I took the MAF out in the spring to clean it, It has never thrown any codes. Took it out again to clean it 6 months later and found this. Car still does not throw codes (other than a cyl 4 missfire at idle when cold outside) and seems to perform well. Any ideas what is that green build up on the hot wire post is and if it will hinder performance?

 

The MAF is not a hot wire MAF. (The wire in a hot wire MAF is so fine it is hard to see.)

The MAF is a hot film MAF and the working "film" is in the small passage way towards the end of the MAF.

Clean with the proper cleaner the MAF if you want and put then put the MAF back where it belongs.

The misfire is probably due to a bad coil. UK Porsche owners seem to go through coils quicker than anywhere. Must be the weather...

Oh, when were the plugs changed?

 

Just did plugs and coils a month ago. It's always cylinder 4, when's it cold outside.