the problem is the pipe being alot wider than the original intake pipe. you need to reduce the volume of air passing over the sensor. something to disturb the flow.

on the 2.4 we had the same issues and the soloution was to reduce the size of the pipe ever so slightly just before the maf.

i did this with a jubilee clip. i tightened it as tight as it would go and this reduced the size by half inch, but was enough to disturb the flow and allow the maf to get the correct readings.

so you still get the benefits of a free flowing pipe without the warnings.

just make sure the maf is exactly the right way round

i started with it pointing slightly away from the airflow, the move it day to day until its in the right position