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 complaints turned into a class action in the United States (Suddreth and Dunton v Mercedes-Benz USA). The court finally dismissed the claim: the gears fail between 60,000 and 80,000 miles (roughly 95,000 to 130,000 km), so outside the warranty period, and the maker was not held liable. According to the case material the maker knew about the fault, issued repair bulletins, and eventually switched to a stronger gear.

If the service book shows a completed balance shaft repair, the stronger gear is fitted.

The case papers also name the M273 V8, but the M273 description does not cover this fault. This applies only to the M272.

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 failure of the seal is reported above 70 000 miles, that is above roughly 113 000 kilometres, and the maker supplies a cooler revised with a PTFE-lined seal in place of the A2730100315, under service bulletin 07.01-P-1089.

This is regularly put down to the valve cover gasket, although replacing the valve cover does not solve it, and the leak carries on. The cooler can only be reached after the intake manifold has been removed, so the labour makes up the larger part of the bill.

Three signs set it apart. The oil stands in the V valley, and there is no trace under the engine. In the coolant tank an oily, brownish layer or a mayonnaise-like mixture is visible. If, however, the front of the engine is wettest low down, then this is not what is leaking but the seal of the front cover, from where the oil runs onto the belt and the exhaust, and comes with smoke and a burnt smell.

The escaping oil can drip onto the hot exhaust, which gives smoke and a burnt smell, and in an extreme case a fire. The visible signs are an oil patch under the engine and an oily engine bay.

The maker's own bulletin gives fault codes 1200 and 1208 for this. What is typical is that the code comes straight back after being erased, on restart, so a freshly cleared fault memory does not hide it. The check engine light comes with it, along with uneven running and misfires.

It is not the mileage that decides but the engine number: the ones below 2739..30 088611 are the affected set, and engines built after that received a wheel made of stronger material.

For the repair the timing chain cover has to come off, but the cylinder heads do not have to be removed. The maker supplies a separate kit for this, number A272 030 03 13, which contains the guide wheel with its bearing, the chain tensioner, the oil pump, the solenoids and the seals. Removing and refitting the cover takes between 10.4 and 16.4 hours according to the maker's labour schedule, so the labour is what makes up the bill.

The same codes can more rarely be caused by a stretched timing chain as well. The symptoms of the two faults overlap, they can occur together and independently of each other, and because they are reached from the same place, both are worth looking at during one repair.

With a broken linkage the flaps do not move, which means the intake manifold stays in one position. The symptom is a noticeable loss of power and a fault lamp coming on.

The cause is therefore not the age of the linkage but the oil: until the leak is stopped, things end up in the same place again after a replacement. Repair kits are available on the market, but they do not deal with the leak that causes it.