Chris -- your CEL issue sounds familiar. I too have a '99 320 with an intermittantly lit 'Check Engine' light. If you did a search on this site regarding the problem, you may have seen a posting from me on 04 10 09 responding to a thread started by Alan S entitled 'Check Engine Light on ML320'.

There is some similarity in our retrieved codes but despite the replacement of the MAF, the light has never stayed out for very long. Actually, it is unlit for longer periods when the ambient temperature is low, i.e., minus 25 degrees Celsius or so.

My point is simply that MAF R&R may not solve the issure for you. Short term saving of your old MAF in case the light comes back on might be prudent. It can always be put back into service -- immediatly or in the future.

Good luck

Geoff