Please, I need some help to fix my first mercedes 190D, 2.5L.
It starts very nice and smooth and after 1/2 minute it starts misfire at idle and below 1500rpm, pinging noise ( this sound is detonation), with blue smoke and knocking noise.
When it is warm, the idle is ok, but a little smoke still with pinging and knocking noise.
Also, I noticed the fuel injector pump return line which connects the injectors return lines has fuel going to the oposite direction with some pressure.
I found that by disconnecting the return line between the fuel filter and number # 1 injector with running engine.
Is this fuel back pressure normal in the return line between the fuel filter and the # 1 injector?
At that time no fuel was coming out from the injectors return lines.

I changed the fuel filter, injectors and prechamber are fine no leaks. I wonder if I have to refurbish the cylinder head or fuel system problems.

Again please, what is the next step should I follow to troubleshooting those problems.
I appreciated any help, thanks you all