Hi
I was driving my 1988 90 19J (LHD by the way) the other day, thinking to myself how well she was running… big mistake as it turns out! Started to hear a knocking noise towards the back of the vehicle and this was accompanied by a loss of power. I pulled over with the engine still running and went to the rear and noticed that the exhaust was spluttering black smoke and depositing a black discharge onto the mud flaps. The noise seemed to be the exhaust rattling due to the spluttering. I turned off the engine for a few minutes and then, given I was only a few kilometres from home tried to start her up again, but each time the car would start up but immediately cut out. Finally managed to get it started by keeping my foot hard on the accelerator when turning the ignition and managed to ‘nurse’ it home. On arriving home I noticed white smoke coming from the drivers side engine bay and the air intake so turned the engine off. The temperature gauge remained in the normal range at all times and there is no obvious signs of any leaks.

Any thoughts would be most welcome.

Minstrel effect
Seriously though white smoke or steam from the engine bay manifold could be water penetration. Your cylinder head gasket for the age if not been replaced is shot. The engine struggle because water and fuel mix. Look at the dip stick for light brown to cream colour. Same with the rocker cover filters for cream deposits.

According to my lawyer I'm not allowed to say minstrel effect.

It does sound like you have a serious feuling issue, i'd not run the engine until you have identified the fault.
Good advice from our own James Gillray above...

Check for water in the oil. If you can, organise/do a compression test.

Many thanks for your help - all sounds a bit concerning, but I suppose that is par for the course! Just checked the dip stick and the oil is jet black despite only having been changed recently. It is also a centimetre below the low level so clearly it is going somewhere - I hadn't checked the level due to the recent oil change. On the positive side at least it's not milky... Could simply running the truck with a low oil level be the cause of the issue do you think?

You did not mention mileage?
Good time when you remove the head to get rebuilt newer valves & new set of bolts.
Assuming the head can be rebuilt my 19J was fine & rebuilt the cylinders were also fine.
With the head off it will be obvious if the cylinders are okay.
200 tdi I have quite the opposite head was shot have not even been able to remove the injectors?
& a cylinder was bad.
What is called water intrusion not good to mix the two.

Not likely you need to look deeper down the rabbit hole.

as it is a 19 J I would start with checking the health of the engine, and ignore all the ancillaries, bar the turbocharger.
If you can get it to run an idle, crack the injectors one at a time and see if it makes the same difference on all four cylinders when you crack the pipes, if there is a cylinder that makes little to no difference then you need to look a little closer.

Do a compression test if you fine one or more of the cylinders is affected by Lynall's test above.