being a diesel novice and this one being my first and only diesel, I don't know all of the sounds or really exactly what a good 601.921 engine should sound like so I apologize if I'm mixing up terms, but I'll be as descriptive as possible.

My '84 190d 2.2 has had a very shaky idle since I've owned it and I've been trying to turn it into a well running daily driver. So far, I have rebuilt and balanced the injectors, replaced the lift pump and replaced the return valve (at the IP) This made the car drive smoother and gave me some extra power, but the rough idle has not been solved.

When listening to it today using a long screwdriver as a stethoscope I found that the only abnormal noise I could hear was from the injector on #4 (cylinder closest to the firewall). The only way I could describe it is a very distinct "pinging" sound that did not go away at any engine speed. Even though the engine smooths out 1200rpms or so, the noise remains at all revs. It also DOES NOT travel with the injector. After switching the injector for the one in #3, the pinging stayed in cylinder #4. I can only hear this with the screwdriver directly on the injector.

I have to believe that whatever is causing this is also causing my poor idle, however, when cracking the injectors, I can't discern much of a difference of impact on the engine between cylinders.

I'm looking for any ideas on what my next troubleshooting steps should be.

It should also be noted that it seems the overflow (fuel return) valve had been installed on the inlet side of the IP instead of the back (return) side possibly starving the IP for fuel... for an unknown amount of time. (at least 20k)

Thanks all for your ideas and input. Hopefully my description is alright... I'm learning every day :-)