OM617 sudden rough running then no idle

[okto]

Yesterday, my 84 300D turbo started running a little rough from just off idle until the turbo spooled. Not enough to cause a real problem, but it persisted throughout the day and started to be a little rough at idle as well. It wasn't consistent; sometimes it would run rough, sometimes it would be fine. Sometimes when it returned to idle it was rough, sometimes not.

Popped the hood, noticed the clear fuel filter had some bubbles in it, so I replaced the hoses between the filter and the lift pump, and between the filter and the hard fuel line.
It got slightly better, then slightly worse, with rough running and loss of power in the 1000-2000 RPM range as before, and would sometimes run rough when returning to idle, but sometimes idle smooth if I gave it a little throttle and let it return to idle slowly.

Popped the hood again, watched the fuel in the filter as I revved it. The filter drained almost empty as the engine revved from idle to about 15-1800 RPM, in parallel with the rough running, then filled rapidly with fuel as the turbo spooled, which began to froth in the filter from there up to WOT.

Drove it back to work, then when I started it to drive home, it idled poorly, still ran rough and lost power in that middle RPM range.
About halfway home it started losing power on the highway, and I had to floor it to keep it running. By the time I got it pulled over on a sidestreet, it was idling extremely rough, visibly shaking an inches or two back and forth, and pedal floored only got the engine up to about 1000 RPM.
Removed the plastic fuel filter and connected the hose from the hard line directly to the fuel pump, thinking it might have a leak. Running didn't improve. Opened the fuel cap to add some Diesel Kleen, thinking tank filter might be sludged up, and the engine died almost immediately.

Restarted the engine, running became gradually worse.

I tried running with the hose from the fuel pump feeding from a bottle of fuel, to isolate the hard lines, tank-side hose, strainer etc. Engine ran equally badly, becoming rapidly worse until the engine can no longer maintain idle for more than a second after starting.

I'm trying injectors first, although I wouldn't think those would fail suddenly, and don't think it would run this rough even with a dead cylinder. Next thoughts are lift pump and injection pump timing, although I don't see how injection timing could work itself loose so suddenly. I've ruled out valvetrain or lower engine damage, as those couldn't really be intermittent, and I don't see how they could improve with increased revs.

New primary and secondary fuel filters installed ~4kmi ago.
Valve lash done ~6kmi ago, then re-checked ~4kmi ago.
New engine and trans mounts ~8kmi ago, visual check shows no damage.
New rack damper bolt installed and tweaked ~3kmi ago.

No smoke on start or at any speed.
No milkshake in coolant or oil.

[sixto]

My money's on the lift pump. If you have the old style primer pump, is the handle secured? I had similar symptoms when the the handle spun loose. Could also be the relief valve sticking open.

Sixto
83 300SD
98 E320 wagon

[okto]

Originally Posted by sixto

My money's on the lift pump. If you have the old style primer pump, is the handle secured?

It's the old style pump, and the handle is screwed down tight. Lifter pump was one of my guesses.
When I pumped the primer to purge the lines after replacing the hoses, I could hear fuel flowing in the cigar hose, so I think the relief valve is okay.

[Alec300SD]

X2 on the old style lift pump,

Also check the tightness of both ends of hoses attached to the primary fuel filter you replaced.
If you can twist the hoses on any of the fittings by hand, air can leak into the fuel path.

Did you use a primary fuel filter with the brass reinforcements?
They work the best.

[kerry]

My money's on the lift pump also.

[okto]

Originally Posted by Alec300SD

Did you use a primary fuel filter with the brass reinforcements?

Just the regular all-plastic one (I was under the impression that spin-on is primary and plastic is secondary in Mercedes nomenclature?)

Heading out to where the car is now with a jumper box and a buddy, gonna crack all the injector lines to make sure there isn't just air in the system that hasn't been purged yet.

What's the diagnostic procedure for a dead lift pump?

[kerry]

Pressure gauge between lift pump and primary filter?

[okto]

It's the lift pump. Disconnected the output line and pumped the primer; fuel spurted out as expected. Cranked the starter; no fuel came out.
Seems pretty conclusive to me.

[rickmay]

I have a 1983 240D, and am having a similar problem, although I am wondering if the fuel pump (that the lift pump is attached to has gone bad).


I did a diesel purge running the fluid through the small plastic filter by the pump (and back to the bottle which was returned to the top of the large fuel filter), and managed to start the car. In minutes, it was running fine. It did, however, idle on the slow side. I could also start the engine while the diesel purge was still connected. When I put everything together again, the car did not want to start. It doesn't seem to be getting enough fuel.


First, I do not see how the primer pump would cause the problems okto and I are having. Any thoughts, please.


I am more inclined to believe the mechanical fuel pump mounted directly to the injection pump is a problem (156,000 mile engine). Any thoughts?

[barry12345]

The lift pump was in service and working when you ran the car on the cleaner. It is more probable that the in tank filter has an issue or you have some issue with the supply line from the tank to the pre filter.


Not written in stone but has to be considered. The fuel pump incidentally is called the lift pump. The manually exercised finger pump the primer pump. This is a just a difference in distinction where in north America we call all fuel pumps. Fuel pumps.