When I got my M109, I had a bad fuel injection pump and I searched all over for info. I ended up replacing it with another one, which I hope you DO NOT have to do! If you do, I would recommend taking the radiator off the truck and driver side fender off of the truck (it will make everything easier to get to), and you can also do other maint, such as flush the radiator, hoses, replace your fan belts, tighten the air compress pully, etc. Timing is important! Be sure to line up the marks on the gear and the pump. GOOD LUCK!
Here is some info that can be useful:
In the center of the top of the hydraulic head of the pump is a plug, it is
surrounded by the fuel distribution lines. Remove the plug. Stand a soft
wood stick up in the hole, and turn engine over while watching stick. The
stick should hop and down, but since you are having a no start condition, it
probably won't (If it does, we have other issues to discuss). Remove stick,
and spray solvent and penetrant in this hole.
Take out two screws and remove the engine shut off cover and gasket. Verify
that the cable from it to handle on dash moves freely.
The remove the two screws you have just exposed that hold on the Control Unit
Retainer (a sorta U-shaped bracket)...if I remember correctly these screws
are safety wired.
Remove the Control Unit Retainer, then slide out the fuel control unit
assembly. Take care that the small, precisely made, friction fit, T shaped
piece of metal on the inner end of the shaft doesn't get lost. It is the
plunger sleeve pin, and that is what all this is about.
Apply generous amounts of solvent, penetrating oil, etc., to make sure all
these pieces are free to move. Once free, spray with lube.
Reassemble fuel control unit into injection pump, being sure to orient the
plunger sleeve pin properly, it should all slide together smoothly and
precisely, like reassembling a gun. If you think you need to force
something, then you have put something together wrong.
As I recall, this fuel control unit lever rocks back and forth when the motor
is turning over, but beware, if you can see this, that means that the shut
off is disconnected, so don't just try this for giggles.
Now, assuming that the plunger on top didn't move, use a brass or similar
soft punch, as close in size to the hole on top of
hydraulic head, rap the plunger a time or two. Beware, too much force will cause a spring clip on the other end of the shaft to disconnect, and the plunger button to fall off.
Repeat test with stick.
You may have to repeat the penetrating oil/solvent/pecking/stick test process
several times before the plunger moves freely.
The above steps 90% of the time solves this problem. Occasionally however
the delivery valve sticks. This is access by removing a plug in the SIDE of
the hydraulic head. Usually this plug is funny looking (12 points), older
engines this plug is hex shaped.
Beware, with the way the pump is oriented in the truck, the delivery valve
and its spring will want to fall out onto the ground, chassis,
where ever....this would not be good as these delivery valves are precisely
matched to the hydraulic heads. So you won't replace just this tiny valve,
but rather the entire, expensive hydraulic head.
Using aerosol solvent/penetrating oil, make sure that the delivery valve is
not sticking.
Reassemble all the things we have discussed.
Truck should now start. However, any time I have fooled with the injection
pump, or a truck that has not been started in a while, I prefer to remove the
air intake mushroom, and have a helper stand by with steel plate or piece of
plywood to shut off air flow in the event the engine tries to run away, or
the shut off won't shut down.
To help with this, for those that don't understand how the injection pump works, I will cover the operation of the pump.
The round part of the pump where the fuel delivery lines are attached is called the hydraulic head. (Which by the way is an easily replaceable part, and is often found on eBay).
Due to the extremely high pressures developed inside the injection system, there are essentially no gaskets (and very few o-rings) used. Extremely close tolerences are relied upon to keep it from leaking, and the head itself is a machined from a single piece of metal.
The 6 small allen head screws perpendicular to the outer surface of the head seal off the outer ends of the fuel passages. (a result of the machining operation)
These passages intersect passages bored down from the ports where the injector lines are attached.
Your foot, or other throttle, moves a lever on the outside of the pump. This slides a sleeve up (or down, when throttle is closed) inside the center of the head (big hex headed plug nestled amongst injector lines covers all this). The sleeve movement up or down covers and uncovers ports in the head which act as valves regulating the amount of fuel admitted. The higher the sleeve is raised, the more fuel is delivered. Inside that sleeve is a sort of dogbone shaped plunger. This plunger is the actual fuel pump, the sleeve is the metering device. In the pumping action described above, the fuel is being pumped on the top of the plunger. It flows from there under high pressure, past the fuel delivery valve (a glorified check valve, accessed through a plug screwed into the side of the head, sometimes the plug is hex shaped, but usually 16 pointed.) This valve is NOT replaceable. From there, it flows into the knecked down central part of the plunger. There is a slot machined in the plunger. The plunger is rotat
ed by a gear drive located in the large lower portion of the fuel injection pump. This slot is then aligned, by the rotation of the plunger, with the fuel delivery passages mentioned so many words ago (with the six allen screws blanking them off). The fuel then travels through the lines to the injector nozzles.
