ELECTRIC FUEL PUMP
TROUBLESHOOTING GUIDE
The following pertains primarily to Mallory COMP Pumps
equipped with Gerotor style pump systems, but also
applies to many other fuel systems.
BASIC INFORMATION
The Mallory Gerotor style COMP Pumps are more forgiving
than vane style pumps, but it is still important to insure
they are properly mounted and receive clean fuel.
The correct mounting position is even with or below the
tank and preferably behind or next to the fuel tank. All
electric pumps push fuel far better than they can pull
(suck) fuel.
Make sure that a low restriction filter is between the tank
and the pump. A 40-micron element it sufficient to pro-
tect the pump from contamination such as dirt, rust, fuel
cell foam, etc. Mallory has them available that will screw
directly into the pump itself.
Electric pumps need a clean and stable power source. Run
a minimum of 12 gauge wires to the pump if it is relatively
close to the power source. Longer runs should utilize 10
gauge wire instead. With a trunk-mounted battery, we
recommend that you use a relay to allow for short power
wires to be run. Do not pull power from a common termi-
nal, especially if the
ignition is wired to the same terminal.
The regulator used must be properly matched to the flow
characteristics of the fuel pump. Refer to the compatibility
information in the current Mallory Catalog. Small regula-
tors and big pumps don’t work. Also, check to make sure
the pump and regulator are designed for the type of fuel
that is going to be used.
For the longest possible life from your Mallory or
any other brand of electric fuel pump, it is highly
recommended that you use a bypass style regulator.
This allows the pump to run cooler, keeps the fuel cooler
reducing the chance of vapor lock, draw less amperage,
and supply constant flow of fuel. The minor inconve-
nience that running a return line to the tank presents is
more than offset by the positives mentioned above.
Fuel line size is based on the length of the lines in
addition to the intended use. The supply line to the
engine is based on the horsepower of the engine. The
return line size is based on the total volume of the fuel
pump. Basically, to insure stable fuel pressure, the
return line should be of sufficient diameter to handle
the full volume of the fuel pump without the engine
running. That means that the return should be as big
or bigger than the supply line.
Use liquid thread sealing or a suitable pipe dope and
not thread sealing tape on all threaded fuel systems
connections. Pieces of the tape can break off as they
are being assembled and plug up lines, get stuck on
small passages, and even seize up a pump if they are
large enough.
Don’t buy cheap fittings. Use low restriction fittings, pref-
erably CNC machined ones like those listed in the Mallory
catalog. Tightly angled fittings can be just as bad of a
restriction as too small of a line size. The larger the radius,
the better and make sure that you buy quality fuel lines.
Look at them closely and make sure they have a large
inside diameter. That especially applies to some of the
low cost dual feed lines being sold today. Some 3/8” OD
lines are 1/4” or less ID.
FREQUENTLY ASKED QUESTIONS
Q.- Why can’t I adjust my fuel pressure down? Even with
the adjuster backed all of the way out, I still have too
much.
A.- Start by checking to make sure the regulator is suit-
able for the pump you have installed. Next, if it is a
bypass system, check the return line size. If the return
line is too small, the regulator isn’t causing the pres-
sure, rather it’s the return line size is too small. This
is especially a problem when the factory return line is
used. Ford Mustangs are probably the most common
offenders.
There are a couple ways to prove it. The easiest way is
to remove the spring from the regulator. Without start-
ing the engine, turn on the pump. The pressure gauge
must read less than 3 PSI on a carburetor-equipped
engine and less than 10 PSI with high-pressure fuel
injection.
Q.- When I first hit the throttle hard, the pressure drops
then comes back up. Why?
A.- The pump has to catch up to the demand. The
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