Fuel pump maintenance 101 explained




Hopefully, after last month’s article on gas tanks, we are all ready to get the fuel out of a good clean and lined tank and to the carburetor where it can do what it is supposed to do in the engine.


            The main component needed to do this is the fuel pump. In some of the real old cars, the gas tank was higher than the carburetor and gravity was the fuel pump. Early versions of the fuel pump were vacuum tanks where the vacuum from the engine brought gas from a low-mounted gas tank into a cam mounted high on the firewall and gravity took it from there.


            In the ’30s, the (modern) mechanical fuel pump was innovated. This is operated by a lobe on the camshaft added for this purpose. This lobe moves a lever that operates a diaphragm between two check valves in a “pull–push” action, pulling gas from the tank and pushing it to the carb at about 3 1/2 to 4 1/2 pounds pressure. The cam only moves the lever about 3/8 inch. This is a good time for a piece of advice on installing new or rebuilt pumps.


            Don’t make the mistake of “testing” the pump by forcing the lever up and down while on the bench. If you move it more than 3/8 inch, it will stretch the diaphragm and you can reduce the pump’s efficiency by as much as 50 percent. Another factor that reduces the efficiency of the fuel pump is wear on the cam lobe that drives the pump. Before you put that new or rebuilt pump back on the engine, be sure you polish the surfaces on the pump lever that rides on the cam.


            At best operation, our favorite old cars tend to have problems with “vapor locking” under many conditions, not just hot weather. Anytime an air or vapor bubble gets into the system, gas won’t pump. The same thing happens if the smallest particle of dirt wedges in one or both of the one-way valves of the fuel pump. This is another reason to put a disposable in-line fuel filter as close to the gas tank as possible.


            Nearly all of the these older cars have added an auxiliary electric fuel pump for safety, efficiency and comfort. Properly and neatly installed, they will not cost you any points in competitive judging. They are a necessity as well as a luxury.


            There are two basic types of electric fuel pumps. First, the most common is the pulsating type that will pump to a certain pressure and electric contact shuts off the current. The biggest disadvantage of this type is the awful noise it makes. It, too, has one-way valves that, if stuck by dirt, loses effectiveness. The second, and far superior, (in my opinion) is the rotary type that is nearly silent and much more dependable.


            This type continues to run and pump as long as the switch is on. With both types you should also have an adjustable pressure regulator in the line. This type pump has an automatic bypass when the set pressure is reached and maintained. Failure to put a pressure regulator in the line with either pump can override the needle valve in the carburetor, causing gas to overflow the carburetor bowl, spilling raw fuel onto the engine or into the cylinders, making the car run poorly.


            When auxiliary electric pumps are installed, there are two ways to install them. The most common and easiest way is to install in series by cutting into the main fuel line somewhere on the frame, thus making all gas go through the pump. When you need the auxiliary pump, you just flip a switch and bingo, you’re running right again.


            Never wire the pump to run all the time…this is very dangerous for obvious and aforementioned reasons. The biggest problem with installing in series this way is that you increase the need to use the electric pump, because the mechanical pump has to work harder to pull the fuel from the tank through the electric pump and its check valves. When this coupled with a slightly weakened diaphragm and/or slightly worn cam lobe and/or fuel pump arm, it seems like you have to use the electric pump most of the time. The best (but more difficult and expensive) way is to install the electric pump in parallel with the mechanical pump.


            You mount the pump on the frame (or other suitable and accessible location). Install a tee fitting on the inlets side of the electric pump. You also install a tee fitting on the outlet side of the mechanical pump. On the electric pump inlet tee, splice a flexible fuel line around the electric pump and attach this line to the original fuel line that goes to the inlet of the mechanical pump. Then install a new and separate line from the outlet side of the electric pump to the tee on the outlet side of the mechanical pump (see diagram). This will allow both pumps direct access to the fuel in the tank independent of each other. Don’t worry, the check valves in both pumps prevent feedback, draining or pressuring.


            The advantage of auxiliary electric pumps are many. One of these advantages is in starting up after sitting a few hours, days, months or even years. You will save much wear on the starter and battery. Also, you now can climb the steepest hill in the highest mountains on the hottest day, and still run at maximum efficiency. One tow bill from a failed mechanical pump will probably pay for two auxiliary electric fuel pumps, installed.


            I doubt if you will find any experienced cross-country tour participant in any make of old car that doesn’t have an auxiliary electric fuel pump installed. Some learn this from their own mistakes and some from other people. Which kind are you? I prefer to learn from other people’s troubles rather than my own, it’s much cheaper and less trouble.


            Time for lights out—see you next month.