Does your "mechanic" actually have any experience with Mercs? He seems to be just grabbing at straws.

With your symptoms the first thing I would do is an EGR functional check if the engine has an EGR system.

Duke

The air pump has a test done on it by the ECU , where it gets energized to see if the o2 sensor reads a lean condition..
You have unplugged the pump, but the Pump SOV is also energized at the same time [ from the same power source] , opening the control valve between the pump and the engine.
BC you say it happens when the pump would come on, I would look for a vac leak at this main hose [ around the control valve], as the valve is still energized even though the pump is not...
I would also see if the check valve is gummed up and stuck open,, this would open the exhaust to pump plumbing when the SOV gets a signal....

I like Paul's idea about the air pump. If it was engaging while warm, it was also probably activating the check valve. If you have since disconnected the pump clutch you still may have the check valve opening. Paul is right that the exhaust will aspirate air in through the pump if that valve goes open. This will fool the O2 sensor into thinking you have a lean mixture and the thing will go full rich trying to compensate.

smog pump rebuilder
 

Fred,

I recently worked on a 1990 190E with a bad smog air pump. It was making a terrible noise whenever it kicked in. It was defective and here in CA we can't pass smog if it's not working. I searched around for a new or rebuilt one and the best price i got was for a rebuilt one from CVJ Axles in Denver. Cost was $295 and has a 3yr warranty.

I'm giving you this info just-in-case someone tries to sell you a $600 pump and you have to put it in for smog purposes.

Thank you to every one for the replys, I love this board!!!!!

The car is now in the hands of Enrique, I'll let you know what the outcome is.