Can suffer electrical problems, so make sure all electrics work before buying.
Two other well known faults which might cause expensive damage are failure of the oil seals to the wiring looms from the top of the cam cover and to the autobox which is electrically operated.
Reports of leaking high pressure diesel pumps on E320 CDIs. Fluid leaks onto cooling system pipes which then also need to be replaced. 'Straight 6' 3.2 litre CDI engine had a common problem with premature injector failure at as little as 60,000 miles. Replacement injectors are about £300-£400 each, plus fitting and calibrating to the ECU - an expensive repair if they all need replaced at about the same time.
Another common problem with the earlier engine is apparent failure of the injector 'fire seals', which allows soot and carbon to basically 'weld' the injector into the cylinder head. If a faulty injector cannot be removed then the cylinder head would have to be replaced.
Turbo ‘boost control regulator’ seems to fail on E320 CDI (pre-2005), putting the engine into limp-home mode and MB dealers diagnose the need for a entire new turbo at £1,600+.
Electronic throttle sensor failures on E320CDI/E280CDI V6. Further reports that this is due to Turbo swirl pot failures. The electronic throttle warning light coming on, closely followed by the engine management light and the engine goes into 'limp home' mode.
Tinkling sound which rises with revs on E320 CDI is from ceramic matrix of catalytic converter.
Also starting problems with E320 CDI. Can suffer battery drain down when left in airport carparks unless upgraded battery pack specified.
Cars fitted with self-levelling suspension are prone to running down their batteries after a long ferry trip or after being left parked for a prolonged period in windy conditions because the system attempts to compensate for movement of the ship or being blown by the wind.
The transmission oil cooler of automatics is sited in the radiator matrix, so failure can lead to coolant entering the transmission and destroying it, leaving the owner with a bill of £4k +. Apparently this fault occurred in cars manufactured between 2000 - 2003 and only when a Valeo radiator is fitted. Two other well known faults which might cause expensive damage are failure of the oil seals to the wiring looms from the top of the cam cover and to the autobox which is electrically operated.
Another common fault of Mercedes diesels is failure of injector seals, allowing fuel/air mixture to be deposited as carbon on top of the engine. The problem can be identified by the smell of neat fuel (like paraffin) entering the passenger cabin, and a "chuffing" sound from the top of the affected cylinder as gas escapes on the compression stroke. Apparently it is so common it has been given the name "black death" within the Mercedes dealerships. Unless spotted early, and seals reground and replaced, the cost of fixing can be up to £500.
Seem to have been a couple of cases of front suspension collapsing on 03s and 04s. That's spring failure, common on all makes and models of European car, not the spring 'perch' failure that afflicted the rust-prone W210 E Class.
Corroded brake pipes are becoming a common problem on 2002 - 2006 W211s. Not a problem on facelift cars with the revised braking system from Summer 2006 on.