Fuel Pump Bubbles

[JMFL36]

Hi guys. I just rebuilt and refitted the mechanical fuel pump on my '54 Merc. Car starts and idles fine, but stumbles on acceleration. I have since noticed some large air bubbles coming from the spot noted in the attached photo. Just a couple of bubbles at idle, more when I rev the motor. The glass bowl stays about half full, but I'm thinking its lowering the fuel pressure to the carb, especially on acceleration. The bubbles seem to be coming from the intake port, so is it safe to assume I have an air leak somewhere upstream from the pump? Or could it be a leak in the pump?

[KULTULZ]

... is it safe to assume I have an air leak somewhere upstream from the pump? Or could it be a leak in the pump?

Actually either is possible.

Remove fuel line inlet at tank and outlet to carb, seal the openings up and see if the line holds vacuum with a MITY-VAC. If good, then possibly fuel pump diaphragm. Is the diaphragm ethanol rated?

[JMFL36]

The diaphragm is new and ethanol rated. The car is also on an ethanol-free diet, as we have the luxury of buying ethanol-free gas at the local gas stations here in Florida.
A couple other notes - I do NOT smell gas from the oil fill tube or the dipstick. I see no gas leaking from the pump or fittings and all the fitting are dry to the touch.

I see the bubbles as soon as I start the motor, so I know its not a gas boiling problem.

Wonder if it could be sucking air from one of the fittings? Do you guys tape or putty your fitting threads or leave them bare?

[KULTULZ]

Originally Posted by JMFL36

The diaphragm is new and ethanol rated.

I see no gas leaking from the pump or fittings and all the fitting are dry to the touch.

I see the bubbles as soon as I start the motor, so I know its not a gas boiling problem.

Wonder if it could be sucking air from one of the fittings? Do you guys tape or putty your fitting threads or leave them bare?

Remove fuel line inlet at tank and outlet to carb, seal the openings up and see if the line holds vacuum with a MITY-VAC (hand held vacuum pump).

If good, then possibly fuel pump diaphragm.

The diaphragm may be defective or not installed correctly.

What is the fuel pressure at the carb?

[KULTULZ]

Looking again -



The air bubble appears to be right at the fuel inlet. Maybe change clamp styles as a worm clamp is not the best for fuel lines.

See Below -

[JMFL36]

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Ok, mystery solved. I plumbed a bottle of gas directly to the fuel pump (1st photo) and started the car. No bubbles in the fuel pump, so it wasn't the pump or diaphragm. It was something upstream causing the bubbles. I then blew some compressed air up the fuel line to see if there were any blockages between the pump and the tank. I felt no resistance and heard air bubbles in the tank, so no blockages. But as soon as I released the air, gas came gushing out of the fuel line, which I quickly plugged back into the fuel pump. I went to the back of the car and took off the gas cap and sure enough I got a whoosh from pressure in the tank. The tank was not being vented properly! These cars do not use a vented gas cap but instead have a separate vent tube (2nd photo) that comes out right above the gas cap, which is hidden behind the spring loaded license plate bracket. The other end of the vent tube connects to the tank via a short length of rubber hose high on the driver's side of the tank. I pulled that hose off the tank and gave it a shot of air but nothing went through. Even at 100 psi I could not get air through the vent hose and tube. I then fished a length of rebar tie wire into both ends of the vent tube and eventually found the blockage about 4 inches in from where the vent pipe exits the car above the gas cap. It took quite a bit of prodding to break up the blockage and I blew the debris out with air applied to the tank end of the vent tube. I'm not sure what it was, but it looked like some hard mud from a small mud wasp or other small critter that was able to crawl up the vent tube. Jeez, what an ordeal, but at least its done. Now on to the next gremlin.

[KULTULZ]

Well ...

I see you have a locking gas cap. The cap on your setup was vented from the factory. The cap has to be vented for the system to work correctly The vent you repaired is for expansion/contraction of the actual fuel in the tank (pre-EVAP).

The cap is a one-way vent. It only opens to the atmosphere as the fuel level is depleted to allow even draw. If pressure lowers/builds in the tank (outside temp change), it is relieved by the vent system you repaired, not the cap. With no cap vent it is possible for the tank to collapse if the pump is strong enough.

You were lucky to notice the bubble(s).

[JMFL36]

My chassis parts catalog shows the cap was non-vented. Only the station wagon (model 79) shows having a vented cap.

There is no check valve on the vent tube that I cleared, so it provides for pressure equalization as fuel is consumed. Only if it is blocked, as mine was, will it build negative pressure as the fuel is used. And in my case it eventually starved the pump of fuel and started sucking air instead.

[KULTULZ]

Sorry I bothered you.

[JMFL36]

No bother at all. Thanks much for the advice. Always appreciated!