Hi all. I have a 2003 k74 4d56 L200 MK3. She's been an absolute beauty so far, only on 58k miles in 20 years. The other day I noticed the engine has started to surge at idle almost like you are lighly tapping your foot up and down on the accelerator pedal. I have bled the fuel filter, cleaned the egr valve and pipe, visually checked all suction piping and cleaned the air filter all to no avail. The surging is only noticeable at idle and otherwise everything through the rev range seems normal. I could live with it but it stalled on me the other day as i downshifted from 5th to 3rd and selected 4wd at the same time. I have been running it off some new synthetic engine oil mixed with diesel that i got for free at about 20:80 - Any suggestions would be greatly appreciated thank you.

Been a while since I posted here! Sounds like an injection pump problem, but worth checking the turbo actuator arm and see if that is moving at idle (it should be stationary).

Nice one Heathen, I'll check it out at brew time and report back 👍🏻

Ok, i have checked the turbo actuator arm and i can confirm that it IS moving a couple of millimeters back and forth as the engine surges at idle. As a side note i can also see the egr valve moving in and out in a similar fashion. This morning on the way to work as i slowed down at a junction the engine cut out and the engine management light came on. I turned it off and quickly started it again as i was blocking the road , and the revs wouldn't pick up and a massive cloud of smoke came out of the exhaust. I turned the engine off again gave it 10 seconds, turned it back on again and waited for the management light to go out and it fired up and drove away as normal !! I gave it a good thrash and it did exactly the same again when I pulled up at the gate at work, same procedure and then it clearef but still surged 🙁

Try unplugging the vacuum hose going into the EGR valve - it should then stay shut. If this doesn't help, you need to check the throttle body (sits above intake manifold with big pipe coming from intercooler) Check the actuator arm on that - it should be fully retracted and not moving at all when engine is running - when you stop the engine you should hear it hiss and go back to the closed position after a couple of seconds).

Ok. The egr valve still open and closes when the vacuum pipe to it is removed AND it also still opens and closes with the pipe removed and the electrical connector block !!!
I have checked the actuator arm on the throttle body and that is seated shut when the engine is running. Unfortunately I couldn't check the hiss bit as i am working on my own. I have spent 2 hours tonight going over everything that i can think of to no avail. I just don't understand why when the engine stalls after cutting out why there is a huge cloud of smoke and no power/revs from the engine and then if you turn the ignition off for 10 seconds, wait for the dash lights to go out inc. engine management light and then start it, it fires up like nothing had happened. Any suggestions welcome thank you.

Think you'll need to check the engine management codes - could be a failing sensor or solenoid. I'd take off the throttle body and have a look see if that is definitely not blocking the air flow of when the engine is running.