Air In Fuel Line

HistoricMustang · Feb 2, 2012

You folks got any ideas on why my see through fuel filter empties while running which causes the engine to die. It is mounted between fuel pump and carb.

Changed the filter and fuel pump but problem remains and fuel tank is one half full. I can actually see bubbles coming into filter on occasions. Any help appreciated.

Phils69 · Feb 2, 2012

Is it a electric or mechanical pump? Have you checked the fuel sender unit?

HistoricMustang · Feb 2, 2012

Phils69 said:
Is it a electric or mechanical pump? Have you checked the fuel sender unit?

Sorry, it is a mechanical pump. I changed to a new pump hoping that was the issue but problem is still there. Never had this issue until the past couple of days.

It is almost like the pump is sucking air from somewhere in the line connections back to the tank. Is it possible for it to suck air without the obvious fuel leak when not running?

Phils69 · Feb 2, 2012

That was my train of thought too if it's doing the same thing since the pump change. Not an expert here but it does sound like its cavitating. I would start at one end and work to the other. Pull the sender off the tank and make sure that the filters not clogged and does not have a pin hole or anything in the tube. Check and replace the rubber lines if needed and blow out the long steel tube under the car. Make sure the carbs needle valve is not stuck closed also. How do you start it back once it cuts off?

HistoricMustang · Feb 2, 2012

Phils69 said:
That was my train of thought too if it's doing the same thing since the pump change. Not an expert here but it does sound like its cavitating. I would start at one end and work to the other. Pull the sender off the tank and make sure that the filters not clogged and does not have a pin hole or anything in the tube. Check and replace the rubber lines if needed and blow out the long steel tube under the car. Make sure the carbs needle valve is not stuck closed also. How do you start it back once it cuts off?

It will start back up after sitting a few minutes. Am guessing the fuel is simply draining from the fuel line and carb. bowls back to the filter which is between the pump and carb. This really does not make sense to me...............if gas is in line and bowls the car should continue to run. Glad the filter is a "clear" unit so I can tell what is taking place.

Will do what you have suggested, which is a good starting point. If cavitating is taking place then that tells me that the pump simply can not keep pressurized and keep line to carb. full. Not really sure..................but will start putting air pressure through the steel lines tomorrow. Thanks!

Any suggestions from anyone else while I got this thing apart?

Phils69 · Feb 3, 2012

Just another thought. If you have or could borrow a low pressure 12v fuel pump you could check the lines without pulling all them apart. Unhook the plumbing at the filter, plumb in the pump with the exit side going to a container. Connect the pump to the battery and see if it flows correctly. If it looks ok take the carb bowl off and make sure the needle valve is not sticking closed.

HistoricMustang · Feb 3, 2012

Phils69 said:
Just another thought. If you have or could borrow a low pressure 12v fuel pump you could check the lines without pulling all them apart. Unhook the plumbing at the filter, plumb in the pump with the exit side going to a container. Connect the pump to the battery and see if it flows correctly. If it looks ok take the carb bowl off and make sure the needle valve is not sticking closed.

Thanks, I will go ahead and take the carb. bowls off while doing this project. The car sat up for a while so the issue could be anywhere in the fuel system. With a new fuel pump installed (assuming it is good) this issue almost has to be in fuel line or carb. Gonna start back at the fuel tank and move through lines to pump and then carb.

Will let everyone know what I find.

PoppyMod · Feb 3, 2012

Hi,
I may have missed this comment, but, did you remove the fuel inlet at the carb and crank the engine to check the pumping capability of the system? If you have a healthy stream at that point, then, I'd check the factory mesh filter inside the inlet connection at the carb and the float level settings.
Good Luck!

rbohm · Feb 3, 2012

HistoricMustang said:
Sorry, it is a mechanical pump. I changed to a new pump hoping that was the issue but problem is still there. Never had this issue until the past couple of days.

It is almost like the pump is sucking air from somewhere in the line connections back to the tank. Is it possible for it to suck air without the obvious fuel leak when not running?

yes in fact it is possible, and quite likely. i went through a similar issue with my old oldsmobile a couple of years ago. when we replaced the rubber line from the tank to the steel feed line, the problem went away.

65ShelbyClone · Feb 3, 2012

Is the tank venting properly?

Phils69 · Feb 3, 2012

Here is a good article.

http://www.fordfe.info/FuelTS.html

HistoricMustang · Feb 5, 2012

Folks, thanks for all the suggestions. Had beautiful weather here in the South so I took the entire fuel system apart. From tank through carb. Replaced rubber lines, blew out tubing, cleaned inside carb and even took the fuel sending unit out of tank to take a look inside.

Not sure where the problem was but it has disappeared. Also, and not sure if this the problem all along, I had plugged the vac. port on carb. since it was not used for pollution equipment...................it had a small split. Perhaps that was the issue.

PoppyMod · Feb 6, 2012

HistoricMustang said:
Folks, thanks for all the suggestions. Had beautiful weather here in the South so I took the entire fuel system apart. From tank through carb. Replaced rubber lines, blew out tubing, cleaned inside carb and even took the fuel sending unit out of tank to take a look inside.

Not sure where the problem was but it has disappeared. Also, and not sure if this the problem all along, I had plugged the vac. port on carb. since it was not used for pollution equipment...................it had a small split. Perhaps that was the issue.

Hi,
Don't think replacing the vac cap had anything to do with "air bubbles in the fuel". However, it will have improved your engines idle characteristics, depending on it's intended usage.
Glad to hear of your success!

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