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Hi All


My first post here so please bear with me but pretty desperate for help.  Have 2004 Boxster S and its, in general, running a bit rough (feels fuel rich).  It also will also, on occasion, not go.   Pulling up at a busy intersection and car reduces to Idle.  Then put the foot down and the car stays at idle.   Normally an engine restart gets things going again (although not always and sometimes requires a few goes).  


Bought a computer reader and found error codes

P0102:  Factory Fault code 115 - Mass air flow sensor - below limit

P2135: Accelerator Pedal - Signal Implausible

P0121: Factor Fault code 117 - Throttle value or accelerator pedal potentiometer - signal implausible

P1266: Factory Fault Code 409 - Fuel shutoff function monitor - signal implausible


Took the car to Porsche.  They suggest a MAF sensor was the root cause and replaced it.   It felt better for a few days then problem returned.  They replaced another MAF sensor.. Same problem.   They then said the MAF reads 0 and have checked the cables so its me DME and it needs replacing (not cheap)


Have taken the car back home and looked at the likely affecting values myself from the computer.


1. MAF

The computer shows to different values for MAF

Mass air flow (HFM) is always 0 regardless of if the car is working correctly or not and at any accelerator level 

Mass air flow seems to move within a range of 18 (idle) to 100 (full throttle)


2. Throttle  (since it had error codes on this)

Pedal encoder potentiometer 1 and 2 always move and the pedal value goes from 0-100% when its working but stays at 0 when its not.  Since the potentiometers always move I am assuming the DME simply can't add things up so sticks the pedal value at 0.


ie.   The MAF sensor reading above confuses the DME and it reduces the throttle to 0.  That feel consistent with the behaviour of the car (its just sticks at idle)   


Any one with any clues?  Certainly don't want to replace the DME unless absolutely necessary....









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I would hope they took the time to clean the electrical connections on the both the throttle valve positioner and the accelerator pedal electrical connector.

Using their PIWIS tester they can check to see if the throttle butterfly is opening all the way when you press the accelerator pedal - if it does not then the e-gas throttle valve positioner or the accelerator pedal sensor is not working to send the signal.


I would expect an MAF fault if the throttle butterfly was not opening.

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Thanks Loren... Might be a lot to assume anything from Porsche service so will check those myself on the weekend.   The "raw value" from my super cheap computer reader from the accelerator potentiometers does change so seems unlikely to be that. i have attached the values and you can see the "Mass air flow (HFM)" is stuck at 0.    It never moves.  The other "Mass air flow" value seems to move in what would be expected with increasing RPM and has a value when at idle of about 18".   Hence i suspect its related to this (see attached)


When i had the computer plugged it i didn't see the throttle values changing but didn't record them.  I will rerun that over the weekend to see if its working roughly as i would expect.


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Looks like the culprit may be the pedal position sensor. The reading of potentiometer 1 needs to be exactly double that of pot 2.

With key ON engine OFF, monitor these two numbers while you press on the gas pedal slowly for the whole range of motion of the pedal, then repeat a few times. Look carefully for spots where these two numbers are not 1:2

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Thanks Ahsai... I took the readings and at no time is POT 1 double POT 2.   I have attached idle, full and somewhere in-between readings (see attached).   Also, the POT 2 value moves.  Its never static, even at idle (at idle it moves between 0.47 and 1.0 while POT 1 is static at 0.78V).    Would that be enough to suspect and replace the accelerator pedal?


There were two mass airflow readings as well

 "Mass air flow (HFM)" is stuck at 0 - always.

"Mass air flow" moves with engine RMP


I was headed in that direction until you mentioned the accelerator pedal










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