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Old 06-24-2019, 02:49 PM   #1
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Boxster with P1124 and P1126

Hey guys,

I have a 99 Boxster 2.5L which has has these two codes for a while. I've been reading a lot on these codes on this forum and other places and I have an idea of things I might want to try next but I wanted to brainstorm what would make the most sense with people here.

Here are some details:
  • 90k miles. Engine runs great, constant power delivery, no rough idle.
  • P1124 and P1126 always come back within a day after clearing codes, always in pair
  • Engine seems to run hot if the car is left running at idle, the temp indicator eventually crosses the middle of the gauge
  • The car fails smog with high NOX
  • I've notice some oil in at least one of the spark plug tubes when I changed the spark plugs a few months ago. It was for the cylinder closest to the cabin on the driver side (cylinder 1?)
  • No apparent excessive smoke
  • Here's what I've tried so far:
  • Replaced MAF. This didn't fix those codes but made the car behave a lot better (lot smoother power delivery, engine seems to run stronger)
  • Replaced gas cap. Didn't fix the issue.
  • Opened the oil filler tube cap with engine hot. RPM dropped and recovered shortly after as expected.

For next steps, here's what I'm thinking:
  • Replace AOS? The oil in the cylinder seems to indicate that the AOS might be bad, but the car doesn't smoke and the old MAF didn't seem particularly dirty when I replaced it so I'm not too sure if that would help. Any thoughts?
  • Air leak? The car seems to be running hot which might be due to an excess of oxygen due to an air leak, making the mixture too rich. This could be causing the high NOX causing the smog to fail too since the car is too hot. The car responded well to removing the oil filler tube cap so I'm not sure if that is the problem. I'm also not so sure how to diagnose that as a DIY-er - I've read about a few techniques like using soapy water but I'm not sure where to start.
  • Replace spark plug tubes? Maybe one of the tube is cracked and is causing the oil to leak through? There's no white smoke though so I'm not convinced of that.

Do you have any additional diagnostic steps I could run? I have an OBDII reader that I can consult. I've read that air flow should be between 13-15 for example. Is that right? Any other values I should monitor?

Thanks so much for your help!

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Old 06-24-2019, 03:21 PM   #2
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Quote:
Originally Posted by sosspistache View Post
Hey guys,

I have a 99 Boxster 2.5L which has has these two codes for a while. I've been reading a lot on these codes on this forum and other places and I have an idea of things I might want to try next but I wanted to brainstorm what would make the most sense with people here.

Here are some details:
  • 90k miles. Engine runs great, constant power delivery, no rough idle.
  • P1124 and P1126 always come back within a day after clearing codes, always in pair
  • Engine seems to run hot if the car is left running at idle, the temp indicator eventually crosses the middle of the gauge
  • The car fails smog with high NOX
  • I've notice some oil in at least one of the spark plug tubes when I changed the spark plugs a few months ago. It was for the cylinder closest to the cabin on the driver side (cylinder 1?)
  • No apparent excessive smoke
  • Here's what I've tried so far:
  • Replaced MAF. This didn't fix those codes but made the car behave a lot better (lot smoother power delivery, engine seems to run stronger)
  • Replaced gas cap. Didn't fix the issue.
  • Opened the oil filler tube cap with engine hot. RPM dropped and recovered shortly after as expected.

For next steps, here's what I'm thinking:
  • Replace AOS? The oil in the cylinder seems to indicate that the AOS might be bad, but the car doesn't smoke and the old MAF didn't seem particularly dirty when I replaced it so I'm not too sure if that would help. Any thoughts?
  • Air leak? The car seems to be running hot which might be due to an excess of oxygen due to an air leak, making the mixture too rich. This could be causing the high NOX causing the smog to fail too since the car is too hot. The car responded well to removing the oil filler tube cap so I'm not sure if that is the problem. I'm also not so sure how to diagnose that as a DIY-er - I've read about a few techniques like using soapy water but I'm not sure where to start.
  • Replace spark plug tubes? Maybe one of the tube is cracked and is causing the oil to leak through? There's no white smoke though so I'm not convinced of that.

Do you have any additional diagnostic steps I could run? I have an OBDII reader that I can consult. I've read that air flow should be between 13-15 for example. Is that right? Any other values I should monitor?
Thanks so much for your help!
The two codes are for p1124 a fuel pump relay output stage problem and p1126 O2 sensor adaption problem.

My first guess is that you either have to much air for the amount of fuel going into the engine.
Or not enough fuel for the amount of air going into the engine.

So lets look for to much air first!!

That would be caused by a vacuum leak.
It could be the "AOS" Or lots of other things that cause vacuum leaks when faulty.
Excessive unmetered air causes a lean fuel condition and can drive temps up.
Very first thing I would do is a proper vacuum test of the intake system.
If you have a vacuum leak then that gives you a starting point.
If you don't have a vacuum leak then your AOS is probably ok as well.
A vacuum gauge is a diagnostic tool that I think should be in every DIY,s tool box.
They cost around $15-$20 and can give you a ton of diagnostic info on your engine
.
Vacuum reading should be 19-21 inches of vacuum with a steady needle at an idle.

Second:
Not enough fuel for the amount of air.
You could have a faulty coolant temp. sensor.
Use your OBDII scanner: to monitor your coolant temp.
Start the car from cold.(make sure scanner shows the cold coolant temp) run engine till it reaches operating temp.
see if scanner reading coincides with dash instrument temp gauge.

If your OBDII scanner shows you live data:
Then look at your "Short term fuel trims" if they are showing "high positive trim"
that may be another indicator of a vacuum leak.

Those are the first few things I would look at:

You need to determine if the root problem is to much air for the amount of fuel.
or not enough fuel for the amount of air.
Then you will know which direction to go.

The oil in the spark plug tubes is unrelated to the CEL codes

To what air flow (air flow between 13-15) are you referring to????
Just trying to get on the same page with you.

Last edited by blue62; 06-24-2019 at 04:26 PM.
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Old 06-26-2019, 07:47 PM   #3
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Blue62 thank you for sharing your diagnostic approach. I donít have this problem but you intel will help understand how better to use the durametric tool. Thanks - Rick
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Old 06-27-2019, 08:07 AM   #4
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Originally Posted by Rickvd View Post
Blue62 thank you for sharing your diagnostic approach. I don’t have this problem but you intel will help understand how better to use the durametric tool. Thanks - Rick
Always happy if I can help someone
Two things I think are very important when trying to diagnose a engine performance problem. (CEL or P codes).
1. Todays engine management systems (OBDII"s) number one priority is Catalytic Converter Performance.
Power, fuel economy, what ever is secondary.
So when ever I get an engine performance problem I try to figure out how it relates to Catalytic Converter Performance.

2. A vacuum test is usually the first test I do because:
It is simple to do: the gauge is under $25.00.
A vacuum leak is unmetered air. Meaning no sensor in the OBDII system is reading that excess air until after the combustion cycle. So a vacuum leak many times does not trigger any codes. It may show up as positive short term fuel trim but other things can cause that as well.
If you don't have a vacuum leak you have just eliminated a ton of potential problems.
The vacuum gauge can give you a ton of other information about engine performance and condition other than just a vacuum leak.


Last edited by blue62; 06-27-2019 at 08:13 AM.
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