I recently had a problem with my OM617 injection pump (from a 300D turbo) that I was able to solve after many many many hours of reading through this forum (especially the "won't start for anything" thread) and pouring over the MB shop manual. I'll summarize it here because I didn't find anyone else posting about the problem and this was the second IP I have come across with the exact same problem. Also, I didn't find anyone who tried the solution that worked for me which might save someone else a little time and others may be able to add their expertise.

Problem: absolutely no fuel flow through any of the IP hard lines. Lift pump circulates fuel through the IP just fine, but nothing will come out any of the injector lines whatsoever even at full throttle.

Diagnosis: IP throttle rack jammed shut

Fix: purge the IP with solvent repeatedly until rack frees up

Contraindications: can also be caused by air in the IP or by a defective vacuum-driven shutoff valve, or mis-configured ignition switch holding the shutoff valve closed. A worn/broken IP bypass valve spring can cause a similar condition though in that case at least a little fuel will usually flow through the hard lines.

The IP rack controls the amount of fuel that the pump delivers to each injector, in the fully closed position no fuel is delivered at all. Normally the running engine is stopped by the shutoff valve or the throttle arm pulling the rack all the way closed. The rack is in the front half of the IP and is pushed and pulled by a rod that extends through the bulkhead and into the rear half of the IP. (the rear half is filled with crankcase oil and contains the governor, the front half is full of fuel and contains the pump pistons). In this situation the rack jammed in the closed position when the shutoff valve pulled the pushrod all the way back.

The reason this can be such a problem is that the throttle arm (the part connected to the foot pedal) only acts on the rack via the governor, which is to say via a system of springs. They are not directly connected. Therefore any significant resistance in the rack cannot be overcome by forcing the throttle arm because it will only compress or release the springs in the governor without actually forcing the rack to move.

I was able to confirm that the rack was jammed by removing the screws of the ALDA cover and rotating it slightly without removing the ALDA. In this position you can look down on the shaft of the rack in the rear compartment. There's a tab on the end where the shutoff valve (which _does_ directly connect to the rack) is clipped to it. Using a screwdriver I was able to force the rack arm back into the idle position (which is toward the front of the vehicle) and confirm that fuel flowed through the hard lines. However, the rack was still very sticky and the governor was still not able to move it.

I suspected contamination in the fuel side by a loose piece of polymerized veggie oil. For those who are not familiar with this, veggie oil forms a sticky plastic-like substance in contact with ferrous metals, the same stuff that makes the black coating on a cast-iron pan. Normally we only see this on the metal components of the fuel system before the filters but there's no reason it could not form after the filters too. I suspected this because I had seen this problem in two IPs, both from vehicles that ran on WVO and had excellent filtration (meaning a foreign object in the IP was unlikely). Clearing something like this would normally require removing the IP, disassembling it and soaking the components in a solvent like gasoline. To save all that work I thought I would first try soaking the internal parts with the IP still installed...

To do this I disconnected the fuel send and return lines and flushed the IP with gasoline using the manual primer pump. I used a screwdriver to manipulate the rack through the gap in the ALDA cover but it did not free right away so I let it sit and repeated after a couple days. After four flushes over the course of two weeks eventually a blob of brown colored fuel came out of the return line. I checked the rack and it was free. I checked the hard lines and they were pushing fuel. I purged the IP with diesel and bled the hard lines plenty to get rid of all the gasoline and the car is back on the road running perfectly.

A note of caution: I suspect that injecting gasoline into a running OM617 would permanently damage it. If you use this technique please do not skip the step of flushing the pump with fuel oil afterwards. A trace of gasoline in the fuel oil is no big deal but injecting straight gasoline would be a problem.

There are more common problems that could cause similar symptoms so to summarize the troubleshooting sequence for a lack of fuel at the injectors:

1. Visually confirm fuel supply by disconnecting the fuel return hose and pumping fuel into a container using the primer pump

2. Confirm lift pump operation by cranking engine and checking that fuel is still flowing into the container

3. Disconnect the vacuum lines to the shutoff valve to confirm it is not engaged.

4. Confirm there is no air in the IP by purging with the primer pump and bleeding the hard lines by cracking them at the injector and cranking the engine.

5. Check for normal movement of the throttle rack by visual inspection through the ALDA cover plate.

Good luck and thanks to all the many brilliant people who contribute to this forum without whom I would be so terribly lost!

Aloha,

-Kanoa

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'84 300d bioturbodiesel