W202 C250TD Intermittent engine knock at idle

[PaddyEnglish86]

An intermittent engine knock at idle. What could this be?

Thanks

 

[carat 3.6]

Depends what kind of knock, but I'll assume air getting into the fuel lines.

 

[guydewdney]

As above.

 

[PaddyEnglish86]

That doesn't sound too scary. I hope.

Any ideas where this could be coming from? Could it be anywhere from the fuel tank onwards?

Thanks

 

[a111r]

Can you see bubbles in the clear pipes?
There's 9 or so O rings, plus the pre-filter and stop solenoid that can be the cause.
If the pipes look dark (brown-ish) they're old and due replacement.
If not, replace the all O rings.

 

[carat 3.6]

As above, the brownish/yellow pipes in this pic are the normal suspects.

I'm slowly replacing mine with banjo fittings.

 

[PaddyEnglish86]

I haven't checked anything yet as I thought it was something deep in the engine. I'll check the fuel lines and let you know.

Does the fact that the noise is intermittent mean it is most probably a fuel issue rather than something metallic banging away in the engine?

 

[guydewdney]

Park it facing uphill overnight. If its a sod to start, you have leaking pipes.

 

[PaddyEnglish86]

Ok I'll try that. It's actually already difficult to start in the mornings when the engine is stone cold. It eventually coughs and splutters into life then it's fine after that. Even if I start it a few hours later, it starts straight away

 

[carat 3.6]

Ok I'll try that. It's actually already difficult to start in the mornings when the engine is stone cold. It eventually coughs and splutters into life then it's fine after that. Even if I start it a few hours later, it starts straight away

In that case, the delivery valve seals are also suspect.

 

[a111r]

In that case, the delivery valve seals are also suspect.

Cheap diy... but easy to get wrong, too = more nailing noise!

The 32 splined sockets are mainstream, cheap items now.

 

[PaddyEnglish86]

I checked the clear fuel lines and there is air in one of them. I also notice a a couple of those white clips were pushed all the way in. Would they cause air to enter the fuel lines?
I should have pushed them in and check but I didn't.

 

[guydewdney]

Clips are out normally, and pushed in to release pipe.

replace all the o rings. Pop out each pipe and replace the o rings on the end. Buy a set off ebay or one of those selection boxes from an automotive store.

major culprit is often the pre filter o ring. And the thermostat (undrr the inlet manifold). And most often , the stop solenoid.

 

[a111r]

I can sell you a set of 9 Viton O rings for a fraction of the Mercedes price for standard NBR O rings.

 

[PaddyEnglish86]

Thanks everyone.

How much for the Viton rings?

 

[a111r]

4 quid, posted.
PM me, if that appeals.

 

[PaddyEnglish86]

I think I've found the cuprit. Hopefully it's the only one. One of the pre filter lines is right up against the end of a plug must have been running for years until it developed a little hole. I've ordered a replacement line. Has anyone else had this problem?

 

[HarioH]

I deleted & bypassed the pre-filter for that reason.

The 10mm feed from the tank is jubilee'd onto the pre heater as well and can always do with a pinch up. (hidden under the IM).