maybeillbuyit

Hello to all, I am new member here. I am attempting to service my buddies new 1998 Boxster VIN 622762. 170000KMs. It had several error codes when he bought it, P1126, P1124 and the mechanic suggested replacing all 4 O2 sensors. I have been working on old 70s-80s BMWs for years so I 'm mechanically inclined but not used to dealing with P codes and a lot of electronic stuff. To me it seemed a bit odd to replace all 4 sensors so we took the car to my mechanic. The battery had died and the codes gone. The car idles rough. My mechanic recommends I do the "water in air cleaner mod". I haven't found any info on this procedure or any reports of anyone ever doing this to a Boxster. Has anyone had any success with this or recommend doing this? This is a copy of the service bulletin from Porsche Thx for looking and commenting

Quadcammer

I believe they are referring to the old debaffle mod. Its an easy thing to do and the car maybe picks up some sound. pretty easy to reverse if you need to.

maybeillbuyit

OK thx for the reply. It seems an odd mod to be doing to cure a rough idle issue. Especially since I haven't read about anyone actually having water getting into the the air cleaner box. Does this happen to many boxsters?

Quadcammer

I wouldn't think so. I don't think this will cure a rough idle though. Thats more than likely a cracked coil.

Macster

The error codes do not come from water in the air box. I drove a 2002 Boxster 16 years and 317K miles in all kinds of weather including heavy rain and with lots of road spray by passing vehicles and never had any water in the air box issues. Save any air box mods to after you have the engine running right again.

The error codes:

P1124 - Oxygen Sensing Adaptation Area 1 (Cylinders 1 - 3) - Rich Threshold
P1126 - Oxygen Sensing Adaptation Area 1 (Cylinders 4 - 6) - Rich Threshold

These codes indicate the fuel mixture is too lean. That is, the engine controller (DME) is at the limit of adding fuel to bring the mixture into spec.

Possible causes:
Intake air system leak.
Fuel pressure too low.
Fuel supply inadequate.
Fouled fuel injectors.

My WAG is there is an intake air system leak. The most common source of an intake leak is a failing (failed) AOS (air oil separator). If the engine is running rough at idle unscrew the oil tube filler cap and remove the cap. In some cases -- if the AOS is in fact bad and has advanced in its failure progression -- one will find the oil cap difficult if not impossible to remove against the pressure difference. The failing AOS subjects the engine crankcase to extremely low pressure.

If the cap comes off without any difficulty the AOS can still be bad just not to that point of badness. One can make a vacuum measurement of the crankcase and if it is too low this strongly hints at a failed AOS. I don't have the details but a search should turn them up. The Pelican parts site might have a white paper on what the measurement should be.

While you are at the oil filler tube cap check that the oil filler tube cap seals properly. Try to wiggle the cap and pull it up. If you can do any of these and the engine reacts the cap is leaking and should be replaced. I had to replace 2 caps over the years. They wear out. (The o-ring wears/shrinks and lets air past. The leak is an idle only leak the amount of air that gets past at higher engine speeds is immaterial.)

Never happened to me but the techs told me it is not unknown for an intake manifold O-ring to leak. A "smoke test" can be used to detect any air leaks at the manifold or at other places, like a vacuum hose.

Down on the list is fuel supply. The car is new to your buddy but is not a new car obviously. Servicing of the car by previous owners is always suspect. I believe the 1998 Boxster had a serviceable fuel filter. (My 2002 didn't have one, the filter was part of the fuel pump housing in the fuel tank.) If the fuel filter is serviceable be sure it is is not due to be replaced. I'd be tempted to just replace it to thus to eliminate it as a possible source of the current problem or one in the future.

maybeillbuyit

Macster, Thx for reply. Some of what you say I've read before when I first started searching. Leaking oil cap and failing AOS. I expect servicing by the previous owner to be suspect for sure .This car looks like its not been loved for a long time. Unfortunately I don't think it will improve much with my buddy, hes not one to lavish time or money on a car. I'm just attempting to get it idling better and clear the codes and warning lights as he plans to sell. In the back of my mind I'm thinking if I like what I see while I'm working on it I may keep it ; ) . We'll see. I take a look at all this, many thx.

