The Stovebolt Site Forums The Shop Area Engine Shop Fuel Pump Issue

The front end of my '48 GMC, stock 228, was up on jack stands while I worked on the king pins and brakes. I liked to run the engine every week for 20 minutes or so and managed to run the truck out of gas. I thought it was the combination of the inop gas gauge and the raised height of the front end.
Finishing my brake work I got the truck off the stands and back on the ground. The stock fuel pump with the glass bowl showed no fuel. 6 gallons of fresh fuel went into the tank, still no fuel showing. I disconnected the fuel line at the tank side of the pump, put my hand powered vacuum pump on the line and had gas after 2 quick pumps.
Cranking the engine over still didn't get fuel to the pump. I started the engine by priming the carb but it only ran for a few moments. I repeated this several times and the truck, with choke, started and ran each time it got gas. Still no gas in the pump.
I disconnected the fuel line at the carb and back filled the line until I could see fuel in the pump. Primed the carb, started the truck, it ran again for a few moments and died. I can still see fuel in the pump but it isn't getting to the carb.
I disconnected the fuel line at the carb again, cranked the engine and saw just a couple of weak squirts of fuel come out of the line.
Reconnecting the line I started the truck again, ran and died, still just a little fuel in the pump.
Questions: what should the fuel level look like in the glass bowl on the pump? Should the pump be able to pull fuel from the tank without having to be filled with gas? What else should I try to do?
As always, thanks in advance! Jim

Well you have done a good job so far. bad pump, intermittent block at tank pick up, gas cap not venting. Do and test these one at a time:
1. Take off cap and try.
2. Pull fuel from a gas can. (Hose attached to pump inlet and stick other end into a plastic 2 gal container)
3. Report

These are quick and easy to do and in 15 minutes you will know what is wrong for sure.
It is possible that running out of gas did cause a "pick up" of gunk into sender tube or hoses.

How much vacuum is the pump developing during cranking, and how long does it stay there when you stop cranking the engine?
Jerry

Forgot one thing to ask. Do you have a filter between tank and pump? If so, take it off and try.
It will take more time to test vacuum and all that, than just doing my isolation/elimination procedure. In my opinion, You can do component testing once you isolate the problem. Pick which way you want to skin this cat. Either way is useful.

OK, here's what I got so far. Gas cap off, no change. Put vacuum to fuel line from the tank to the pump and it was holding pressure, no gas moving from the tank! That's different than when I did the same test before. Good, seems like progress.
Got out my little compressor and put some air back through the line, re-tested with vacuum and got fuel flowing through the line, like I was able to move an obstruction.
Disconnected the fitting at the carb, put vacuum to the line and was able to watch the fuel fill up the pump and was able to get fuel all the way the carb end of the fuel line.
The fuel level visible in the glass dome on the pump went from almost full back down to about 1/4" in the bottom of the pump when I removed the vacuum pump. I could hear the slightest air leak, I think coming from near the pump.
Running problem remained the same as before.
I wanted to try running from the auxiliary gas can to test the pump but can't figure out how to temporarily connect it up. The pump has female threads but I don't have a fitting that I could attach a rubber fuel line to. I can try the fuel pump pressure test tomorrow.
Just for fun, I opened the petcock valve on the tank to see if any dirt or gunk would come out but nothing at all came out and now the petcock has a small drip!
Regardless, it seems I can easily pull fuel from the tank, through the pump and all the way up to the carb with just a few pounds of pressure on my vacuum pump.
Leaning towards a bad pump but tomorrow's another day!
If I understood all the functions of the pump better it would be easier for me to figure things out!

The pump is a rubber diaphragm operated by a lobe on the camshaft. The cam pushes the operating lever outward against a spring and the diaphragm causes a vacuum to be put on the inlet line which draws fuel into the pump. As the cam lobe moves away from the pump the spring pushes the diaphragm back to its original position and forces gas into the line going to the carburetor. There are one-way check valves which are supposed to keep the fuel from flowing back to the tank. A good pump will develop 10 inches of vacuum or more on the inlet side just from cranking the engine, and hold it for a few seconds after the cranking stops. If there is fuel being pushed through the outlet line, it should have a pressure of 3-5 PSI, an d also hold pressure for a while if the outlet check valve is working.

A good pump will deliver about 1 pint of fuel in 30 seconds to a container from the fuel line at the carburetor if you run the engine at idle speed with the line disconnected. Use a long piece of rubber fuel line to keep the catch container well out of the way of any backfires, etc. If the engine won't idle for 30 seconds on the fuel in the float chamber, fix the carburetor!
Jerry

You probably won't see the glass bowl fill completely with motor running. The proof of pump operation is the flow test you did and as described by Jerry. Good, pulsing, strong, squirts.

To run from a can you need a simple hose barb fitting. Like This sized to your pump threads and hose. You may have one somewhere in your fuel system you can "borrow".

Thanks Bartamos, for your practical diagnostics and reference for the barbed fitting. I don't have one but my parts store does! It's easier to test the pump output than pull it. I'm pretty sure the fault lies with the pump thanks to Jerry's excellent tutorial.

