The more serious trouble is at the connector. Through it the engine oil gets into the engine wiring harness and travels under the insulation by capillary action, so it reaches the engine control unit and other important parts as well. The loss of the control unit is the end of that road, and serious engine damage can follow; alongside the contaminated harness the most varied fault messages can appear. Cases have been reported where the oil ran out of the connector, and the consequential damage after that was already large.

The check is simple: the connector of the solenoid has to be taken off and looked at to see whether it is oily. In the fault memory a faulty solenoid gives one of the codes 0059, 0060, 0063, 0064, 0271, 0272, 0275 and 0276. One thing is worth knowing: according to the maker only the solenoid that has a corresponding code has to be replaced, so if a workshop has changed all four of them alongside two codes, that is worth asking about.

The engine number decides what is in the car. Above engine number 2729..30 759427 (M272) and 2739..30 193592 (M273) the maker already fitted the improved solenoid, under part number A272 051 01 77. The oil-leaking solenoid is a problem on engine numbers below 2729..30 895140 and 2739..30 236360.

The prevention is the oil stop cable, which has to be fitted between the solenoid and the harness: the two wires contain a sealed copper sleeve covered with heat-shrink tubing, and that stops the oil from travelling further. It is available from the factory and is not expensive: the two solenoids were around 33 euros and the two oil stop cables around 31 euros, while replacing the control unit is a four-figure sum.

The rattle lasts one to two seconds and then goes quiet, and that length is exactly what matters. A rattle of a couple of seconds points to the tensioner and the adjuster, not to a chain that has jumped. The control unit logs a code to match: a timing deviation, not a jumped chain.

It is not only the mileage that marks out the affected cars. The maker issued a service document for this in June 2013, and in it the examples with an engine number below 276 8xx 30 001281 or 276 9xx 30 406603 are the affected ones; that runs up to production in February 2013. The fault is also reported between 60 000 and 100 000 miles, that is between roughly 96 000 and 160 000 kilometres. Repairing the chain tensioner costs between 900 and 1300 pounds and the camshaft adjuster between 700 and 1100 pounds, and long cranking and misfires belong to the second job as well.

If it is left like this, the timing fault codes become permanent, the idle turns uneven, power drops and the repair grows larger. The diagnosis runs on live data: the workshop reads the real timing angle and looks at the freeze frame of the fault code.

The repair is typically replacing the mechatronics unit or the conductor plate together with an oil change: not the whole gearbox. The complaint is most common on the longitudinal platforms from around 2005 to 2015 (C/E/ML and the early GLC).

The 9G-Tronic (725.0) is a different design; this applies only to the 7G.

The signs of the mixing are clearly visible. A mayonnaise-like, frothy deposit appears on the oil filler cap and on the dipstick, an oily film settles in the expansion tank, and the coolant level falls with no external leak. White smoke from the exhaust can come with it, overheating without any visible loss of fluid, and uneven running.

Once the seal has started, the leak accelerates. A few weeks of inattention is the difference between a seal replacement and a rebuild with connecting rod bearings, because oil diluted with coolant does not lubricate the bearing.

The repair costs between 1700 and 3800 euros and is a one to two day job: the cooler can only be reached after the intake manifold has been removed and the engine partly disassembled. The labour makes up the price, not the part.