[olddog]

Depending on the cam shaft profile, you do get into issues with a lot of valve overlap. There can be reversion air flow. The piston will push some exhaust up into the manifold, before sucking air back down, at an idle. The mass air meter does not have a clue which direction air is traveling. It only knows that air movement cooled a heated wire, and it measures the current required to keep the wire at a desire setpoint. The current is proportional to the mass of air that passed by.

If you have reversion air flow the MAF will measure more air entering the engine than what actually entered the cylinders. The huffing and puffing is measuring air passing back and fourth. This will cause the engine to idle rich.

This can be tuned out by commanding a leaner mixture, however it is better to solve the mechanical issue. If you add more pipe between the MAF and the manifold, it should smooth the pulsations out.