It sounds to me like you have more than one issue going on at the same time.
When you open the petcock, all of the fuel should pour out of the tank. You say that on one test, "nothing at all came out and now the petcock has a small drip."
You have crud in the tank. It may be moving around causing intermittent fuel delivery stoppage.
Until you address that issue, you cannot diagnose anything else.
Carl

You could, as I say, have blockage as your problem. That is why you use a process of eliminate/isolate. This is useful if you don't have a lot of testing equipment. It is sometimes much faster and less chance for error or misinterpretation.
With the gas can you are eliminating the tank/blockage, the tank pick up, the screen, petcock, fittings, filters, cap vent and hoses and isolating one component, the pump. This is a quick way to focus on what area of the system has a problem. If pump does not squirt, it's the pump. If pump begins to squirt out the outlet, you hook up outlet to carb and run the motor from the can. (BTW: Set the can on the ground). If motor runs, it's a line/tank problem. If it does not run, probably a carb problem. With just a gas can and a hose barb fitting, you analyze the entire problem. No vacuum testing, taking tank off, replacing pump, lines, filters, forcing gas into lines, rebuilding carb and so forth. This isolates and eliminates a good portion of the fuel supply system. If it turn out to be lines/tank, then you isolate and eliminate that area where possible. More on that as first results are known.
I don't know too much about the sight glass fuel pump. I don't use them or like the idea. They can possibly leak gas, spill gas or suck air and are not necessary, except for looks. It is slightly possible it is doing something to cause this. They say it's a filter, a sediment bowl, a water separator and assurance. I say, that's a bunch of "horse and buggy". Filter your gas BEFORE the pump, a completely sealed pump!

("nothing at all came out")
I had the same concern as Carl but decided you meant no gunk came out. Not that no gas came out. Only small particles will come out, larger may float around and maybe block when flow/suction starts, as Carl says.

No, I did mean that no gas came out of the petcock. All the way in and all the way out, no gas or crud.
The petcock is attached to the same fitting as the fuel line from the tank and I am pulling nice, clean gas with my hand vacuum pump.
This is a 1 ton truck with the under the flatbed tank. The fuel line and the petcock are mounted on the front side and are up a few inches from the bottom of the tank.
I'll assume there is a pick up tube and filter inside the tank. It's almost certain that the tank filter or line was blocked by debris and that by blowing a little air back through the line I relocated the crud, at least temporarily.
I'm not by nature a conspiracy theorist but it does seem I may have more than one problem ganging up on me!

Aren't petcocks at the very bottom? If it is up, then less likely to plug up? Do you have a drain at bottom of tank? Don't know 1 ton gas tanks BUT if the fuel is being drawn from a spot a few inches above bottom, I fear that 6 gallons is not enough. But can't believe it's being drawn from that point if that is what you are saying. On the other hand, if no gas came out, I am totally confused. No gas came out but it drips? Seems like a blockage. If you keep pulling a vacuum on the tank/lines you will plug them up good. If you would draw from a can, we can move forward. (or backwards, in this case )

Progress report. Fuel pump will not draw from a gas can. If I gravity feed to the pump it barely pumps any fuel at all.
Regarding fuel tank, the fuel outlet line is connected to a fitting at the front of the tank that also has a petcock valve. These are located 4" above the bottom of the tank. At the rear there is a drain plug on the bottom of the tank.
I don't know what the inside of the tank looks like so I'll have to imagine that there is a filter and pickup tube similar to an oil pump setup in an oil pan. Otherwise why would the fuel outlet be located so high above the bottom of the tank?
I wondered if the tank was original to this truck, a '48GMC, so I checked my other 1 Ton, a '48Chevy, and it has the same tank setup.

Isn't there a fuel gauge sending unit at the top of the tank? I'd suggest removing it and shining a bright flashlight down into the tank, or maybe using a borescope to inspect the tank for debris. I have one with a built-in LED light that I use for inspection of engine cylinders that will fit through any hole 1/4" or larger diameter, and it projects a full-screen image on my laptop computer. Very useful item, and they're available on Ebay for less than $50.00.
Jerry

If fuel pump will not draw from a gas can then sounds like your in the market for a new pump.

I love how a simple job gets so involved! Jerry, my truck is a flatbed and one of the jobs on the 'list' is to make an access panel on the bed for the gauge sending unit.
In one of my earlier posts on this thread I mentioned an inoperative fuel gauge but more accurately it's a bad sending unit. Fixing it was already on the 'list' but other things got ahead of it. Just as well because now I can justify buying one of those inspection scopes, see into my tank after I cut an access opening and repair/replace the sending unit.
And Martin, a new pump is on the way already!

Thanks, Jim

OK Jim, That is a step forward. Good. Run the motor with new pump out of can. Runs good? Awesome. As you know, you don't want to run it with the old hoses, filter, tank and etc. This now must all be new or renewed.
If new pump does not draw from can. The pump arm is not getting actuated? or there is a crab stuck in the carb inlet.

OK, new pump installed. Truck runs fine from a can but doesn't pull fuel from the tank. Time to drop the tank and clean everything out.

It's a blessing in disguise that I managed to break down in my driveway instead of out on the road somewhere!

Thanks everyone for your help! Jim

You got this!