My car is revving uncontrollably

No, yea i turned the key.

Okay, i'll get the compression tester out tomorrow.

What did you do with the engine? swap it, or repair it?

ProcrastinatorFraser said:

I think i may need an oil change though. Because the filler cap, and what was inside, when tiny amounts of smoke were rising through, it smelled like diesel fumes or full on diesel.
So i'll get that done ASAP, then i'll see if it's okay.

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Sounds exactly like a fuel leak into the engine, most likely from the lift pump. Fit a new one - fairly easy and not expensive.

ProcrastinatorFraser said:

Okay, well this morning that's what i did. Very little smoke coming out of the filler, and also very little blue smoke coming out the exhaust, bearing in mind i am running it on red diesel, because it is currently not a road vehicle.

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No reason red diesel will make any difference to running, it is exactly the same as road diesel now, except for the marker dye.

On a naturally aspirated engine, if your throttle linkages and fuel pump are working well, two things can cause a runaway.

One is the crankcase breather putting oil into intake tract.

The other is rising oil in the engine, always caused by either fuel or coolant running into sump.

As above ^^^^^^^^ lift pumps are cheap and easy change, would solve or eliminate fuel as a cause.

ProcrastinatorFraser said:

What did you do with the engine? swap it, or repair it?

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If that was a question for me, I changed it for a 200 TDI.

As other have said, the revving is also a symptom of fuel starvation. If you've got any doubts about your fuel lift pump, I would change it for a new Delphi one and bleed the system well.

If you've got an air leak that you can't track down, think about replacing the injection pipes. Mine would never seal correctly and a long standing air issue (difficult starting) was resolved by replacing them. They are very cheap.

Check your oil level in case it's rising.