What's that knocking?
 

Hi guys,

A buddy of mine has a 1979 240D with a 4 speed manual. We were out driving it yesterday, and when it is fully warmed-up and at idle, it occasionally will start making a 'knocking' sound. The sound will come and go.

We got it to the house, and it started up again. We popped the hood, and used the engine stethoscope. It's kinda hard to pinpoint, but it is definitely coming from the engine (i.e. not from the AC, or some accessory), possibly from injectors 3 and maybe 4. (I'm not positive, but it seems a bit louder when placing tip of stethoscope on the injectors for 3 and 4)

It goes away when you rev the engine, and doesn't consistently come back. It just appears every now and then. Performance seems unaffected.

Would anyone care to hazard a guess as to what it might be?

Sincerely,

PE

Air in the fuel Supply System.

Yes, that was my first guess as well. I'll check it out. Any hints on finding the cause?

Thanks so much,

PE

Try slightly opening each injector line one at a time when it's making the noise and see if it goes away. This will tell you if/and which injector(s) are making the noise.