My W211 recently developed a rough idle at certain times, and on two occasions a check engine light came on. Reading the fault codes with my iCarsoft CR Pro+ pointed to a faulty ignition coil, specifically the one of cylinder 5. Luckily replacements aren’t that expensive. I went for Delphi as one of the six was already a replacement Delphi, and I ordered two to have a spare, as this is a pretty common issue.
Replacement starts with the removal of the engine cover. First pry off the front part, which is held on with two clips. Then disconnect the hose to the secondary air pump and the two hoses to the air filters. Be careful not to damage the flexible part – grab them by the plastic and turn and wiggle to get them off. The main engine cover/air filter housing is clicked in place in four locations and can simply be lifted upward with a bit of force.
This reveals the valve covers. In order to replace the right coil, you need to know how the cylinders are numbered. Mercedes uses standard German numbering, which starts at the right front. Looking into the engine bay, these are the cylinder numbers:
The ignition coils are held in place with two Torx T30 screws. First disconnect the connector, then unscrew the two screws. Pull up on the ignition coil, which may require a bit of force.
From there on it’s the reverse of disassembly. Place the new coil, screw it in place, and connect it. Place the air box (make sure that it is properly seated on all four points), then the three hoses, and finally the front cover.