Thanks MB Doc:
Problem solved.
The dealer ran another smoke test and could not come up with a definitive diagnostic. Maybe this, maybe that, we'll see
Next step he said was to replace most opf the valves and hoses in the vac. system. Cost estimate: «between $1,200. and $1,500.... but not really sure.....

I went back to my old back yard shade tree mecanic and spent 45 minutes explaning the problem and what had been done at the dealer.
Within a hour, he found out that the vapor canister purge valve was stucked.
($245. at the dealer + labour)
The valve was dismantled, cleaned, oiled and put back on the car.
Total cost, less than $100.
it has been two weeks now, the check engine light has not reapeared yet.

I would avoid this dealer in the future. Sounds like they are good parts replacers and nothing more.

__________________
1989 300CE

I have the same car with almost identical miles.

Could this part be related to another problem that I occasionally have? Sometimes I cannot completely fill the tank. The hose from the gas station will keep shutting off. I used to think that this was related to the different nozzles at the different stations but it happens pretty randomly.

How do you access this part?

Thanks,

glenmore
1991 300CE
1999 C280
2000 C280
2000 C230

This part is behind the inner plastic fender on the rear left side of the car and is connected to a kind of black plastic tank.
I don't really know if your problem could be related you are refering to.
For your info, that little cheap plastic tank I am refering to retails for $450.

I'll wait another few weeks before going back to my FORMER dealer and have a talk with the service manager....