Posted to Technical Theory Forum on 12/7/2012
MAF Sensor / Engine breathing
While I am working on a 2007 Mini Cooper S with a Turbo
(N14), this post is more about general theory. This Mini has
a pretty consistently reoccurring P115C / 2B5C AIRFLOW
PLAUSIBILITY code. It has no other codes. According to my
information this code indicates actual air flow is lower
than expected. The vehicle has an intermittently varying
harmonic surge at idle, but no other drivability symptoms.
The freeze frame is always after it has been running quite a
while (1500+ seconds) and always at about 800rpm with
vehicle speed being 3-7mph. The fuel trims in the freeze
frame rarely are more than +/- 1. I would love to have a PID
that indicates air flow error, or something similar, but I
don't seem to. In all my test drives the OBDII fuel trims
are well balanced. I'm not familiar enough the Mini factory
side of the data to make much use of the fuel trim
information available to me with my aftermarket tools.
Lastly, this vehicle has a brand new factory MAF on it.
I suppose a quick question is: Is OBDII fuel trim
reliable/accurate for this vehicle?
My main question about theory relates to this MAF
plausibility code and engine breathing on a single bank
In any engine with a throttle blade I would imagine that a
majority of the time the throttle blade is the greatest
restriction. At some point of throttle opening I would also
imagine that valve ports may become the greatest
If there is a leak between the MAF and the back of an intake
valve then there should be evidence of this in the fuel
trims. If there was enough of this issue I would suspect the
engine management system would set a fuel adaption code.
Low MAF reading versus expected could also be caused by an
intake or an exhaust restriction. Anything that reduces the
air flow of the engine, itself. In both of those cases there
shouldn't be a fuel trim problem as the MAF is reading what
is actually getting into the engine. Having a turbocharger
or supercharger shouldn't make a difference, even if there
was a problem with it, because it would be behind the MAF,
right? Would an issue with the turbo/supercharger generating
boost cause lower than expected MAF readings?
A backpressure test, of some sort, should reveal a
restricted exhaust. I would also think there may be a
power/drivability issue evident.
A restricted intake would also lower actual air flow. A
restricted intake from carbon, for example, on many of these
GDI engines. In the event of a restricted intake on a direct
injection engine there shouldn't be a fuel trim/adaption
issue. Wouldn't that intake have to be pretty severely
restricted to be the greatest restriction, in most cases?
Especially at idle/near idle RPMs? Wouldn't an intake
restriction have a more severe effect with increased engine
RPM? Also, I would think an intake that restricted would
This vehicle does have a previously identified moderately
carbon'd intake manifold.
Lastly, any valve timing and/or variable valve timing issue
could change the breathing characteristics enough to set a
plausibility code. I would think these conditions would also
set additional codes. I'm not entirely sure if there would
be a fuel trim issue present, but if the MAF is reading the
actual air flow, I don't see why the fuel trims would be
I am trying to diagnose this Mini, that is true. I'm also
making sure my MAF and air flow theory is sound and correct.
Every so often we run across a vehicle that challenges the
way we think about things, and what we think we know. This
vehicle is like that for me, and that's why I posted this in
TTF instead of TDF.
Shannon from California
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57 Replies Received