Ben,

From my chair it can be anything? Get your self a OBDII tester, and run the codes..

A car that old with so few miles says a lot of " stop n go" miles in the city, and the emissions recirculation systems are heavily taxed. In my opinion it always lead to plugged EGR passages/ Charcoal canister / canister valve / air injection / open vacuum line passage. It can be the driver circuit driving these systems from the computer(This was my case on one of my cars).

Really your problem can be anything, but I would not have keep running with a check engine as the one problem that gets worse with time is the plugged EGR passage.. It gets harder to unplug as it gets more plugged. Some cars have a pipe that do this, but the cars that channel the exhaust through the engine block have the risk of some times having to replace the engine, or getting rid of the car because the coat hanger trick on drill to unplug will not work on the one channeling EGR gases back into the intake.

I have 2002 E320 W210 wagon with 170k miles with the same engine as your ML, but highway miles do not cause this as the EGR kicks very little. I have never had check engine on the wagon..

Run your codes...It is the real only suggestion

FYI, the engine gets better running as the EGR gets plugged because the exhaust gases cannot mix into the intake.

All the best,

Martin