THEORY OF OPERATION
The following vehicle repair strategy is to be used when
encountering a NO FUEL PRESSURE/VOLUME symptom. The fact that fuel pressure
is not present does not automatically mean that the fuel pump is bad. The
knowledge gained here will help in present and future system specially on
newer variable speed fuel pumps that are controlled by the ECM.
FUEL SYSTEM OPERATION
Modern automotive fuel systems are composed of the following
• FUEL PUMP
• FUEL FILTER
• FUEL PUMP
• FUEL PRESSURE
At present time there are 3 basic fuel delivery systems
employed in automobiles, RETURNED FUEL SYSTEM, VARIABLE SPEED FUEL SYSTEM,
and RETURNLESS FUEL SYSTEM. It is important to understand that fuel pressure
and fuel volume are NOT one and the same. The ability of a fuel delivery
system to maintain a specific pressure (in PSI) is an important factor when
analyzing a fuel system. It should not be confused with volumetric or
ability to deliver a specific amount (in Gal. per minute or Litters per
minute) of fuel. As a rule of thumb, when a system can not maintain a
specific pressure, the vehicle will probably run very bad or not run at all.
On the other hand, it is quite common to see vehicles with good fuel
pressure and have performance problems due to a lack of fuel volume. Good
fuel volume is the system’s ability to maintain a specific quantity or
volume regardless of vehicle operation. Such quantity or volume is measured
in GALLONS or LITTERS per minute. The use of a fuel volume gauge is highly
recommended to diagnose modern fuel systems. These units have a small
specific gravity scale that tells the technician the amount of fuel flow
going through it in gallons or Litters per minute.
• The RETURNED
FUEL SYSTEM is the average system used on most older and late model cars.
These systems use a fuel pump, filter, dedicated fuel lines, fuel pressure
regulator, and return fuel lines. In this system excess fuel that is not
needed by the engine is dumped back to the tank. The main drawback of this
system is the heating of the returned fuel by the engine; causing excessive
fuel vapor inside the tank. This is the main reason why this system is being
phased out in favor of the returnless fuel system.
RETURNLESS fuel system is being employed in late model cars superceding the
older RETURNED system. It consists of a fuel pump, in-tank fuel pressure
regulator, filter, fuel lines and NO fuel return lines. Given the ever
stricter emission control laws, the advantage of this system is that no
heated fuel is returned back to the fuel tank. This system greatly reduces
fuel vapors inside the fuel tank, making for an easier and cheaper to design
EVAP system, thereby, reducing the amount of fuel vapors being released to
the atmosphere. Sometimes the fuel regulation is accomplished by the use of
electronics as explained next.
• The VARIABLE
SPEED SYSTEM is basically an adaptation to either the RETURNED or the
RETURN-LESS system. Fuel delivery is normally accomplished with a regular
fuel pump that is being controlled by a fuel pump module or the ECM itself.
The fuel pump module actually controls the available current going to the
pump, therefore, varying the pump’s speed.
Two basic variations of this system are available the dual speed and the duty cycle controlled systems. The
dual speed system has two voltage settings feeding the fuel pump. The system
supplies full battery voltage (maximum current) to the fuel pump during high
fuel demand conditions (as in cranking, warm up, and acceleration) and a
lesser voltage (low current) being applied during less demanding engine
conditions (as in cruising and deceleration). Duty cycle controlled means
that the fuel pump itself is constantly being controlled by the fuel pump
module or ECM, by way of a pulsating DC current. This is accomplished by
applying a variable width
square wave (duty cycle) to the fuel pump. In other words, by varying
the ON-TIME pulse being applied to the fuel pump the pump’s rotational speed
can be regulated. The higher the demand for fuel the longer the on-time and
the lower the demand the lesser the on-time. Again, this system can be found
with either the returned or the return-less fuel system. Some of the new
systems actually have the fuel control module, fuel pump and fuel sending
unit housed in the tank, thereby, doing away with the fuel pressure
regulator all together. Not only is this system good for the environment,
but also extends the life of the fuel pump itself.
