Apparently it happens... people put in a new air filter cartridge and do it upside-down. The effect is that the airlifter is way too close to the MAF making the engine grasp for air since just a tiny bit of the filter is used effectively. The engine will start but run badly as soon as some rpm are gained. Air starvation is the word.

Don't get fooled by the print of the rubber material... it can be wrong as this picture shows.

Be aware: the convex is ALWAYS up.

The MAF meters airflow, whatever it is, so a restrictive filter or high altitude won't change the air/fuel mixture: less air means less fuel.

Where things do get messed up is that the MAF only measures airflow in the little venturi gizmo, in the central 1" or so of the MAF housing. If the flow is uniform across the whole MAF then this central measurement is an accurate estimate of the total airflow. And uniform airflow requires some plenum volume upstream of the MAF, so that the air has a chance to settle down and flow smoothly.

Which is Blake's point, and is exactly right-- the hump in the filter does reduce the "headroom" above the MAF intake, which does restrict the airflow to a degree. But it also messes up the distribution of that air flowing through the MAF. If the flow is now weaker in the center then the MAF will under-estimate airflow and reduce fuel to match, which results a weak mixture.

Bottom line: Do it like the factory did it, stock filter and right-side up... Or change it completely (velocity stacks through the hood?) and re-tune.

Jim Corendon


Yes, the hump creates wrong output at the MAF and especially since the sensor is very sensitive in the lower range the LH ecu gets confused. Fuel mixture gets very lean and the engine misses power or dies suddenly when using the throttle.


Theo Jenniskens

1992- 928 GTS Midnight Blue