Go With the Flow: BASIC FUEL SYSTEM ANALYSIS

Motor, Jul 2007 by Thompson, John

While basic fuel system diagnosis may begin with a fuel pressure measurement, it certainly shouldn't end there. The system's fuel volume and flow also must be measured and evaluated.

The fuel injectors are at the end of the line in any EFI system. The entire fuel system and each of its components exist to provide proper injector flow rate through the injector nozzles and into the engine's cylinders. Basic fuel system diagnosis should always be performed bearing this in mind. Diagnosing basic fuel system problems requires an understanding of components, fuel system design, pressure and flow theory, as well as diagnostic techniques. Lets start with fuel system components, beginning with the last component first, to explain how an injectors flow rate is calculated.

Fuel injectors are designed and rated for the quantity of fuel that can flow through them at a given fuel pressure and duty cycle at mean sea level. The amount of fuel that an injector can flow is measured in pounds per hour. For rating purposes, most manufacturers specify a standard operating pressure of 43.5 psi. One exception is Ford, which rates its injectors at 39.5 psi as the standard pressure.

Injector design flow ratings are measured in a static condition, which means they're held open continuously. This is referred to as a 100% duty cycle. However, once injectors are installed in an engine they'll be pulsed with a varying duty cycle (depending on engine load requirements) measured in millisecond time increments. Operating injectors at 100% duty cycle would build up excessive heat within the injector windings, leading to premature failure. So in typical OE applications, injectors are never duty cycled above an 80% to 85% on-time.

Injector flow ratings are factored when an OE manufacturer designs a fuel system for a specific engine size. Specific pressure and flow expectations as well as a dynamic fuel map based on rpm and load of a particular engine are calculated. This fuel map is the primary control factor of injector duty cycle. However, the fuel map assumes that the system's design specifications will deliver an expected pressure and volume of fuel to supply the injectors.

Once installed in an engine, injector flow output depends on three factors-the quantity of fuel flowing to the injector (volume), the force behind the volume of fuel flowing to the injector (pressure) and the injector duty cycle or on-time command from the PCM (pulse width).

If the designed pressure or flow volume is altered by a defect in a mechanical component of the fuel system, or if the injector duty cycle is altered by the PCM due to an incorrect sensor input, injector flow rates will also be altered, ultimately affecting the goal of the fuel system, which is to deliver the required injector output flow based on engine rpm and load.

Fuel filters trap harmful contaminants and are passive components that, when restricted, can cause immediate system problems by reducing fuel flow. Delayed system problems also will occur if a filter is no longer able to trap contaminating particles, which will then travel further down the line and affect other system components (usually the fuel injectors).

Fuel pressure regulators restrict the return of fuel to the tank by a calibrated amount in order to maintain desired fuel rail pressure. If calibrated rail system pressure is exceeded, excess fuel will be permitted to return to the tank.

Regulators typically fail by a ruptured diaphragm resulting in engine vacuum drawing raw fuel directly into the intake manifold, poor seating of the fuel-pressure regulator resulting in fuel leakage to the return side or no return flow to the tank whatsoever when a regulator sticks closed.

To give a practical example of how injector flow rate can be altered by multiple factors, let's assume an increase in fuel rail pressure at idle due to a stuck pressure regulator. An increase in pressure will result in increased injector output volume. The PCM does not have control over the fuel volume being pumped in the system, nor can it control the pressure in the fuel rail. So how could the PCM attempt to prevent overfueling of the engines cylinders? Duty cycle. Faced with this scenario, a PCM (in closed loop) could reduce injector flow by reducing the injector pulse width.

Conventional EFI systems utilize a submersible fuel pump with a permanent magnet electric motor, a vibration damper and a relief valve to prevent system damage from overpressure. Fuel enters the pump inlet tube by passing through a sock-style filter and is pushed through the pump to the outlet by the motor.

Conventional EFI systems also rely on the fuel pressure regulator, not the pump itself, to control pressure in the fuel rail. Any fuel not required by engine demand is diverted back to the fuel tank via the pressure regulator. Therefore, it's important to remember that fuel pumps themselves only supply fuel volume; they do not create pressure in the fuel lines.

Fuel pump current analysis is a technique that's used to identify a deteriorating or defective fuel pump. It utilizes a low amperage probe to first calculate the current drawn from a fuel pump's electric motor, then transfer that information to a lab scope waveform (Fig. 1 above) for visual analysis. This technique may allow you to decide if the amperage drawn by the circuit is typical. It's normal for a pump's initial current draw to be higher when the pump is first energized from a dead stop. As the pump starts to turn and push fuel through the system, the amperage should drop and level off.