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Old 08-28-2002
Mark Matthews Mark Matthews is offline
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Bleeding Diesels

I am replacing the fuel tank and fuel lines in my PS 34. There is going to be a lot of air in the new lines, I assume too much to bleed. What is the best way to get all this air out?

Mark Matthews responds:
If you are replacing the tank and fuel lines, they'll obviously be empty to begin with, so yes, there will be a lot of air in the fuel lines. You can use the manual lift pump to clear the air out and get the diesel in, which may take a while, although eventually it will work. It will help to fill the primary and secondary filters with clean diesel, taking care not to introduce anything else into the system at this point.

Some fuel systems also have a small, 12-volt lift pump that can do the pumping for you, and now might be a great time to install such a device. I've also heard of even simpler systems—namely installing the bulb from an outboard fuel line to aid in bleeding fuel lines, although care needs to be taken as the rubber in these bulbs can degrade over the years and clog the fuel system.

You may also want to consider a trick gleaned from Don Casey's book This Old Boat. (It's conveniently for sale in SailNet's online Store, and it can help unshroud this and many more boat maintenance mysteries in the future.) He recommends pressurizing the fuel tank by forcing air into the vent with a bicycle pump, which pushes the fuel into the fuel lines, and then opening one of the bleed screws to allow the trapped air to escape. When the bleed screw starts squirting straight diesel without bubbles, you can work your way up to the next screw. Eventually you'll work your way up to the injector nut and bleed these in sequence. You're likely to have to repeat the sequence several times before all the air is out, and based on my experience, it's always a good idea to run the engine for a good bit at the dock, just to make sure it won't act up as soon as you cast the lines off. Good luck.