The Stovebolt Site Forums The Shop Area Engine Shop Symptoms of a bad fuel pump

Wondering what you guys would consider to be symptoms of a failing fuel pump... I can start my truck and it runs, ok, random slight miss at idle. Runs ok with an occasional miss, that everbody seems to believe to be fuel starvation.

While under the hood and I can watch the inline filter, and the pump barely keeps fuel in it at idle. Also appears that quite a bit of air bubbles get pushed in to the carb. It seems to me that some of this air getting injected into the engine would create a slight miss, and if the engine is missing under load from fuel starvation then the pump may not be able to keep up with demand...

What think you? For the record the carb was recently rebuilt by myself, and the one thing I wasn't sure on that might contribute to this, was setting the float, I wasn't exactly sure where to bend the arms...

First is to look for obvious leaks. If gas or vapor spray is comeing out the vents on the pump while it is running its a sure sign of a bad pump, or gas getting into the oil. You can check it with a pressure gauge, it should put out 3-5 lbs pressure. You can take the line loose at the carb & put a glass or can under it. Start the engine. It should fill the container in a few seconds. Those transparent filters always act like that. I dont know why. From your syptoms it sounds to me like its an ign problem. Take out a spark plug or 2. See if there is a black carbon buildup on them. That can cause an intermitant miss.

asilverblazer,

Air cannot get injected into the engine on a carburated system. If the pump were to transfer air somehow, as soon as it hit the fuel bowl in the carburetor, it would rise to the top in the form of a bubble and vent to the atmosphere.

Wrenchbender speaks the truth..those little clear glass or plastic inline filters will appear only half full of fuel, but that's the nature of the beast. They are truly passing the full output of the fuel pump. I guess because there is no back pressure in the pump to carburetor plumbing, they won't fill completely full as you would expect.

Typically, when an engine starves for fuel (runs out of fuel), it will just plain shut down, usually accompanied by backfires and snorts. Missing isn't normally associated with fuel starvation.

You could have a suction side leak on the pump, but it would almost always drip when the rig was parked in the gararge or at the curb. Don't let the visible fuel filter deal send you down the wrong troubleshooting path.

Follow Georges recipe above!

Stuart

If I were to find a sooty spark plug, what should I suspect?

A slight miss at idle, no detectable missing while driving, random miss after its fully warmed up under moderate load, which can not be duplicated?

In fact it usually runs great on my way out, but coming home it runs with less gusto, and with the a random miss.

asilverblazer,

Obviously a sooty plug on a single cylinder tells you that it is missfiring for some reason. Fuel problems would involve all cylinders and plugs.

Basic sleuthing would require a compression test and a look at the valve train. The ignition system with it cap and wires could cause problems specific to one cylinder only.

A good basic tune up, timing check and the above mentioned items would narrow down the problem.

The coil is know to be temperture sensative as well as the valves. Too tight and one might not be closing all the way as things warm up.

A vacuum leak in a intake runner could cause problems too...there are so many 'maybes' that you need to do the basics to wipe all the variables from the chart. Do a full tune-up, run the valves and do a compression check. You will cure the problem or narrow your search so you can pinpoint it more easily.

Stuart

It has a recent tune up on it, points, condenser, plugs, wires, rebuilt carb, cap, rotor, coil, the only thing left is adjusting the valves, checking for vacuum leaks and compression test.

Hmm, a trouble code would sure be handy.

What were the symptoms of a weak or failing fuel pump?
I'm having similar problems with my 216. It happens lately only when I'm going uphill. It coughs, sputters and then dies. If I turn it around and head down hill or level it seems to run okay.
It's full of gas. My engine also has to have the choke partially out to run at both start up and while driving. Plugs are dry black. Fuel pump appears to be relatively new and the bowl is full. No inline filter.

Thanks,
Spot

Spot,

While I'm sure fuel pumps can slowly die, my experience has been that they work, or don't work at all. Your problem almost sounds like a float that is set improperly.

I sure would check that, but I would also make sure the fuel pump had a unrestricted supply of gas from the tank. These old truck can accumulate gobs of junk in the tank.

When you head up the hill, you are loading the engine, and this would be a perfect time for it to start using a lot of fuel.

If you can give the line all the way from tank to carburetor a clean bill of health, I sure would suspect the carburetor itself.

If you have a short flexible hose near the discharge of the gas tank, make sure it is in perfect condition. These can collapse internally and strangle the flow of fuel to the pump.

The fact that you have to use the choke to run the engine is another sign of a low float setting. Vacuum leaks are a possibility, but check the float level first.

Stuart

Here's the test procedure we used for a lot of years:

1. Disconnect the gas line at the carburetor, and plumb it into a catch container of some sort. The most common one was a 1-qt. oil can, back when they were round, and made of metal. Start the engine and idle. The pump should flow a minimum of 1 pint in 30 seconds. The gas in the carburetor float bowl should be plenty for a 30-second idle time without stalling.

2. Attach a pressure gauge to the fuel line. Run the engine. The pressure should rise to a steady 3-5 lbs. and stay there for a short time after the engine is stopped. (10 seconds or more)

3. Attach a vacuum gauge to the inlet side of the pump. Run the engine. The vacuum should go to 12-15 inches of mercury, and stay there for a few seconds after shutdown. The above tests check out the inlet and output check valves. Replace the pump if it fails either of the check valve tests.

4. If the pump is low on volume, but passes the check valve tests, look for a restriction or an air leak on the input side. Attach the vacuum gauge to every fuel line connection from the pump to the tank- - - -at the flex line, and every line connection all the way back to the tank outlet. If it holds vacuum at the tank outlet line, the problem is inside the tank. I once found a 2-inch long grasshopper lodged in the tank pickup line on my first car, a 47 Mercury convertible. That car would run 1 mile and starve out, repeatedly.
Jerry

Thanks to both Atomarc and Hotrod Lincoln for your help!

Atomarc.....when I'm running the truck it's usually just around the neighborhood and I'm never carrying anything. No load.
Usually a big cough and it when I pump the gas it dies out like it's starved for gas. Turn it downhill and it usually starts.

I'm changing out the bedwood and noticed yesterday that short, flexible piece of hose coming out of the gas tank you spoke of.
It looks original and therefore dryrotted, and pretty rough. I told myself I should replace that piece, so I'll definately do that.

Hotrod Lincoln....... Great test procedure info, so thanks!!

Kind regards,

Spot