You did not say if you replaced the 5mm O-ring on the Fuel Supply/Lift Pump. People buy the Mercedes Fuel Supply/Lift Pump valve kit because it is cheap and there is no O-ring in that kit.

I don't know if this is your issue or not because I have no experience with it.

Around 2010 I was looking up some injector stuff and I found a picture of a Injector set up so that the Nozzle end was sealed and under pressurized air.

What was being checked was the seating area inside of the Injector Nozzle. If it was not seated bubbles would come out of the top of the Injector.

I assume the air pressure was mimicking the pressure of compression
If this test is true if your Injector nozzles are worn air from compression can pass through the nozzle and out of the Injector and into the return fuel.

I don't know if that is what is happening or not. I worked in a Fuel Injection Shop for 5 years and no one ever said there was even such a machine for that type of test.

If air is not going into the Fuel Injection Pump I don't see it causing idle issues.

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84 300D, 82 Volvo 244Gl Diesel