Water Manometer

Introduction

Mechanical Difficulty:

When diagnosing and repairing fuel system leaks, having a controlled low pressure air source is invaluable.

The Maintenance Manual call for pressure checking the fuel tanks with an air pressure of 0.5 psi, which is almost impossible to achieve using the typically available compressed air sources and pressure regulators. Trying to use such sources can also endanger a fuel tank, as over-pressuring the tank can break bond lines and severely damage or destroy the tank.

Using a water manometer as a pressure gauge and a low-volume air source gives one much better control over the process. There are a number of design floating around the web, a good example is this one.

Here's some pictures of one assembled in about 15 minutes from stuff laying about the shop:

The tubing goes from the pump, to a tee fitting, to the tank vent of the tank to pressurize. A U-shaped length of tubing goes from the tee fitting, down to the bottom of the ruler and back to the top, where the open end of that tubing is used to fill part of the U section with water.

Here you can see the water in the tubing at ambient pressure. If you have some food coloring or other water-based dye handy, it makes the water level easier to see, but it's not strictly necessary.

To attach the bicycle pump to the manometer tubing, cut a Presta valve out of an old road bike tire. It will thread snugly into 3/16" ID tubing. Presta valves are preferred over Schrader valves because they don't have a spring closure that has to be over-pressured to force the valve open. But use what's available.

The 3/16" ID tubing has just about the right size to slide snugly into the tank vent opening. Slide the tubing in the vent and temporarily seal the connection with modeling clay, Play-Doh™, plumber's putty, something similar.

To use, make sure the fuel cap is on the tank, and the fuel selector valve is off, or selecting the other tank. With the bicycle pump attached, slowly pump in air to increase the pressure in the tank. If you try to pump too fast, the transient overpressure can force the water out of the U shaped tubing and ruin your test. Try putting a finger over the open end of the tubing and closing off most (but not all) of the air escaping that tubing while pumping.

For 0.5 psi, look for a difference between the low end of the water column and the high end of the column of about 13.9". Use the ruler to subtract the level of the high end from the level of the low end of the column. The important number is the difference between those two levels, not the absolute value of either level.

Once the desired pressure is achieved, remove the pump and close the valve to seal off the tank.

There are at least three good uses of the manometer when servicing the fuel system:

• When draining the tank for service, pressurizing the tank to get the last couple of gallons out of the sump really speeds up the process.
• When diagnosing the source of the fuel leak, pressurizing the empty tank and dabbing suspect locations with a 50-50 mix of liquid detergent and water and watching for bubbles helps to zero in on the actual locations of the leaks(s).
• Pressurizing the tank after repair is easier to confirm success without having to fill the tank with fuel, and subsequently have to drain the tank again if the repair wasn't successful.