Macster

To better know if the car is a good car to own should your buddy decide to move the car on it is even more important to get the engine running better and eliminate the error codes which means you'll have to address the P1124/P1126 errors.

I believe the error codes arise from a fueling problem when the engine is operating at low engine speed so my WAG is an intake leak of a mild sort -- the cap pardon the expression "fits" this classification -- is going on. The AOS can start out a leak of a mild sort based on my experience after going through 3 AOSs over the years but it wont' remain a mild leak for long. At some point the engine is at risk of serious damage, possible even fatal damage, as the failed AOS can result in the engine ingesting oil and suffering from hydraulic lock.

As the possible causes list makes clear a fuel supply/pressure problem is down the list. If the fuel supply/pressure was a problem the engine would run lousy at higher RPMs and I don't believe you mentioned that it does this.

Because the history of the car is probably a big unknown there's a chance the car was visited by rodents. So as you work on the car keep a sharp eye out for rodent sign. Trash on top of the engine, on top of the plastic panels under the car. Feel the rubber hoses/wiring harnesses for signs of gnawing. I've seen them chew through the plastic fuel vapor lines that run between the engine and fuel tank. They have also destroyed engine wiring harnesses and created bare patches in the carpet and chewed through cabin air vent plastic tubes and well, just made a real mess of a car.

maybeillbuyit

Yes what you say is correct. The car seems to drive fine at speed. Its just at idle theres a stumble. I've seen and heard of lots of rodent stories. The car been looked at by two mechanics and no report of rodent damage etc. Although given that its next to impossible to even see the engine on this car with out removing covers etc I'm not sure how much investigation has really gone on. I'll be doing than soon. Sorry I have to ask, what is WAG? I've not heard that term before. Thx again

mikefocke

To help you with acronyms, a list specific to Porsche is here. In this case WAG = Wild *** Guess. Guestimate, SWAG, etc.

I suggest you follow the diagnostic steps in the repair manual. And not just throw parts at an not yet thoroughly diagnosed problem. A great Porsche mechanic schooled and experienced with the M96 engine will be your friend. One with the proper Porsche diagnostic equipment. You are looking for something that affects both sides of the engine.

Post where you live and maybe someone has a recommendation of a mechanic for you.

maybeillbuyit

Mike, sounds like good advise, I'll look for an online copy of the repair manual

Macster

"WAG" : wild *** guess.

If there was rodent damage almost certainly the techs would have spotted it. They might not see the actual damage at first because sometimes the damage is not readily visible but the signs of rodents having been in the car is quite distinct and if rodents have been in the car there's rodent damage. Also, in the cases I'm aware of the car was brought in due to a CEL and in some cases the engine not running right so when the tech began investigating he spotted right away signs of rodents then checked for damage and found it.

With no rodent damage this takes us back to the error codes and the possible causes:

Intake air system leak.
Fuel pressure too low.
Fuel supply inadequate.
Fouled fuel injectors.

You, or someone, who's more qualified, needs to eliminate the above causes until he discovers the cause. And I'd start at the top of the list. There's a reason why Porsche listed intake air system leak as first...That's the most common cause of these low engine speed air/fuel mixture problems.

maybeillbuyit

OK thx , I'm planning to take a look for intake system leaks and the AOS this week sometime. You've given me some great leads. I agree it makes more sense to have someone fully qualified to diagnose, which is what I did when we took it to the second mechanic, unfortunately that didn't seem to pan out with this strange " water in airbox" diagnosis. Thx

maybeillbuyit

I thought I would add to this thread. I ended up finding that the oil separator was gunked up with oil and the lower bellows wasn't even connected properly. ( lose gear clamp) . There was oil streaming down that side of the engine. Replaced the oil separator. Check engine light and error codes gone. Car idles and runs much better. Thx to all who replied. Only about 10 more deferred maintenance issues to go. Thx . Dave

Macster

Thanks for the update.

Good old AOS. I went through 3 of them with my time with my Boxster.