What the owner notices is that the coolant goes down, but there is no puddle. If the seal has let go, the loss becomes rapid, and the low coolant warning appears as well. The fault appears above 120 000 kilometres and can lead to overheating.

The factory repair is outstandingly expensive on this engine: to get the pipe out, the timing chain cover, the chain, the intake manifold and the cylinder head all have to be disturbed, and that is over 40 hours of work, with a bill above 4000 dollars. That is exactly why two aftermarket routes exist: a liner pushed into the old pipe, at around 1000 dollars, and a collapsible pipe that can be fitted without opening the engine, at between 1500 and 2500 dollars.

One thing is worth asking. Workshops usually replace the gasket of the alternator bracket and the seals of the valley pan in the same job, because they are reached from the same place. If the pipe has been done, the other two are worth looking for on the invoice.

This fault is dangerous because it gives no advance sign. There is no warning signal from a disintegrating impeller, and the engine temperature runs up faster than the gauge can follow; by the time it shows, the cylinder head can already be damaged. The breaking impeller is a fault of its own on the six-cylinder built between 1992 and 1995, which is why the maker went back to a metal impeller, and then used a composite impeller in the later engines, which proved more durable. A pump with a metal impeller is still available today, as a durability replacement.

There is, however, one sign that can be caught before the failure: play developing in the shaft of the pump. That means the bearing is wearing, and because the impeller sits on this shaft, play in the shaft signals the coming end of the pump. With the belt off it can be felt by hand.

On the M54 it appears at around 120 000 and on the N62 at around 150 000 kilometres. Once the engine has overheated, the consequence stays: a warped cylinder head and a damaged gasket are no longer put right by replacing the pump.

The characteristic sign is a puff of blue smoke, and not only at a cold start: it also comes when the car has idled for a long time at a light, or when it is started after standing for a while. Oil consumption rises slowly but steadily alongside it. The valve guide often wears together with the seal, so the repair does not necessarily end with replacing the seals. The fault appears above 130 000 kilometres.

The cylinder head does not have to come off for the repair. If the workshop has a valve spring compressor that can be used with the head in place, the camshaft and the chain do not have to be disturbed either, but the job is still a long one. The shop price is between 1800 and 3000 dollars, while the parts themselves are 300 to 500 dollars by comparison.

The symptom is a visible trace of oil around the alternator, and this is the rarer case: here the leak also appears from outside, instead of only collecting in the V valley. Fresh, shiny oil around the alternator belongs here.

The gasket itself is under 10 dollars, but the labour is not. For the repair the alternator has to come off, the power steering pump has to be unbolted, the engine has to be lifted and the driver's side engine mount has to be removed as well. That is between 7 and 10 hours of work, with a bill of between 1000 and 1500 dollars, and workshops also allow for removing the intake manifold.

Because they are reached from the same place, the coolant pipe running in the V valley and the seals of the valley pan are usually done in the same job. If only one of them is on the invoice, it is worth asking why not the other two.

The early sign is a rattle audible at a cold start from the front of the engine. If the chain jumps, the valves and the pistons meet, and that is already an engine rebuild.

The condition of the guide can only be seen with the engine apart, so the rule is that whenever the upper timing chain cover is off, the guides have to be looked at as well. That comes up during the repair of the coolant pipe running in the V valley and of the alternator bracket, because those require opening up too.