. No way to non-destructively test/inspect it; there's a thin plastic springloaded membrane in there that's designed to allow air to pass through but no liquid oil, but it's sealed. If it failed, you would draw vacuum on the crankcase and would know it.

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Ok... So assuming the AOS is fine, I am going to replace the hose connecting the oil filler to the block. There is a tiny leak (a quarter sized oil slick each night). After that I have no idea what else would be causing my hunting idle...

Replacing the oil filler tube for sure if it's leaking. It looks like a PITA from google so I'm not surprised if they charge a lot of labor. Agree the oil vapor in the j tube is normal. To find air leaks, the best is to use a smoke machine.

As an interim fix, you can wrap whatever you suspect is leaking with electrical tape. If the problem goes away, you've found your culprit.

Thanks guys. I'll play around with electrical tape this weekend. After having a catastrophic engine failure in my first boxster I am always suspecting a major issue when it could just be something minor. Would an air leak in the oil filler be causing the TB to be really dirty? Maybe the excess flow through the AOS is bringing just a little extra oil than is normal? I'll let you know over the weekend how it goes. I may have to make a smoke machine out of my shop vac to figure it out...