W123 OM617 missing on a cylinder. Test injectors?
 

I have a 1985 W123 which first developed a rough idle and now seems to be sporadically "missing" on one cylinder at all RPM. I've had clogged fuel filters before and the symptoms seem different. I thought the next thing to check might be the injectors and I'm looking for a simple way to test to see if one in particular is bad. I'm finding lots of instructions about replacing all of the injectors or cleaning all of the injectors or building a "pop" tester but I'd like to start with something simple and fast. I see lots of people talking about about "cracking a line" but I've burned out Google trying to get the specifics of what that literally and specifically means. I'm guessing I open the line for each injector (where, exactly in the line, though) and see if the idle gets worse or not? If it doesn't get worse, that's the bad injector? Anybody have a link or tips for me?

Cracking a line: Loosening the nut that holds the line to the injector just enough that fuel spurts out instead of getting injected into the cylinder.

In your case, get the engine running, then crack injector lines one at a time. When you crack the line on a good cylinder, the idle will drop and become more uneven. When you crack the line on a bad cylinder, nothing happens to the idle.

Cracking open the line at the injector will make the injector not build enough pressure for it to inject fuel in the cylinder, thus effectively making the cylinder dead. Basically you crack the line on a dead cylinder, nothing changes in the way the engine runs. But you crack the line on a working cylinder, the cylinder becomes dead and a noticeable change on how the engine runs happens.

Causes include a bad injector, poor compression, air getting in the lines or issues with the injection pump.

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Here is My version:Injector cut out test-Loosen one Fuel Injection Line Nut at a time and see what happens to the Engine. If you loosen a Fuel Injection Line Nut and there is no change in how the Engine runs you have identified a problem on that Cylinder.
Cutting the Fuel off to a good Cylinder shouuld make the Engine run rougher. Cutting the Fuel off to a bad Cylinder does not have as much of an effect on the Engine.

If you found a bad Cylinder remove the Injector from that Cylinder and swap it with an Injector from a good Cylinder.

Do the Injector cut out test on those 2 Injectors that were swapped. If the problem follows the Injector from the bad Cylinder you have a Bad Injector.

If the Problem stays with the original bad Cylinder you have a problem in the Engine.

Would not hurt to do a Valve Adjustment before deciding to rebuild the Injectors.

before doing any of that, I'd first verify the valve clearances...

http://dieselgiant.com/valveadjustment.htm

Don't do extra work. If/when you determine which cylinder isn't firing and you pull the injector, compression test that cylinder.

Below 200 p.s.i. is bad, 200-250 marginally bad, 250-300 normal and above 300 is optimal. Test at room temperature not after engine is at operating temperature. "Why?" you ask, because the car is not at operating temperature when you start it the first time of the day.

Adjust the valves before doing anything else