Mercedes W123 200D - serious lack of power

Fuel filter would be my first thought. Does yours have a drain on it? My 200tdi did the same once because the filter was full of water from a dodgy batch of diesel.

Also clean out the sedimenter if you have one.

Blocked air filter next. Did you try dumping the filter totally and trying it for a short while?

Cam & pump timing is up next. Plenty of scope for a numpty changing the cam belt and getting the pump timing wrong.

Its a good looking beastie, persevere with it!

Also, it doesn't turn off very easily - I think the vacuum pipe from the on-off knob is knackered. Could this have anything to do with it - perhaps the engine is running 'half off' despite the knob being pulled all the way out? Is there a way to bypass this?

Drop an OM606 in it!! :-)

make sure diesel pump is fully open when throtle is flat out , if i rember they had like a p'clip effort to hold cable near pump , you may not be getting flat out

unclefester, I was going to have a look at the throttle linkage, thanks for the technical info as I know nothing about this car (don't even have a Haynes manual).

dodgerover - yes, perhaps it is overfilled. This may explain the high oil pressure too (I see your ninja edit there ). It's running on pump diesel; it's never had veg oil in it afaik.

'Don't panic' is probably the best advice. I had a curse word experience within half an hour of buying it, but I shouldn't let that put me off of even attempting to fix it. As my boss said "For heavens' sake, stop being such a wimp about it. It's just a machine!"

Heh, me too. The Merc is arriving tomorrow morning so I'll get to work on it.

Could anyone talk me through how to bleed and prime the fuel system on these? There's a primer pump, but is there a bleed valve anywhere, or should I unhook the injector lines whilst pumping to bleed the air out? Or both?

The usual causes are; fuel filters, fuel tank strainer and unadjusted valves. Also leaky lines will cause problems (air lock etc.). Whilst your at it, adjust the valves too. It's often neglected on these engines (you'll know why once you try ;D). Valve clearances should be on a smal sticker on the slam panel.

IIRC these old pumps are bled by the hand-pump ontop of the lift pump. Turn to unlock and pump away...

some of those bulbs on certain engines can suck in on themselves and stop fuel flow. My mates caddy was doing this when doing higher speeds.

i have had one on mine for ages with no issues, altho when i tried fitting the same one to a ford td engine the thing wouldnt even run, it was stopping fuel flow completely.

This is very likely since the valve clearances have to be adjusted regularly and many garages seem to not know this.

There are indeed a number of possible reasons, which can be cross-checked.

Valve clearances: if the valves are too tight the OM61X-engines will be hard to start, too.

Injection pump diaphram: usually comes with lots of black smoke from the exhaust. They are easy to swap and don't cost an arm and a leg, but they aren't exactly cheap. Merc took about 50 Euros from my wallet last time I had to do this.

Leaky fuel system: is you can see air-bubbles in the fuel lines connected to the filter housing something's amiss. The most likely culprit should be manual pump at the injection pump. Merc sells you a new one on the cheap.

Clogged up fuel system: going for the filters is always a very good start. If you find your in-line filter clogging up regularly the mesh-filter inside the fuel-tank should be checked over.

Oh and in regards to your oil-pressure: call yourself lucky! An indicated pressure of 3 bar indicates a very healthy engine with very good main bearings.