Hi all... I've been dealing with a strange idle issue lately (2003 C4S) and I've done a little research and some of the conclusions point towards possible bad cat converters. My question is... what are the indicators that you have bad cat converters? Is it an obvious thing (like it won't rev over X or won't idle)? I'm running some Seafoam through the system, I've cleaned the throttle body and reset the idle per the instructions I've read online (key on for 60 second, off for 5 secs and on again for 15 secs). It is running a little better and hasn't thrown any more codes today.

Thanks

 

The exhaust flow would be so minimal at idle, what makes you think it is the cats without the codes? I would think a vacuum leak or idle control first....

 

Where is the idle control on E-gas 996's? I thought they didn't have one.
Thanks

 

Converters can fail in several ways. One way with which I have direct experience is the converter brick gets loose -- I think an impact from road debris some years ago caused this.

In this case every once in a while the CEL would come on and there would be a P0430 error code. I ended up replacing the bad converter (and the other side which was just fine) with a pair of exhaust manifolds sourced from a salvage yard.

Besides the P0430 the only other signs of a bad converter was a buzzing -- when the converter was hot -- that sounded kind of like a loose heat shield. Towards the end though the thing knocked when cold.

Another way is the converter comes apart. This can be due to structural failure or excess heat. In this case exhaust gas flow can be impeded to the point the engine may not run right. Certainly not revving freely to red line could be one sign of a bad converter. Possibly even a bad idle, too, if the DME was having to adjust fueling to try to get the expected #2 O2 sensor readings from the bad converter.

But in all cases if the converter was truly bad I would think you'd see the CEL on and there would be one or more error codes, with one pointing to the converter.

Have you checked for any pending codes? The behavior doesn't read much like a bad converter (unless there is a pending code that suggests otherwise) but something else.

My WAG is the AOS is probably in its early stages of going bad. This can start out with an idle problem that can appear to fix itself only to return just when one thinks he's solved the problem.

Be sure the oil filler tube cap isn't leaking. I had one leak on my Boxster. At idle see if you can move the cap about, even pull it up a bit, and see if the engine reacts.

 

The idle control is the E-gas system and the position of the butterfly valve.

This system needs periodic calibration. The techs suggest this be done at least every service. I do this in my cars every approx. 2500 miles.

The steps do this should be on the owners manual. From my memory: With the key off and your foot off the gas pedal turn the key to the on position. Leave on for at least 60 seconds. My memory is then turn the key off for at least *10* seconds. The next time the engine is started the calibration is done.

 

Thanks for the info! It's sounding more and more like an AOS issue. How much of a PITA is it to change one of those?

 

Can't help you regarding the level of PITA difficulty replacing the AOS. I've never replaced any (3 now) on my Boxster -- had these all done at a dealer service department -- and my 2nd hand info is the 996 AOS is a bit more work than the Boxster AOS.

 

Rough times. The 3.8L is easy but that really doesn't help much.
I've read that getting it to seat can be a pain and then you have to deal with all the connections.

 

How much did the dealer charge? I'm pretty good at doing all the maintenance on my vehicles and have worked on cars with tight engine bays (Audi TT comes to mind), but this sounds like it could be a major PITA and worth paying someone else to do if it's not crazy money.

I've heard that pulling an engine in the 996 can be done by a DIY'er but is it really something that can be tackled?

Thanks

 

Pelican parts has a DIY how to. Just do a search, you have to remove one of the intake manifolds for room.

 

I can't recall now how much the last AOS replacement cost. The 1st one was back in early 2004, the 2nd one about 4/5 years later, and the prices I paid back -- both done at the Merriam KS Porsche dealer -- probably have no bearing now, while the 3rd/last one was back in 2014 best I can recall.

I'd have to go through my Boxster file to find the invoice for the last one.

Have to point out that even between dealers prices vary. My local dealer charges at least $150/hour and it might be even higher now. And a 996 AOS is more work and that is for a manual. For a Tip equipped 996 the AOS is even more work than for a manual equipped 996.

A plus is a dealer, well, a dealer's techs, have probably done a number of these by now and they probably have this procedure down but it is more work.

 

An AOS replacement in a 996 C4S is complicated by the AOS being located behind the engine and several things needing to be removed. Depending upon the condition of the hoses/fasteners it is sometimes better to just drop the engine to do the AOS replacement.

A local dealer quoted me a rough estimate of $2000 for this work.

 

So I finally broke down and took it to Chris' German Auto in Redmond WA. They found a bad throttle body and two vacuum leaks. They also found a coolant leak that I didn't even know I had. All fixed now and she's running great now. They also upgraded the ECU software to help smooth out the "hit" that happens when the variocam kicks in at 3000rpm. Great place to take your P cars!

 

Thanks for the update.

Just curious: How was the throttle body id'ed as bad? Where were the two vacuum leaks?

 

They tested the TB to determine that it was bad. It was an eGas model and they said that there's a spring inside that sometimes fails on those. The vacuum leaks were on the intake. Not sure exactly where, but once they fixed those all the rough running/high idle issues stopped.