Thanks to Gordon on .net he found another potential air leak into the inlet manifold on the M54 engines but not sure if it applies to the M52's or not.
There is a small blanking plug over a pipe on the rear of the inlet manifold near to the dip stick. When I took mine off it just about fell off as it was split both sides for nearly half it's length and another small split in the middle.
I used superglue to glue the rubber edges together, just as we do when making one off 'O' rings, and then wrapped tape around to make a temporary job. When refitted was a much snugger fit.
Pt No 11611727176 @ £1.45, BMW OEM
Here is general location, yellow pen pointing.

Closer look at the plug.

There is a small blanking plug over a pipe on the rear of the inlet manifold near to the dip stick. When I took mine off it just about fell off as it was split both sides for nearly half it's length and another small split in the middle.
I used superglue to glue the rubber edges together, just as we do when making one off 'O' rings, and then wrapped tape around to make a temporary job. When refitted was a much snugger fit.
Pt No 11611727176 @ £1.45, BMW OEM
Here is general location, yellow pen pointing.

Closer look at the plug.
