Beginner's guide to bleeding

SOMEONE WITH A GOOD MEMORY says she remembers that I wrote a column years ago about how to bleed a diesel engine. “I didn’t take much notice,” she admits, “because I never thought I’d have to do anything like that. But now, having inherited a sailboat with a diesel that won’t start . . . well, I wonder if you could possibly repeat that column for my benefit.”

Well, certainly madam. Anything that saves me from writing a new column on the day after Thanksgiving, when my brain cells have gone all numb and weak, is very welcome. Here (I hope) is what you are looking for: 

ONE OF THE STRANGEST facts about sailboats is that a tiny bubble of air can bring a hulking auxiliary diesel engine to a sudden stop. It hardly seems possible, yet it happens all the time — and usually at the most inconvenient moments, never when you’re safely tied up in your slip.

If you know anything at all about diesel engines — say, enough to turn the key to start one – you’ll know that they work by compressing air in the cylinders until it’s red-hot. Into these ruddy infernos, a high-pressure pump squirts a mist of diesel fuel.

The mighty explosion that follows drives the piston down in the cylinder and turns a big heavy thingummy round and round. This big thingummy is attached to a box of gears at the back that turns the propeller shaft. And then the shaft turns the propeller and makes the boat go forward. At least, that’s how it’s supposed to work.

You will appreciate, therefore, that a working diesel engine is a ferocious box of tricks, noisy, vibrating, smelly, and husky as all get-go – a real macho piece of work.

So how can this monster be halted in its tracks by a tiny, girly, bubble of air? Well, it turns out that air is compressible. Let us pause here for a moment to reflect upon the significance of that last sentence. Maybe we need to backtrack a bit.

When the fuel pump sends diesel to the cylinder, the fuel pressure obviously has to be high enough to counter the pressure of the air that has been compressed in the cylinder. I mean, if the fuel pump pressure was less than the cylinder pressure, the cylinder would blow fuel back along the line to the pump, which would be just plain silly, not to mention stupid.

Now, to make sure this kind of blow-back can’t happen, there’s a little sort of check-valve thing that will only let fuel through to the cylinder if it’s highly pressurized. If it isn’t, the little valve thing simply won’t open. And that’s exactly what happens if air gets into the fuel stream. Diesel fuel is a liquid and is not compressible; so when it’s under pressure it’s forced to squeeze past the valve thing. It has no choice. But air is compressible. You can pressurize it, but it won’t expand enough to open the valve thing (which some people call an injector, I believe).

That means you can turn the key and let the engine go whumpa-whumpa-whumpa for as long as you like, but no diesel fuel is going to reach the cylinders as long as there’s air in front of the injectors.

To cure this problem, you have to bleed the engine. Bleeding a diesel is like burping a baby. Air has somehow got into its insides and has to be wheedled out. It can be a tedious, messy job. First, you have to know which end to start at. In the case of a diesel, it’s usually the nether regions because diesel burps usually travel from bottom to top.

Here is what they teach you in Bleeding 101 in auxiliary diesel college:

•  Make certain there’s fuel in the tank and that the shutoff valve is open.

•  If you suspect your fuel pump has a solenoid, switch the “ignition” key on.

•  Undo the bleed fitting on top of the fuel filters and operate the priming lever on the fuel lift pump. When pure fuel is oozing out (no bubbles) tighten the fittings again.

•  Loosen the bleed fitting on the body of the fuel injection pump and do the same.

Now, if that doesn’t cure the problem, you’ll have to take the advanced course:

•  Open the throttle wide and switch on the “ignition” key.

•  Partially undo the high-pressure fuel line nuts at the injectors.

•  Turn the engine over slowly — use the decompresser valve if you have one — until clear fuel comes out of the fittings.

•  Tighten the nuts again.

•  Locate the clean rags and clean up the mess.

I’m happy to say that some engines such as my Westerbeast 13 are self-bleeding. Cynical as I am, I have not yet been given reason to doubt that claim, and I am very grateful.

If your bleeding problem is chronic, you might want to check all the hose clamps and nuts in the fuel line for slackness before you get into the more serious stuff. You might just luck out and find the cause of the problem.

Meanwhile, here are five reasons why there’s air in your fuel lines:

--You’re out of fuel.

--Fuel is very low, and the pipe in the fuel tank is sucking air as your boat rolls.

--The fuel tank shutoff valve is closed.

--There’s a leak in the piping, or connections are loose, so air can be sucked in.

--You just changed a fuel filter and air got in the line.

Finally, if nothing has worked, get out the darned owner’s manual and read it. I know, I know, It’s tough. But you’re out of options now. Be brave. Open it at Page 1 and start reading. Good luck.

Today’s Thought

A solemn, strange, and mingled air

’Twas sad by fits, by starts ’twas wild.

— William Collins, The Passions, 1.25. 

Tailpiece

The works manager phoned the railroad station.

“Are you the passenger section?” he asked.

“No, honey,” purred a female voice, “I’m the goods.”

(Drop by every Monday, Wednesday, Friday for a new Mainly about Boats column.)