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MAF Sensor / Engine breathing
Posted to Technical Theory Forum on 12/7/2012 57 Replies

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 system.

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 restriction.

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 cause misfires.

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 skewed.

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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car Vehicle Data

2007 Mini Cooper S 1.6L

Engine1.6 L
Trans6-speed Standard