Failed spark plug wires and/or ignition coils can cause intermittent misfires (rough running); the Check Engine Light may also illuminate. Diagnoses of the ignition system will be necessary to determine which components are at fault.Google+
Car Problem Reports
Mercedes-Benz R500 Bad Spark Plug Wires/Ignition Coils May Cause Misfires and Possible CEL
Mercedes-Benz R500 Problem
Visitor, 2007 Mercedes-Benz R500, 124,000 mi
Check engine light accompanied by rough engine, especially at idle. Symptoms seem to correlate with low fuel. As fuel level goes below 1/2 tank, the likelihood of the symptoms returning increase with the symptoms alway returning when fuel level goes below 1/4 tank. When fuel tank is refilled to full, symptoms always disappear. Replaced #7 fuel injector which eliminated the rough engine but did not eliminate the check engine light, which continues to illuminate as fuel is used. Mercedes technicians suspect either one of two fuel pumps or the logic that shifts between the two may be an issue. More troubleshooting required...