sigh.. CEL

[stateofidleness]

checked and "cleaned" the MAF yesterday and it looked brand new. car is running top notch. go to start it this morning and minor hunt at idle again. last 3 cold starts were perfect, no hesitation or hunt at all.

half way to work, CEL comes back on (after 3 days of solid driving). Car is running great though.

so I guess my next step is to have the Air Change-over valve and the Electronic Change-over valve replaced.

I had read elsewhere that the Tiptronic models have an extra vacuum valve or something that can also go bad... since it requires taking the alternator out to replace these, I want to replace as much as possible all at once.

what can i expect to pay in labor? and should i replace the seconday air system hoses as well?

[Daniel R]

I have had all the same fun and games you are describing with my 2000S. It turned out to be an MAF on the way out and a pair of oxygen sensors in a really bad way, one so bad that that bank was bringing up misfire codes. When I replaced both the MAF and O2 sensors it was back to normal.

On a side note, if your car has been doing this for some time you may want to check the catalysts for damage. I switched to bypass pipes when I replaced the sensors as it was a less expensive option than new cats. I am now catless, as in Australia we only have to run one set of cats so the Boxsters only have the main cats and one set of O2 sensors.

Good luck.

[stateofidleness]

this all started within about a month.
tonight on the way home from work, i threw another code (only 1 this time)
p1128 which looks like a rich threshold on banks 1-3

i'm suspecting some kind of vacuum leak at this point but will have to dive in tomorrow morning. how does a "driveway mechanic" spot a vacuum leak? any tricks?

i was all ready to just have my air changeover valve and ECO valve replaced, but this code might indicate something different. I scoured the net, but haven't found too many people with just the ONE code, many had the accompanying 1130 code. What does having one code indicate?

*thanks for all the help and suggestions, hopefully i'll get her running top notch again soon*

[Bladecutter]

Originally Posted by stateofidleness

this all started within about a month.
tonight on the way home from work, i threw another code (only 1 this time)
p1128 which looks like a rich threshold on banks 1-3

I believe if you look for a vacuum leak, you will be looking in the wrong direction.
I believe that this code is going to point more in the direction of the two O2 sensors on that bank of the exhaust going bad.

When the exhaust passes through the headers and past the O2 sensors, the computer receives a signal that tells it if there is or isn't alot of Oxygen in the exhaust. If there's too much Oxygen, then the computer richens up the mixture. If there's too little, then the computer leans out the mixture.

Right now, the O2 sensors are saying that the one bank is running too lean, and the computer can't richen up the mixture for that bank any more than it already has, hence the CEL being lit.

If you replace those two O2 sensors, that will most likely resolve the issue.

Some recommend just replacing all 4 at the same time for the best results.

BC.

[stateofidleness]

consensus seems to be that it is the O2 sensors that are REPORTING the problem, but are NOT the problem. They don't really affect the mixture, they just report it.

[Bladecutter]

Originally Posted by stateofidleness

consensus seems to be that it is the O2 sensors that are REPORTING the problem, but are NOT the problem. They don't really affect the mixture, they just report it.

You're most likely going to find out that your impression is incorrect.

If the sensors are faulty, then they will send the wrong signal to the computer, which causes the computer to richen the mixture.

Here, try this:

Switch the O2 sensors from the left exhaust to the right exhaust, and vice-versa.
If the problem follows the O2 sensors, then my suggestion is correct.

If the problem stays with the bank, then you are on the correct path.

It won't cost you anything more than about an hour's time to swap the sensors, and a day to drive the car around.

BC.

[stateofidleness]

sounds logical. i'll give that a try! googling some more and I keep coming across people stating the MAF.. yada yada... When I looked at mine, it looked BRAND NEW! Does a failed/failing MAF have any visible signs of failure or do they just "not work" after some point? also, during reinstall of it, could that have introduced a vacuum leak where it connects? I've read that if you get BOTH codes, it is most likely the MAF, one code and it means something is wrong on that side... (might go along with the 02 sensor theory, but i haven't had ANY indicators up to this point that an O2 was on the way out)

Also, and this may be far-fetched, but I have been using 92 octane (Valero) gas for the past few weeks only because my Shell (only 93 octane in town) is not on my daily route. Could this cause some kind of lean issue as well?

Would filling up with 93 and maybe some additive do anything for me?

[Bladecutter]

Originally Posted by stateofidleness

sounds logical. i'll give that a try! googling some more and I keep coming across people stating the MAF.. yada yada... When I looked at mine, it looked BRAND NEW! Does a failed/failing MAF have any visible signs of failure or do they just "not work" after some point? also, during reinstall of it, could that have introduced a vacuum leak where it connects? I've read that if you get BOTH codes, it is most likely the MAF, one code and it means something is wrong on that side... (might go along with the 02 sensor theory, but i haven't had ANY indicators up to this point that an O2 was on the way out)

Also, and this may be far-fetched, but I have been using 92 octane (Valero) gas for the past few weeks only because my Shell (only 93 octane in town) is not on my daily route. Could this cause some kind of lean issue as well?

