617 sudden miss / down on power
 

I was pulling into my driveway when the engine suddenly sounded like it was missing. It was down on power, so I pulled in, looked under the hood, and I have diesel coming out of the diesel return line that goes from the #1 injector back to the fuel filter (it leaks out of the end attached to the injector).

Would the leak at the diesel return line cause the #1 cylinder to miss? Or do I probably have a clog somewhere?

No. That will not cause a miss.

Your symptoms may suggest a fuel supply issue; most likely air entering the system.

Any fuel leak has the potential of introducing air into the injection system. This will cause poor atomization of fuel and could be the cause of "missing". It is a very inexpensive repair and takes about 30 min. to resolve.;)

Answer
 

It can in some circumstances.
I went through this on the 300SD last year.
After replacing the looped fuel return line, further testing found:
* The lift pump valves where bad = weak supply pressure.
* The fuel pressure relief valve spring was broken in three pieces, supply pressure at idle was 0.8 psi, the injection pump return line was reading variable vacuum levels at higher RPM, and the engine had serious (unsafe to drive) power loss.
* The oil, fuel, air filters where replaced as normal maintenance.

Feel free to call me with questions on this issue.

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In summary, the leaking return line is not the cause. If repairing the return line fixes the symptom, you have much larger issues to deal with on the that are the actual Cause.

Well, you were correct. I got my new return lines today and installed them. It started right up, but still misfiring (although it did fix the leak). I bled the hard lines to each injector starting at the back and worked my way forward. When I would loosen a line from an injector, it would squirt a bit of fuel and the idle would drop. This happened on each one until I got to the front. I got a more powerful spray of fuel, and the idle didn't drop.

Bad injector? You just replaced the head on this motor didn't you?

Maybe, I did replace the head, but I swapped the injectors over from the old head without having them rebuilt.

Swap the injector with another cylinder and see if the problem moves with it.

I'll do that in the morning!

Swapped the #1 & #5 injectors, it didn't move the problem. I pulled the 1st hard line, wasn't clogged. I turned the engine over without the 1st & 2nd hard lines so I could see the fuel coming out of the injector pump. I get good fuel out of the 2nd port of the injector pump, the 1st port puts out an anemic amount.

I've been reading and it sounds to me like I should buy a good used IP, as it's not really something I can just pull apart and clean on my bench. Does that sound like the right course of action?

I ran a can of diesel purge through it, and it cleared up the miss.

Good
 

That means there was sludge in the injection system, cleaned out by the diesel purge.

A simple cheap repair. :D

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