Diesel Time: How to Bleed the Lines

If you’re going to try to start a homestead (outside of an urban environment), there are two power tools that will no doubt make the process substantially easier (especially if you’re planning on having a woodstove): a chainsaw and a tractor. I bought my tractor not long after we moved into our house. I was hesitant to make such a big purchase, but the idea of paying someone with a riding mower to mow our massive lawn was a huge incentive. ( Don’t worry, we’re slowly letting some of it grow back).

Before actually purchasing the tractor, we considered many things which I’ll cover in the future. For now, all you need to now is that we ended up with a Yanmar 2000D – a 2-cylinder, four-wheel-drive, 1970s, diesel import from Japan. And in that whole litany of descriptors, the key adjective is diesel. By no means am I a gear head, but coming up through high school (and even now to some degree) I always worked on my cars, but they were gasoline, not diesel. So while I have a vague idea of the systems, it’s a bit of a new field for me.

There are a few “tricks” to diesel machines that almost anybody with one will tell you. Firstly, diesels don’t use a traditional spark plug to create combustion, but instead rely on compression to create a spark. When you compress air fast enough it creates heat and can even create a spark. (Next time you use a hand pump to inflate a bicycle tire, feel the cylinder and notice the warmth.) If you live in a cold-weather climate, this means that you’ll be attempting to create combustion using cold air: the air has to heat a significant amount to create a spark and sometimes, this just doesn’t happen. (There is something called a glow plug that aids in the warming of the air, but when it’s the dead of a New England winter, this isn’t always enough.) Most new diesel cars have a plug on the engine that allows you to plug into an outlet and warm your engine, forty year old tractors do not, so you may need to warm the engine with a flood lamp, blow dryer or some other heat source.

The second rule of thumb is to never let a diesel run out of fuel. When you run a diesel out of fuel you start sucking air – instead of diesel – into the fuel line. When the air reaches the end of the fuel injector lines and enters the cylinders they essentially stop working. They’ll work again, but they need fuel which means you’ll have to remove the air out of the fuel system. Depending on the complexity of the system, this can be difficult. Luckily on my little 2-cylinder, it’s fairly easy.

Unfortunately, I’ve run dry a few times. The first time I actually ran my tank empty.

Diesel fuel filter
The knob-end points to the “c” when it is closed.

(Don’t ask me how. I was gone on vacation for a couple of weeks and when I came home, my tank was empty, and there was no evidence of a leak.) The other two times I got air in my system was because I can’t remember if the fuel shut-off lever should be up or down. You’d think it would be an easy thing to remember, but I always forget whether it’s the lever end or the knob end that indicates open or shut.

If you’ve managed to drain your tank, go buy some fuel and fill her up. Now you have a tank full of fuel, a line coming off of it and traveling around and ending in your cylinders which is all well and good, except that that line is full of air and that air is now trapped in the line.

Diesel Fuel Supply System
A basic Diesel Fuel Supply system.

Next step: bleed that air. Start at the tank and find the hose coming out of the bottom of the tank, this is the supply line. Follow it along until you find your first impediment in the flow. On my tractor this is the fuel filter valve (the one I can’t figure out): make sure it’s open.

Filter Line Mark Up
The actual fuel supply line on the Yanmar.

You’ll also notice on top of the filter are two bleeder screws. A bleeder screw is a hollow screw with a hole going perpendicular to the

bleed screw
A bleed screw.

threads; when these screws are loosened they allow air to escape the system to be replaced with whatever fluid should follow. You’ll know when

FIlter and Injector Marked Up
The Bleed Screws.

the air has fully escaped because fuel will begin to weep by the threads of the screw. It’s important not to remove the screw completely or loosen it too much. If you take the screw out, the air will rush out and the diesel will follow and then getting the screw back in will be a messy task. Also, if you loosen the screw to the point where you can fully see the perpendicular hole, the fuel will come rushing out; you only need to crack the screw slightly.

Bleed Screws Wet Dry
The bleed screw on the left is open too far, if the shut-off valve were open, we’d have a mess. The screw on the right is open the appropriate amount and you can see the fuel weeping by.

Once fuel is weeping by the first screw, shut it and move onto the next. On my system this will take a little longer as the fuel fills the fuel globe. Loosen the second screw until the fuel begins to weep by the threads, and again, screw it shut. At this point, the fuel filter is now full of fuel, and the fuel line leaves the filter and goes to the fuel injector pump. This is the next impediment in my fuel line. There is only one bleeder screw on the top of my pump and as the line is very short, fuel appears quickly. My Yanmar is only a 2-cylinder which means that coming out of the injector pump are two injector lines that wrap around the front of the tractor and enter into the engine block on the opposite side of the filter and pump. Each injector line goes into a cylinder, and this is where the job can get a little tricky.

Diesel Return Mark UpIf you look at the end of each line where it enters the tractor, there is a nut that the injector line runs through. This is essentially a collar that holds the end of the injection line in place. Mine is a 17mm, if you don’t have a set of open end wrenches, you’ll need them for this. If you unscrew the collar completely, you can remove the end of the line; if you’ve run your system dry, the cavity will be dry also. If you’re totally dry, you may need to loosen both injector collars, though probably not.

Injector End loose
The collar is about the right looseness for bleeding.

In a properly working system the injector pump is connected to the cam shaft or the timing belt and when they turn, the pump sends fuel to the cylinders. When the lines are filled with air, the pump is attempting to send fuel, but it can’t get around the air in the system, and consequently the cylinders aren’t able to do their job. In order to get fuel to the cylinders, you’ll need to crack the collar on the injector end where it enters the cylinder. You don’t want to open it all the way. And whether or not it’s true, my experience has been that when it is too loose, it doesn’t work as I presume there is too much air and not enough fuel getting to the cylinders. I seem to have the best luck when I crack the collar a little less than halfway. Once you’ve loosened the collar appropriately, it’s time to hop on up into the drivers seat and fire her up.

Before actually turning it over, make sure you’re wrench that you used to loosen the collar is handy, as you’ll need it once the tractor gets going. Pull the throttle almost all the way open and turn the engine over. It may take a couple of tries, but eventually the fuel will get there and the engine will catch. (If you’ve tried it twice and it’s not working, try loosening or tightening the collar.)

Injector End Diesel
Fuel Injector sucking fuel.

Once the engine catches, you’ll hear that it doesn’t sound quite right, this is because the fuel-air mixture is not correct and the engine is running super lean. Hop down, get on that injector collar, and tighten it down. As you tighten the collar, excess air will no longer be entering the system, and the tractor will start to idle appropriately. Congratulations, you bled you line. Now, don’t do it again.