Would filling up with 93 and maybe some additive do anything for me?

Well, I just changed my MAF on Friday.
The old one was just as clean as the new one I put on.

In fact, when using the Durametrics program, I was getting the same Air Flow readings and Air temp readings with the new sensor as the old sensor. The only difference was in the Voltage reading, which was 1 volt constantly across the rpm range, while the new sensor increases as the rpms rise.

As for the gas, it would affect both banks, not just one.

If you wanted to get technical, we could break down the different fuel systems piece by piece:

MAF sensor - tells computer amount of air being ingested by the entire engine (doesn't differentiate between cylinder banks)

Air temp sensor (in MAF) - Tells computer temp of air being ingested by the entire engine (doesn't differentiate between cylinder banks)

Throttle body - Controls amount of air ingested by entire engine (doesn't differentiate between cylinder banks)

Vario-cam valve timing - Adjusts opening and closing of valves in relation to crankshaft position and engine speed. Both banks are activated at the same time at certain rpm points, but controls just an individual cylinder head. Should not affect fuel mixture in any way.

Fuel injectors - Each one is assigned to a cylinder. If one was sticking open, could cause a bank of O2 sensors to report an error, but would also be associated with a cylinder misfire, and bad engine performance. (opposite of what you are experiencing)

Spark plugs and Ignition coil sticks - One per cylinder. If one is failed, cylinder misfire, bad engine performance, and too rich of a mixture would be reported on that bank of cylinders. (opposite of what you are experiencing)

Cat converters - Two per bank - the second set of O2 sensors will be able to point out if the primary cat converter is not scrubbing the exhaust optimally.

O2 sensors - Two per bank. First set of O2 sensors controls the mixture as the computer uses its signal to determine the Oxygen content of the exhaust stream. Second set is primarily used to check the efficiency of the cat convertor, and determine if it has fully warmed up, based on the differences between the two sensors in that bank.

The O2 sensor swap is probably your best bet for quick and easy, with no cost involved.

BC.

[stateofidleness]

wow great info!

ill try to swap them when i get home tonight.

any thoughts on what the "thunking" sound is on cold starts? like hitting the side of an empty 50 gallon drum with a hammer.. or a low steel drum.. i hear it randomly about 3 times on cold (not driven in 10+ hrs) starts, if i turn car off and then restart, it's fine, but idles at 1k steady. When i put it in R, it goes down to 500 like normal.

[Bladecutter]

Originally Posted by stateofidleness

wow great info!

ill try to swap them when i get home tonight.

any thoughts on what the "thunking" sound is on cold starts? like hitting the side of an empty 50 gallon drum with a hammer.. or a low steel drum.. i hear it randomly about 3 times on cold (not driven in 10+ hrs) starts, if i turn car off and then restart, it's fine, but idles at 1k steady. When i put it in R, it goes down to 500 like normal.

That I don't know.
I would have to hear the noise from the cabin to hazard a decent guess.

Personally, I would check the condition of the engine mount first, because if it is really bad, the engine could be flopping around when the engine cranks to life. That's easy to do just by removing the engine cover, and driving up and down the street in first gear, flooring the car to see how much the engine moves.

Do you park on a slight incline of decline when it makes the noise?

BC.

[stateofidleness]

flat surface when parked and Tiptronic trans. I might try cold-starting it with the engine cover off and viewing it topside to see if I can better place the source of the noise. Wouldn't I hear the noise on warm starts too though if it was the engine mounts?

seen a lot of posts recently about engine mounts so I was getting curious about mine.. I might try to get a "look" at them if it is even possible without dismantling stuff.

[Daniel R]

Again, I had more or less the same problems. The new O2 sensors fixed everything. I actually could not believe how much the computer relies on their readings for the air fuel mixture. If they are bad they also throw other electronics out and cause multiple error codes.

Try changing them and see what happens.

[stateofidleness]

it's back!

car running fine for quite some time (since last post) with no CEL (only thing is it seems like the idle might be high at startup, 1k)

driving to school like normal today and raced a yellow light
when i stepped on it (read, accelerated real fast) about a minute later, the solid CEL came on... I haven't had a chance to read the codes yet, but I will when I get out of class

again, I started scouring the net for symptoms.. I have a pre-existing exhaust leak that is "minor" but should be noted. Is there any way to "bench test" an MAF? mine looked brand new when taken out to inspect, so I'd hate to replace a good one..

also, my mechanic has offered to smoke the exhaust system and attempt to fix the leak.. what does it cost to have this 'smoking' done?

thanks!

EDIT: I should note that the AOS was replaced since last post as well. Wasn't sure if I mentioned it. No smoke is present now.