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Old 11-18-2014, 07:36 AM   #1
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STFT spikes of -98.4% causing P1128/P1130 on 2001 Porsche Boxster. Need Help!

I have a 2001 Porsche Boxster 2.7L with about 55,000 miles on it. Recently I have been getting CEL for P1128 and P1130. I have an ODBII with some recording capabilities and was able to record a driving session with STFT for both banks. I looked through the data and for the most part it is within the +- 10% mark. LTFT was set to +3.3% on both banks. What I found in the data that was very interesting is that there are short spikes of -99% trim on both Bank 1 and Bank 2. This only occurred once on each bank over a 20 minute drive

I believe this is what is causing the P1128 and P1130 codes to occur. I just don't know why the trim would spike to this for short times. Below is an example of the data:

LTFT1 +3.3%
LTFT2 +3.2%

TIME STFT1 STFT2 MAF Air Flow
115.5 -3.1 -3.9 0.73
115.953 -3.1 -1.6 0.73
119.297 -3.1 -98.4 0.73
119.703 -3.1 -98.4 1.92
120.094 -0.8 -98.4 1.92
120.5 -0.8 -2.3 1.92
120.891 -0.8 -2.3 2.08
121.297 -3.1 -2.3 2.08
122.594 -3.1 -5.5 2.08
124.391 -3.1 -5.5 2.55
......
768.453 1.6 3.9 1.49
768.797 2.3 3.9 1.49
769.141 2.3 0.8 1.49
772.203 -98.4 0.8 1.49
772.5 -98.4 0.8 1.49
772.797 -98.4 0.8 0.6
773.094 -0.8 0.8 0.6
773.453 -0.8 -2.3 0.6
773.797 -0.8 -2.3 0.56
774.141 0 -2.3 0.56
774.453 0 0.8 0.56

The car does not run poorly in any way. Here's what I have tried so far from suggestion in this and other forums to resolve P1128/P1130:
  • Replace AOS (replaced 2000 miles ago for unrelated issue)
  • Check Oil cap for air leaks
  • Clean MAF sensor with MAF sensor cleaner (engine runs more smoothly now)
  • Check air filter (replaced 2000 miles ago)
  • Checked for disconnected vacuum hoses
  • Check that oil cap seal was not leaking

From the chart above, it seems like the MAF is doing its job. Perhaps there is some faulty wiring that is causing a short to battery or short to ground causing the intermittent spike. Has anyone seen something like this before, or have any ideas what could be causing it.
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Old 11-18-2014, 08:45 AM   #2
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Those two codes are for a lean condition at idle (both banks).

Since you have some kind of OBD2 tool, does it provide Freeze Frame data? FF data gets captured when the CEL is set and has most of all the sensor parameters in memory. If you can obtain that info, post it here so we can understand what's going on. Also, does your tool have Mode 6 capabilities?

BTW, the fault areas for those codes are:

1) Incorrect signal from MAF sensor
2) Intake air system leaking
3) Fuel pressure too low
4) Volume supply of fuel pump too low
5) Fuel injectors fouled
6) Exhaust system leaking

Regards,
paul...
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Old 11-18-2014, 09:15 AM   #3
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Hello,

Here is an image from what my app will provide as far as the freeze frame when the P1128 code appeared.



I'm not sure what Mode 6 capabilities are, but it does scan for all available modes. Also I can perform more captures while driving but I am limited to capturing 3 at a time.

I also may be able to view more things using a different application on my laptop.
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Old 11-18-2014, 10:30 AM   #4
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I would look for an intermittent intake or exhaust air leak first. This seems most likely.
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Old 11-18-2014, 12:05 PM   #5
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I have one last thing to try. I purchased a new oil cap to make sure that it wasn't that seal that is leaking. It didn't sound like it was leaking but it's hard to tell. It had a light vacuum when I pulled it off during idle.

I will most likely be taking it in for a smoke test to see if anything is leaking.
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Old 11-18-2014, 12:18 PM   #6
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Also, pinch off the hose at the throttle body that connects to a hose going under the driver side intake tubes. If the trim values go to normal, the problem is the valve connected to the hose.
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Old 11-18-2014, 01:16 PM   #7
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Quote:
Originally Posted by san rensho View Post
Also, pinch off the hose at the throttle body that connects to a hose going under the driver side intake tubes. If the trim values go to normal, the problem is the valve connected to the hose.
I will give that I try. I wonder if that may have something to do with it. During the AOS repair I removed the air intake manifold on the passenger side. There was some oil residue inside it most likely due to the AOS being on it's last legs. Perhaps that oil damaged or clogged up the one way valve attached to the driver side vacuum. I can try to pull it and see if there is oil within the hose going to the secondary vacuum system.
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Old 11-20-2014, 04:35 AM   #8
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I would agree with what Topless recommended -- the freeze frame data looks good with respect to fuel trims and the code was set at idle/low load and not at high rpms/high load.

Although it's not obvious, even a pin hole in the exhaust between the engine and upstream O2 sensor will suck in enough air to make the O2 sensor detect more O2 and make the fuel trims go way out in adapting.

Regards,
paul...
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Old 11-20-2014, 06:16 AM   #9
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LTFT in the +5 range don't seem normal to me.
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Old 12-01-2014, 12:52 PM   #10
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Thanks for all your replies.

I took it to my local mechanic to have a smoke test done.

Here's what he found:

- I had replaced the Air-Oil Separator myself and it was a PITA. The mechanic said I did a good job getting it all back together but one of the clamps at the bottom was on a little crooked and wasn't sealing properly. So that was causing a small vacuum leak

- There were some leaves that had gotten sucked up into the air box that were preventing some of the air flow to the engine, so he vacuumed that out and got rid of them.

- The throttle body wasn't super clean (no surprise there) so he went ahead and cleaned it all out for me

- The MAF sensor while perfectly clean, did not operate as well as their test MAF in the system. He recommended getting a new MAF and the car should be back to perfect. I'm glad they were able to do the test because a new BOSCH OEM part is $280. I didn't want to spend that unless it was necessary.

Again, I appreciate all your help. Hopefully someone will come on here and it will help them. Without the smoke test, I don't think I would have been able to locate the problem.
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Old 12-02-2014, 04:52 AM   #11
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I got a new MAF from Amazon, just be sure that you have the right part number. The 101Projects on MAF has a good description of the different part numbers and what years they map to. Then search Amazon for the part number.
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Old 12-02-2014, 08:13 AM   #12
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I ordered it from Pelican Parts because I have used them many times in the past. Usually my part arrives next day with free 2-day shipping.
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