W210 P170 & P173 Codes. Help!

[Bill '90 300E]

I have an ongoing CEL situation with my 2000 E320. The codes suggest the MAF. I first tried cleaning the existing MAF to no avail and have since replaced the MAF with what appears to be a "good" replacement that I procured from the local pick 'n pull. I cleared the codes and installed the "new" MAF and drove the car for about 50 miles, shut it down for a half hour or so, started back up and no CEL. YES!!! Fixed...oh, but no. The CEL came back on again. I am due for state emissions inspection and this is becoming an exercise in frustration. In searching, some said the cure was in the MAF replacement, some with a gas cap cleaning or a new cap. A friend suggested that it may be a vacuum leak issue. I'm baffled and need to get an inspection. Any suggestions? Thanks in advance...