Disco 1 Knock at idle.

Hoping someone can help me diagnose this noise.

It's a 300 tdi auto edc.

It started making a loud knocking noise when the engine revs drop too just before idle. Being a compleate bastard it's compleatly intermittent, but in an odd way. It will do it every time the revs drop almost on command then sometimes you can turn it off and back on again and it won't make any noise for as long as then engine is running sometimes it will keep coming back. Basically whatever it decides it going to do when it starts up it will continue to do as long as the engines running. It did however give a little rattle when started yesterday and cleared but was fine this morning and ran absolutely fine for four hours.

The noise itself is not rev related but sounds more like something ratting. It's coming from around the injector pump. It is a very angry metallic sound though.

So far I have eliminated the vacuum pump, the lift pump, run it without the aux belt so I'm certain it's nothing that run off that. The lifters and push rods all seem fine and I can't find any thing loose.

I have a spare injector pump so I may change that but I really would like to avoid randomly spending money on guess work.

Any one?

I have a theory this may be severe diesel knock but may be barking up the wrong tree.

Could be injector knock ( loosen injector fuel pipe at injector while engine is running to where it leaks and see if noise changes do each injector one at a time if the noise goes away it is that injector that is bad )

Have a listen under the vehicle along the exhaust system, the innards of middle muffler can fall apart and rattle/knock.

torque converter drive plate causes the same symptoms when it starts to crack

Sounds like over fueling, It could be one of many things, try the injector test by slackening each injector in turn to see if the noise disappears, it the noise disappears on a particular injector swap it with the one next to it and try again, if the noise is still there on that cylinder then its an internal fault but if the noise disappears it will be that injector. Another item that fails is the diaphragm on top of the injector pump, there is a plunger below it that sometimes sticks allowing less or more fuel constantly, this can be checked in place by removing the four screws that holes the diaphragm cover in place and gently removing the diaphragm and plunger.

jamesmartin said:

torque converter drive plate causes the same symptoms when it starts to crack

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I'm guess there's only one way of checking this and that's taking the box off?

MrMayze said:

I'm guess there's only one way of checking this and that's taking the box off?

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yes or engine out and removal of drive plate ,cracks are often around the boss and arent allways visible

MrMayze said:

I'm guess there's only one way of checking this and that's taking the box off?

Click to expand...

I'm not sure if it is at all possible to see in there, (bell housing), on an auto, maybe through the timing pin hole, but the presence of fine red/brown dust, product of "fret" corrosion, may indicate that there are cracks in the converter drive. Fretting of the edges of the cracks occurs due to slight movements between the two bits, the same movement that can make the noises.The same could be due to loose/broken bolts. Any of the above means separation of engine/transmission.