The Stovebolt Site Forums The Shop Area Engine Shop Testing a mechanical fuel pump (235)

Is there anyway to test a mechanical fuel pump with it disconnected from the engine? I assume that operating the lever by hand would work, but not being able to do it as fast as it would attached to the engine leaves my assumption in question.

Any help here?

You can test it by hand. A vacuum gauge works best but not nessesary. mount it in a vise. Hold your finger over the inlet port. Work the lever. You should feel a suction or the gauge should show a vacuum. The vacuum ought to last for a few seconds. Put your finger on the outlet side & work the lever. It should build up pressure. This is pretty general. It still might leak when on the engine.

in my experience the pumps are pretty much go/no go - start with the sound when you actuate the lever, will be a definite suck and blow - hold a finger over the inlet and work the lever, you'll feel the suction - then do the outlet, feel the pressure .... if it's the style where the body halves are held together with screws it's easy to open it up and look at the diapragm to insure it's clean and pliable - used to be kits available to replace those

the other thing to watch is if the end of the lever is worn a lot where it contacts the cam - what acts as failure sometimes is just that the arm isn't being moved a full stroke so apparent pressure is low

Bill

Sorry to be bumping an old thread like this, but I am new to truck repair and had a similar question.

I have been trying to get my '54 3100 running well enough to drive and a friend told me that he thought I wasn't getting enough gas to the carb because the liquid level in the inline filter dropped too much when the truck was running. He thought it might be the fuel pump.

I have pulled the fuel pump off and it seems to work fine. The first time I operated the pump manually I didn't realize that there was still fuel in it and gas shot out of the discharge, coating my workbench. I assumed that this means I am getting enough flow through my pump, but is there a way to check to make sure?

If I did put a pressure gauge on the discharge, what range of pressure should I be looking for?

Do I need to check the discharge pressure or does the evidence of shoting gas mean I am probably fine?

BTW - I currently have the gas tank out to clean it, am replacing the sending unit, have rebuilt the carb and have blown out the fuel lines. I haven't put it all back together yet, but I haven't seen anything that would make me think that I had a clogged line somewhere.

My fuel pump is sealed so I can't take it apart without bending metal.

Welcome to the 'bolt! If your pump is capable of making a mess on your workbench it's probably good. As far as output pressure, you won't see much, probably in the 3# or 4# area.

As for the low level in the inline filter, mine looks like that too. There's always an air bubble in there.

I read in your blog that it will run with the choke partially out. You may have a vacuum leak somewhere that is causing that condition. Check your wiper motor and vacuum advance cannister to see if they hold vacuum. Also look for cracked or broken vacuum hoses. Timing may be an issue too. Put your light on it next time you have it running, check your point gap also.

Keep us posted, hopefully you'll get 'er purring like a kitten soon.

Thanks Waldo. I know that I definitely need to adjust the timing. I will let you know how it goes after this weekend.

hi all
Similar question - Just picked up a new pump from NAPA, got home, and cant feel any pressure when I wiggle the arm by hand.
Is the finger test reliable enough that I should return the pump? Or should I install it anyway to give it a try?

I'don't think the arm should wiggle. I should be a pretty firm push when the pump is secure.

When i first fired up my 58 that had been sitting for about 30 years, the fuel pump shot gas EVERYWHERE. I pulled it off and just slapped a new one on there, if you want to keep your original pump i know chevs of the 40's has rebuild kits. As for the arm wiggling or not..it shouldnt wiggle, there should be a significant amount of force needed to push the lever in or else i don't think your goign to get any fuel pressure.

you will have to further define "wiggle", are you pushing it fully or just "wiggling" it, I would assume it will have a bit of wiggle/slop, its not really that precision of a piece. There is a spring you are working against, it isn't really stiff, but you definitely know if you are compressing it or not. you should get suction and pressure if you are operating it by hand. but I am not so sure you are running it through its paces completely. The word "wiggle: is the one that gets me, there should be no action that resembles wiggling that would make the pump work or not work

The working pressure should be around 3 to 4 psi.

Should I be able to blow by mouth through the fuel pump both inlet side and outlet side? Wouldn't this indicate the pump is no good?

Bad pump. There are two check valves that should only allow flow from inlet to outlet, not the other way around. Could be a bad outlet check valve and broken diaphragm also, but either way, the pump is bad.