Originally Posted by Splitpin

What makes you think that, and what model and model year is this car?



If it's running lean under boost, no. Under boost the mixture is calculated via rpm and the airflow meter only. The O2 sensor only affects the mixture during off boost/low speed cruise.



The "unplug the AFM and see if the engine stalls test" is complete bullsh*t. Unfortunately it has been repeated by enough people on here long enough that it has come to be regarded as "fact", which is a problem when it gives false confidence and can (both ways) do more harm than good.

It proves nothing, one way or the other so if you are using this as evidence that your air flow meter is "good", I'm afraid it simply doesn't work in practice. At this point, for all you know, the MAF might be good, or it might not be, this "test" doesn't help you determine either way.



Do all four plugs look the same? From what you've said so far we know a couple of things. First of all you have an AFR meter, that, presumably, piggybacks onto the standard lambda sensor for its input, yes? Is this telling you that your AFR's has recently leaned off under load? If so there are two possible explanations:

First of all, your lambda sensor has degraded a little (or reads wrong when it gets very hot) and as a result, the reading on your AFR meter is showing lean but the actual mixture is correct.

The other possible explanation is that either due to the engine being able to ingest unmetered air somewhere (check the induction pipework again), or due to a failing airflow meter, the mixture is being set too lean.

Are you monitoring knock? If so, has the activity you see there also increased?



The stock narrowband sensors tend to become unresponsive and/or inaccurate when got really hot (i.e. foot down for extended periods of time) so this may explain what you're seeing - the reading recovers as the sensor cools back down.

However, as mentioned the lambda sensor has nothing to do with setting the mixture once into positive boost/high revs. If the plugs tell you that it's lean, then your best bet would be to beg, borrow or steal someone's DeltaDash (etc) and actually monitor the readings from the MAF sensor (and the final mixture value) as the car is being driven.