Diesel Injection Pumps

The following is for educational purposes only:

 Pumps are not complicated, but they are precision devices.

I remember asking an older guy about injection pumps, he said... "People who know that stuff don't tell, they pull the shades when they do their work".

For those that believe knowledge brings a lever of independence, I can share this.

Most failures result in contamination, a very few result in marginal tolerances from the factory, a pump can be a little too tight, and can run for a while before the plunger sticks in the bore, or other problems surface (rare).

I received a great education when I got a call from a friend who had an injector with a stuck plunger in a new engine that had been setting for a year or two. This happened in some brand I don't know much about, but it is a common looking Chinese single.

After pulling the pump, we found the 'rack' frozen, another problem was the diesel fuel that had leaked into the engine sump. How did that get there? I have found only one reason for this, the Chinese use a less than generous copper washer to seal around the retaining screw for the barrel that the plunger travels in. The screw bottoms out before the copper washer is compressed. If you do not have a thicker washer of the same size, put the washer on a screw driver shank, hold the washer vertical and roll it across a hard flat surface, this will fatten the washer on the inside and outside rim and it will seal. You can identify the fuel leak problem by forming a vacuum at the fuel delivery outlet side, and putting your thumb over the hole the retaining screw fits in, you'll feel the vacuum leak.

As for the stuck plunger problem?

The insides of this pump was amazingly dirty, and the fuel filter was clear and clean as can be. Considering how easily the diesel got by the unseated retaining screw, is it possible that the lube oil was forced past this fitting as the piston headed towards B.D.C.? At this time, I think this is the source of all the dirt we found.

But how do you disassemble a Chinese pump with a stuck plunger? Here's a trick, remove the retaining ring, remove the little pin under the retaining ring. remove the end piece that rides on the cam lobe, if this is stuck, find a piece of hard wood, and bring the injector body down on it at an angle so the injector  meets the wood on the body of the injector, not on the roller end. The kinetic energy should remove it, if not, treat the area with Kroil, and try again in a few minutes, if it doesn't fee up, bring it down harder. when this piece frees up, remove it, note any shims behind this piece, and set it aside. Get some Kroil inside and allow it to set for whatever time you have. Now, bring the end of the injector body down hard on the wood, this is exactly how reloaders pull the bullets from questionable ammo, they use a kinetic hammer that holds the cartridge, and after a few blows the bullet is removed.

In our case, I had to get real serious, and brought the pump down hard several times before the plunger retracted from the barrel. when these parts where put under the microscope, it was amazing how dirty they were. This contamination came from the test run, and had to come from the lube oil side (IMHO).

Thanks to my friend who works with extremely fine tolerances, and has all the tools to do so, we identified another problem. After the parts were soaked in simple green, and scrubbed, they went into an ultrasonic bath, when they came out, we put them back under the microscope and they were still dirty, more scrubbing, more cleaning. Finally, we were in a position to 'feel' the clean plunger, in the clean bore. In this pump, we found a tight spot about half way thru the travel of the plunger. We chucked the plunger in a lathe, and polished it, we also checked the barrel and polished it. 400 or 600 grit paper is way too much for this kind of work, don't even try this without the special paper you can find at the machine tool supply outfits.

After polishing, cleaning, and refitting, we finally got a smooth travel of the plunder in the barrel. All parts were soaked in clean diesel, and the pump was re-assembled in a clean environment.

I positioned the governor linkage where the pawl was in the middle, I then set the rack in the middle, so when I plugged the pump back in it's hole, the two would align. I double checked that the shims were all in place to assure proper timing.

We turned on the diesel fuel, and allowed it to spill from the low pressure side before tightening the banjo bolt. I set the engine to the full throttle position, and we cranked the engine about  60 or 70 times before the fuel started spurting from the HP side of the pump as the plunger went forward in its' bore. We hooked up the High Pressure pipe to the injector, left the injector side loose, and then turned the engine over again till we had fuel at the injector. We then continued to turn the engine and slowly tighten the fitting at the injector.

The engine started immediately, we had to restart it twice before all the air was out of the system, but the engine now runs well.

For me, the lesson is... if you want to know what's going on in an injection pump, you need a microscope, and you need a way of cleaning these parts without damage, without a microscope, you never know when the parts are clean, and if they're not clean, you will not be able to check the plunger and barrel for a proper fit.

New plungers and barrels are often parts found in rebuild kits, don't fit them unless you create the environment necessary to work clean, and to clean all the other internals in the pump.

All the best,

George B

Special thanks to Mike McDonald for teaching me the basics of fitting high tolerance parts.

ADDED NOTE as of 12/2005, the utterpower CD has an article that documents a complete tear down and assembly of the Mico pump used on all Lister types,

read this and you will likely rebuild your own pump with confidence!