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65 Mustang Fuel Pump Leaking?

10520 Views 10 Replies 7 Participants Last post by  29-ford-ccpu
I suspect that my fuel pump is leaking. I read a bunch of posts that seem to indicate that my symptoms (hard to start without feathering the throttle) mean the fuel pump is letting gas flow backwards from the carburetor. I also have air showing in the in-line clear fuel filter after a day or two. There is no gas smell in the oil however.

My question: Is there a way to test the fuel pump before I replace it? I don't mind buying a new one, but want to make sure that is the problem.

Thank you very much for your thoughts.

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Hello 29-ford-ccpu,
Nice that you are checking before swapping pieces.

I doubt it is fuel pump related. You would smell gas in the oil
and even if the gas were leaking back - there would still be enough
gas in the fuel bowl to get you running.

My thought - -CHOKE related - -What type of carb and electric or manual choke?

If unsure - no prob - we will help.

I would NOT swap the fuel pump

Print Dad
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as in leaking into the oil? i would think that your oil wouldnt feel very viscous anymore if this were the case and it would appear quite dirty. i have a holley puel pump i just installed and after a few days it will sometimes allow backflow and othertimes not. i beleive that this could be because of where the pump arm is in relation to the stroke of the diaphram thus having an open circuit. just a theory of mine i have nothing to back that up.

also remember you have vented float bowls that are subject to evap. evaps enough to drop the float and open the seats then evap will happen in the lines as well, thus air will get into it as well. modern gas has a tendancy to evaporate quite quickly so it doesnt take much to evap that small amount between the carb and pump.

thats all theory. if you want the facts you could disconnect it from the carb and turn the engine over with a wrench, if fuel pumps out you then know it has some flow to say the least. next you could actually pull the pump off, hold it in your hands and push the pump arm down. how much resistance? a little? a lot?

at that point when it is already off you might as well just replace it since they are only 20 to 30 bucks and it will alleviate all doubt and stress. this is what i would do and have done in the past. you dont want gas in your oil so spend the money for the reasurance. (as i said i assumed you ment leaking into the engine from the pump arm)

lastly dont get discouraged if air appears in the line. you know your pump is sound and like i said it could be simple evap through the bowl vents. maybe someone else can verify this possibility.

when cold i always have to give my engines one pump before i turn them over. never thought this to be concerning. if your choke is set to conservatively then you will have to tap the gas a bit more to start the engine as well most likely having to hold it till it reaches a reasonable rpm to idle on its own. when you start it does it have a very low idle with your foot off the gas until it warms up? if so its choke related.

hope this helps some its a bit long.
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:) Hi Bob

P-Dad and Gearheadhick429 make some good points. Your problem could be several things and all solvable.

Here's a simple test to rule out the fuel pump as the problem. Disconnect the fuel line that feeds the fuel pump. (1st pic). Take a length of rubber fuel line hose and reconnect it to the pump and put the other end in a gas can. Then pull the coil wire to the distributor (2nd pic). Disconnect the fuel line into the carb. Put a rag or suitable container under the disconnected fuel line. Have a helper turn the engine over for 3-5 seconds. You should have fuel squirting out of the line. If little or no fuel, I would then disconnect the fuel line at the pump and blow it out to ensure there was no blockage. If you still have little or no fuel coming through, I would think your fuel pump has had it. If all is good, we can help you trouble shoot the rest to find the problem. Good Luck. :bigthumbsup


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Thanks for the replies so far. Here is some additional information based upon the replies:

I put a new carburetor on this summer as the one that came with the car had a manual choke, and it was not the right size for the engine. I bought a Holley with an electric choke and the choke is working fine. In other words, it is fully closed when cold and takes about 90 - 120 seconds to open.

When I try to start the car after ~ 1 week, it does not seem to go to high idle until the engine warms up a bit. It is on the high idle cam when I go to start it. I usually have to start it 4 - 5 times or sit in the car with my foot feathering the gas pedal.

The fuel line from the pump to the carb is brand new, so I know that is not plugged. How much fuel should come out from the fuel line? It will take me a couple of days to check some of the things that were suggested, but I will report back.

Thanks for all the help so far!
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hope you didnt buy a street avenger thats all i can say. if it is NOT rich(too big of carb, thick exhaust smell, almost girgly exhaust sound and blackish plugs with deposite) which would cause hard starting symptoms, then it is definitly lean. pull a plug and check its color.

set the choke so that it is off and see how it starts. choke could actually be hurting its ability to start. (not likely but something quick and easy to check before dissasembly) rich mix would require more air.

id just change that pump out. save yourself a lot of trouble and mess as well as eliminating it from the equation. it only takes about ten minutes. if you do, remember to stick a drill bit or something in the line from the tank as it will continue to gravity feed out.

make sure you do not have 2 fuel filters. this is quite an added restriction and can sometimes cause vapor lock lean symptoms.

could be simple as a low or high float setting. (there can be fuel in the carb and not in the line the level in the carb would just be low)

relay your findings. check the things i mentioned. remember that evap theory mentioned earlier, it only takes a little to open those seats up and expose the line to air.
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There is a weep hole on the fuel pump. If the diaphragm has a hole in it, fuel will be coming out the weep hole.
I sounds like your choke needs to be adjusted, factory settings may work and may need different settings for your location, altitude, and any performance changes. So you need to determine which way to go, remove the air breather, press the gas pedal once to set the choke, start the engine, the choke pulloff diaphragm should open the choke just a little to give the engine some air, if the engine is not running right, then use your finger to open the choke butterfly a little more, if the engine speeds up the choke butterfly is closed too far, either loosen the choke or adjust the choke pulloff to open the plate more, if it didn't speed up, then may be open too much, so do the opposite of the above. Post your results. Good luck.
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Thanks to all for your help. It will likely be Saturday before I have any additional information, but I will surely let everyone know what I tried and what worked (hopefully).

90 to 120 seconds for the choke to open sounds too short of time. When you say it opens do you mean fully open? If so, then you need to adjust choke to stay on longer. Should say rich & lean on the choke cap, just move it a notch or two.
Well, I spent a few hours over the last few days tinkering with some of the items that everyone suggested and decided that the fuel pump was allowing the fuel to slowly flow backwards, but not into the crank case. After sitting for one week, the fuel line from the pump to the carburetor was completely dry.

I first tried playing with the choke settings to lean it out or make it richer and that did not seem to help. I then pulled off the pump and tested it to make sure it was pumping adequate fuel, which it seemed like it was. Finally, I tried putting a very small amount of pressure on the outlet and could hear wisping coming out the inlet. The replacement pump I bought did not do this, so I ended up installing the new pump.

I disconnected the coil wire and cranked the engine for a bit so the system would fill with fuel and then started it. It killed once but the second time (with a cold engine) it started and stayed running. Time will tell as I will let it sit for another four or 5 days and then try it before deciding if it is really fixed. I will let you know what happens.

One good thing to come of this is I found out the existing fuel pump was from a 66 or 67 (no fuel filter canister) and the replacement I got is the correct one. Definitely looks better!

Thanks to everyone for your comments and suggestions.

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