The MGA With An Attitude
BUBBLES In The Fuel Line -- FU-106

At 02:40 PM 12/11/05, Dave Plantz wrote:
"The pump just runs continuously and never shuts off. I couldn't find any leaks on the output side of the fuel pump and the needle valves are holding. I pulled the fuel line at the carb and submerged it in fuel. I then switched on the ignition and noticed air bubbles coming from the line."


"Doesn't this mean that there is a leak on the suction side of the pump?"

Very likely, yes.

"The line from the tank to the pump looks fine, so I assume the leak is in the line inside of the tank. .... Could the metal line inside the tank have a hole in it?"

Of course the first question is, is there fuel in the tank? If all is otherwise well but the tank is empty, the fuel pump will pump some air through the lines.

It may be leaking at the flare nut fitting where the steel line connects to the tank, or any place along the length of the line from tank to pump. There is an outside chance that the pump diaphragm is ruptured. You can test these things individually.

To check the line inside of the tank, disconnect the pipe and attach about two feet of clear plastic hose to the tank fitting with a good hose clamp for an air tight connection. Then hold the end of the hose above the tank and draw a slight vacuum on the hose to pull some fuel up into the hose where you can see it. If there is a leak in the pipe inside the tank above the fuel level, there would be some air bubbles rising in the test hose. No bubbles, no internal leak.

If no leak inside the tank, reconnect the steel line to the tank and disconnect the front end from the pump. Connect the clear hose here, hold it high, draw a vacuum to raise the fuel in the hose, and check for bubbles. Bubbles would mean you have a leak in the steel line or end fittings. No bubbles, no leak.

If no leak there, then test the pump by itself. Disconnect both lines from the pump and attach two hoses. Pressure hose needs to be clear plastic hose. Put the suction hose in some fuel. Put your thumb over the end of the output hose. Switch on the pump to make it run. Slowly let your thumb off the end of the hose to let the air out, and hopefully you will have fuel rise in the output hose. Assuming it does, put your thumb back on the hose to keep the pressure, and watch for bubbles as you allow a little fluid to escape from the hose. If it bubbles here, you have an internal leak in the pump, possible a ruptured diaphram. Otherwise you have some leak at the inlet fitting, or inlet screen assembly, or a very remote chance of a crack in the pump housing.

If it looks like the pump will work, then put a pressure gauge in the output line and see if it will make between 1.5 and 3.5 PSI static pressure at the pump outlet. If an SU pump clicks it should make correct static pressure (good condition) or virtually no pressure at all (bad pump). If you have pressure greater than 4-1/2 psi, you have a non-standard fuel pump. Then you would need to install a pressure regulator between the pump and the carburetor(s).

If all that checks out, then reconnect the line from tank to pump, check output flow and static pressure, and watch for bubbles in the output line.

If that checks out, then reconnect the output line to the pump, and disconnect the line up front at the carburetor. Repeat check of flow volume and static pressure. This should be about the same as it was at the pump output fitting. If the flow is significantly less up front than it was at the pump outlet, then look for a kink or a dent in the steel line going forward from the pump.

Also do a vacuum or pressure test on the hose which connects the steel line to the carburetor, and check flow through the front hose(s) to be sure they are not clogged.

Hopefully you will find the problem much earlier.

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