FUEL SYSTEM TESTING
The following steps should be used to properly diagnose fuel
system problems. These steps are arranged in a logical sequential order.
Connect a fuel pressure/volume gauge to determine if there is a problem.
Fuel problems are categorized into two possibilities, fuel pressure and fuel
volume problems. Proceed to the correct testing procedures bellow according
to which of the two specifications is at fault.
FUEL PRESSURE is the ability of a fuel system to maintain an
adequate PSI value or pressure regardless of engine conditions. Fuel
pressure specifications are widely available and fairly easy to find. It is
important to remember that a specific amount of pressure can be maintained
regardless of volume. In fact it is possible to have adequate pressure and
no fuel going onto the engine. A good example of that is when the fuel pump
is primed and dead-headed, by crimping the return line. (It is not recommended to dead-head
the pump for a long period of time). In this case the fuel gauge will
read maximum pressure without any fuel flow at all. Fuel pressure problems
could be caused by the following:
or weak fuel pump. (Fuel pumps can be checked by using current ramping
techniques while they are in operation, as well as a fuel
fuel pressure regulator. (Regulators can either fail shut causing
excessive pressure or open up letting too much fuel go back to the tank
through the return line. A punctured regulator will cause a rich condition).
resistance in the fuel pump relay thereby, reducing available current
going to the pump. (It is important to always measure the available voltage
and current at the pump. Performing a voltage drop reading across the fuel
pump relay is a good indication of the fuel pump relay’s general condition.
placing a headlight in place of the fuel pump, while activating the fuel
pump relay, will also reveal a lack of current problem. The brightly lit
headlight will draw an adequate amount of current of 3 to 6 amps, pointing
to good circuit conditions).
fuel lines. (This will cause either very low or very high fuel pressure,
depending on where the lines are kinked).
check fuel flow with the engine running. A running engine will have 14.2
volts available to the fuel pump as opposed to 12.6 volts. The difference in
voltage between KOEO & KOER will skew off the readings making you assume
that the fuel pump is actually defective.
NOTE: It is important to understand that
a clogged fuel filter, in real life, may not cause fuel pressure problems
unless it is totally shut. On the other hand, it will cause very low fuel
volume or almost none at all.
FUEL VOLUME delivery is the ability of a fuel system to
maintain an adequate amount or quantity of fuel going to the engine
regardless of engine operating conditions. Fuel volume specifications are
very hard to find and they are usually acquired by trial and testing the
vehicles that the technician actually works on. A fuel volume gauge is a
highly recommended tool when analyzing fuel systems. The gauge works by
passing the fuel going into the injector rail through a glass tube with a
small metal ball inside. That ball has a specific gravity (exact weight) and
as the fuel moves up the glass tube so does the small ball. The exact amount
of fuel flow being delivered by the fuel pump can be ascertained with such a
gauge. Fuel volume problems could be caused by the following:
fuel filter. (A clogged filter will cause a low fuel flow condition. A
reading of under 0.3 Gal. per minute is usually considered inadequate to run
fuel pump. (Will cause low volume). Contact resistance in the fuel pump
relay reduces the available voltage going to the pump. (Low voltage at the
pump will cause low fuel volume, since the pump is rotating at a lower RPM
fuel pressure regulator if stuck shut. (Will cause low or no volume,
while the pressure will be very high).
fuel lines after the fuel rail. (Low volume and high pressure since fuel
can not get through. If the kink is before the fuel inlet side, the result
will be low volume with low or normal pressure). These steps should point
you in the right direction and prevent you from replacing unnecessary parts.
NOTE: Some variable speed fuel pumps will
have fuel flow reading lower than 0.3 gal. per minutes at idle. This is
acceptable, since the demand for fuel at idle is quite low.