lots of bubbles appear in glass inline fuel filter

[Robert O.]

i have a 1969 corvette 427 with holley sitting on c427 square-port intake. i have only owned the car for a few months. car runs absolutely great. today, i was warming up the engine and all appeared OK. as i walked to the passenger side, i noticed LOTS of bubbles in the glass fuel filter which is mounted between the pump and carb. i turned the engine off and watched as fuel slowly drained out of the fuel filter until it was empty. i hit the key to start and it started immediately. bubbles quickly returned although engine did not appear to be affected. forums i have reviewed today indicate that this is "normal" because of the low boiling point of gas. other comments indicate a possible ruptured fuel pump diaphram. pls advise as to best way to proceed. bobbydean411@gmail.com

[Mark E.]

Re: lots of bubbles appear in glass inline fuel filter

Robert,

From what you describe, it's possible the fuel pump diaphragm is leaking. A leaky diaphragm can allow fuel in the line between the pump and carb to drain back after the engine is shut down. It can also introduce air into the fuel causing the bubbles you see (I doubt the bubbles are from boiling).

Do you see or smell any leaks? A diaphragm can leak fuel externally. It can also leak fuel into the engine, which you can't see. You can imagine how dangerous gasoline in the crankcase can be, so it's critical to immediately rule this out as a possibility.

You can perform flow and pressure tests on the pump. But since a new replacement AC fuel pump is only about $25 for your car, you might just replace it and see if your symptoms improve.

For safety reasons, avoid splicing the factory steel fuel line between the pump and carb to add aftermarket filters or rubber hose. That's the pressurized segment of the line, so it's best to keep it all steel with no hose clamps.

If you determine the diaphragm was leaking gas into the crankcase, after repairs are made change the oil and filter as well.

[Robert O.]

Re: lots of bubbles appear in glass inline fuel filter

Mark, THANK YOU SO MUCH....

I will run a flow/pressure test on the pump Friday. i will go ahead and order a fuel pump for it - and try to find a replacement steel line from pump to carb. When i purchased the vette, it already had a rubber hose splice and aftermarket glass filter installed. The outflow line to the carb feeds into a Summit fuel pressure gage. And, get this... the rubber hose was fastened directly to a bolt on the front of the head with a piece of wire and the hose was in direct contact with the head.

There does not appear to be any leaks externally; and the oil still looks brand new; however, i will go ahead and change the oil and filter as a precaution.

When i raised the hood today, the fuel filter was full. I cranked the engine and ran it until it reached 160 or so and did not see any bubbles form in the filter. pressure at idle was 8 lbs according to the summit gage.

Regardless, i am going to make all the changes you recommended; and hopefully fine a replacement steel line.

thanks and regards,

bob owens
bobbydean411@gmail.com