Talk about Timing
There’s been much talk about timing, and yet there are many who do not understand the basic concept about timing. I decided to write it up so that I can just refer to it rather than explain it all each time someone asks. There are really 2 aspects to the timing within a diesel engine. The “time” that the fuel is injected into the cylinders, which is decided by the relationship of the injection pump to the crankshaft via the camshaft, and the relationship of the valves to the pistons, as decided by the positioning of the camshaft in relationship to the crankshaft.
When the engine is assembled, the crankshaft is positioned so that the #1 piston is at the top of it’s travel, and the camshaft is placed so that both the intake valve and the exhaust valve are closed, which in the cycling of the 4 cycle engine would be considered the top of the compression cycle and is referred to as “TDC” or Top Dead Center. There are marks (Dots) on the front faces of the crankshaft and the camshaft gears, which are aligned at assembly, and once aligned, cannot move. On the Camshaft, opposite to (180Degrees rotation of) the “Dot” mark, is a “y” marking. When the crankshaft is placed at TDC, this “y” mark is at the top of the camshaft gear, and it is this mark that is used to align the gear which drives the injection pump, which has a corresponding “y” mark on it. The whole thing looks like this:
If you have set the #1 piston to TDC on exhaust stroke, the “y” mark on the camshaft will actually be at the bottom, and the “dot” will be at the top, and you will have set your injection pump timing incorrectly by 180deg of engine travel, and the engine will not start or run.
As each piston comes up to the top of it’s compression stroke, the injection pump is setup to deliver it’s “shot” of fuel just before the piston reaches TDC. The Momentum of the engine carries the piston past TDC, and the heat from the compression causes the fuel to ignite, which expands rapidly, driving the piston downward producing power. The point in time that the fuel is injected into the cylinder relative to the crankshaft, is known as injection timing. The injection pump can be rotated slightly clockwise or counterclockwise on it’s mountings, which allows for small adjustments of the injection timing which can make a huge difference to how the engine runs in terms of power, efficiency, and fuel economy.
One major problem that happens is that occasionally someone removes the injection pump to change it or do other work. The pump can be removed without lifting off the injection pump gear cover. If this is done, the gear will not move in relationship to the camshaft. However, if one removes the injection pump gear cover, then the gear can easily be lifted off the camshaft, and the relationship disturbed. The real problem is that the “Dots” and “y” marks on the front of the gears cannot be seen from above because the front plate of the engine is only about 1/8” away from the gears, so alignment of the gears again can seem virtually impossible. Some guys have had some luck with sliding a thin dental type mirror down behind the front plate and looking at the camshaft gear with a flashlight while placing the injection pump gear into place. I’ve tried this and it is nearly impossible, and when I tried it, I couldn’t see well, and couldn’t start the truck afterwards, so obviously I didn’t get the gears aligned correctly. I also heard of one guy who dropped a tin plate on a stick down the front of the engine!!
There is a way to re-align the timing fairly easily though. Firstly, the engine has to be brought to TDC. There are several ways to do this. The vibration dampener on the crankshaft front has a line on it. The front plate has a small plate sticking out under the vacuum pump area which also has a line on it and marked with a “0”. When the line on the dampener and the timing plate are in alignment with each other, the #1 piston is at the top of it’s travel. It could be on exhaust stroke or compression though, and it must be on the compression stroke to set the timing. To figure it out, you should be able to feel strong resistance to the engine turning by hand (clockwise) just as the marks are about to align up, and to confirm this, you can loosen #1 glowplug and you should hear air (compression) escaping while you are turning the engine and just before the marks line up. It should look like this, but with the line on the dampener exactly lined up with the “0” line on the plate:
Once the engine is at TDC, the crankshaft and camshaft are aligned which sets the valve timing, now you need to put the injection pump gear back in place so that it is also aligned correctly to set up the fuel injection timing. To do this, you must locate the “y” mark on the injection pump gear. Stand the gear up so that the “y” is at the 6 oclock position. Now count up on either side of the mark to the 19th tooth, and put a mark on the 19th tooth up from either side of the “y” mark. Next, you must mark a visible line with a marker across the gear face from the 19th tooth on one side of the gear to the 19th tooth on the other side of the gear. Now, mark another line vertically from the “y” mark up through the exact center of the gear, so that where the 2 lines intersect, it forms a 90deg angle. It should look like this:
Injection pump gear
Finally, you now place the injection pump gear back onto the camshaft gear. If the engine is properly at TDC, and you’ve marked the lines correctly, then the horizontal line between the 2 teeth will be exactly parallel with the top of the plate on the front of the engine, and the vertical line will be going right up the center of the notch in the top of the front plate. If the gear is out by even one tooth, or you haven’t got the engine exactly at TDC, then things will never line up. It should look like this:
Injection pump gear relation to block and front plate
All that is left is to place the injection pump gear cover back in place without disturbing the injection pump gear, then re-assemble all the rest of the injection pump etc and start up the engine. If you follow this procedure, the injection pump (fuel) timing will be set approximately (static timing) and should only require fine tuning (rotating the whole pump assembly slightly) to set it accurately. (Dynamic timing) Some people feel they can get the timing set quite accurately by “ear” – listening to the sound of the engine, but this takes years of practice and knowledge and most don’t believe it is very accurate and correct timing can be only set using meters and probes.
One final word about timing. This relates to the fuel injection timing. The injection pump and injectors work at high pressures and are fairly precise. As they wear out, the pressures change and precision is lost. As the injection pump wears, it can no longer deliver the high fuel pressure required to drive the injectors, and eventually fails. As the injectors wear, they will no longer require as much pressure to open among other things. New injectors for our engines usually require around 2,000psi to “pop” open. Worn ones will “pop” open at lower than 1500psi. Since the pump delivers the same pressure to all injectors and since the pump is timed in relationship to the crank/camshaft, then it follows that if one injector is popping open at 1500psi, while another one doesn’t pop open till 2,000psi is reached, then the lower pressure injector will deliver fuel sooner in the cycle than the higher pressure one. What this means is that as the fuel injectors wear, each cylinder can be running with different fuel timing, which can cause a whole host of symptoms from rough idle to uneven power, poor starting, imbalance, smoking, poor fuel economy etc. So, for this reason, it is important to the health of an engine that when fuel injectors are out, they should be all tested for “pop” pressure and spray patterns. When they are replaced, they should be replaced as a set, so that they are all working the same. Lastly, if the injectors are all replaced with new high pressure ones, and the pump is still old and tired, then the pump may not be able to work efficiently against the new injectors and the result could still be a poor running engine despite new injectors, so again, it is common practice to replace the pump and injectors as a set complete.