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Unread 05-19-2010, 11:56 PM   #1

Name: kazamali
Title: United Newb
Status: Offline
Join Date: Apr 2009
Location: San Jose, CA
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Your Ride: 99 M3
Calling all fuel trim experts!!

Hey guys,

I've been chasing a fuel trim issue for quite some time and need some expert help. I'm including a lot of data, so hopefully it will accelerate and narrow the areas of investigation.

The facts:

1999 E36 M3, 96k miles, Supercharged via AA Twin-screw (9lb psi), stock exhaust

-O2, MAF, Fuel filter and regulator replaced 1 year ago
-Spark plugs and fuel injectors replaced a few weeks ago
-Car throws 1188/1189 codes (fuel trim) consistently after a few "trips"
-Plugging in an OBDII data logger shows positive fuel trim(10% - %20+) all the way from idle, hard acceleration and cruising speed.
-Car has had supercharger for 4 years now and initially had trim issues when first installed(vacuum leak), but has run fine since for 2 solid years without issue
-Car is showing larger positive trim at cruising speed, not at idle (the latter usually indicates a vacuum leak as vacuum is larger at idle)
-Car shows no physical signs of issues (i.e. no limping, hesitation on acceleration, nor rough idle)

I just need some direction on where to go next since these codes are preventing a successful smog check (resetting the ECU doesn't work because the "monitors" in the ECU will reset). This is really looking like a vacuum leak issue (which, at this point, is going to require a smoke test because I've been over every hose and seal a million times), but I wanted to make sure if this is possibly pointing to a fuel pump failing or another MAF issue.

I'm hyperlinking graphs of a 5 mile trip that show values of short term fuel trim for both banks at different RPM/Throttle Position plot points AND MAF volume reports at the same plot points. (note that idle is at around 800 RPM and 15% Throttle angle and cruising speed is 3200 RPM and %30 Throttle Angle) On top of that, I also have a link that shows the same trip, but in a line graph showing the same values in a different way. Note that I've reset the adaptations to start off fresh and that's why the Long Term Fuel Trims are small (they WILL grow to %12 percent if I drive long enough).

I need someone to take a look and see if there's a definite smoking gun here so I don't blindly go and check a bunch of other things unnecessarily.

I'm trying to be as thorough here for you guys as much as possible, but let me know if I've missed anything.

Table of STFT and MAF readings

Graph of data (for the same trip)



Last edited by kazamali; 05-19-2010 at 11:59 PM..
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