This is a typical problem with CAV DPS/DPC fuel pumps. The fuel leaks along the edge of the solenoid housing (a round part with a huge nut on its housing, screwed into the side of the pump). Remove it by unscrewing it with the appropriate wrench, having unplugged the electrics first, of course. The leak develops where the back of the solenoid is joined with the rest of the body, and the replacement is very expensive (cca 160 Euros).
A new type of head gasket was introduced in 1993. The older one was a mica press with metal inserts, the new variant a steel-aluminum-steel sandwich construction. The old one failed for me at 130,000 km: part of the gasket lining burned out, leaking gases into the coolant as well as the adjacent cylinder. Such failure is not possible with the newer gasket.
Some mechanics would tell you that the camshaft housing has to be removed (together with the camshaft and all that this implies, timing belt, etc.) to change the adjusters. This is not true, nothing more is needed than removing the camshaft cover and, naturally, knowing what to do.
There were the wrong type of engine supports fitted to early diesel XMs, supposedly up to Q1 of 1992). The replacement top ones are softer and taller, and have a different shape. In fact, the original ones made the engine sit at an angle (the gearbox end was higher by about 1/3 of an inch). Post-1993 microfiches show this replacement, pre-1993 ones do not.
Steam and water cleaning older engines with fuel injection is always a little bit risky. Humidity getting into some parts can make re-starting a challenge. Note that we don't speak of permanent damage, only that you'll have to wait or dry those parts with compressed air. If you do this yourself, there's no problem, but at a garage, when other cars queue up behind you, it can be bothersome.
The advice is plain and simple: never use them! There are many oil additives and so called engine treatments (often sarcastically referred to as snake oils) available on the market but they all share one thing in common: none of them was ever proven to be advantageous, actual tests found quite the contrary more than once. Manufacturers routinely claim that independent laboratories tested their products scientifically but they don't rush to disclose the names of those laboratories or the circumstances under which the tests were conducted.
The primary job of the oil you put into your engine is to stop the various metal surfaces from grinding together, causing rapid wear of the parts. At the same time it has to dissipate the heat generated from friction, to transfer part of the heat of combustion away, to hold the byproducts of the combustion in suspension, not allowing them to stick to the engine parts. The oil has to comply with all these requirements under significant pressure and a wide range of temperatures ranging from the chilled engine in a winter morning to the high temperatures in an operating engine.
The XM was introduced in 1989 as the successor of the legendary CX series and its production stopped in 2000.
The XM saw one major model change during its production life, the introduction of the Series II in 1995. Although there are numerous differences between the older and newer models, two changes are very eloquent and help tell the two series apart without trouble: