Tuesday, July 23, 2024 Detailed Auto Topics
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A fuel pressure gauge is one of the most useful diagnostic tools we can own. With this simple tool, we read fuel pressure and fuel volume. This provides far more information than many people realize.

Beyond the simple pressure test

Testing fuel pressure with our engine idling and accelerating gives good diagnostic information. Fuel pressure that drops when we accelerate infers a low fuel volume. This may be due to a restriction, such as a plugged fuel filter or a weak fuel pump.

Testing fuel volume

A fuel pump may supply adequate pressure at an idle and still cause problems under a load. This is because the engine uses much more fuel when accelerating. Volume is how much fuel we deliver in a specific period. For example, consider a fuel pump that produces one-pint of flow per minute. If the fuel injectors require one-half pint of fuel per minute at an idle, the pump can supply this. The additional one-half pint of flow shows as pressure in the system. At 3,500 RPM, the engine may require one-pint flow per minute. Pressure drops to zero because we use all fuel supplied by the pump.

Testing fuel pump volume

We measure fuel volume with a graduated container, suitable for gasoline. Obviously we must be careful when working with gasoline in an open container. With our gauge setup as in checking pressure at an engine idle, we depress the volume test button on the fuel pressure gauge. We allow fuel to flow into the container for 15 seconds. Most fuel pumps will produce at least one-pint of flow in this time. During the test, fuel pressure should not drop.

If fuel pressure drops during the test, or we collect less than one pint of fuel, volume is insufficient. We can block off the fuel system, as in the dead head test and check again with the engine off. Adequate volume with the system blocked suggest leakage in the system, such as a bad fuel pressure regulator. Volume that remains low means a restricted filter, line or a bad fuel pump. We can replace the fuel filter and repeat the test or with the proper adaptors check volume at the filter inlet.

The electrical factors

When we turn on the ignition switch, the PCM energizes the fuel pump for only a few seconds. When the engine does not start, the computer turns the fuel pump off. To test fuel pressure without the engine running, we need an alternative means, to power the fuel pump.

A relay bypass tool used for testing

The technician uses a wiring diagram to learn how to supply voltage without the engine running. Even more handy are relay bypass devises that fit in the place of the standard relay. We flip the switch and it bypasses the relay for us. These are very handy for testing.

Before we condemn a fuel pump that has a low volume, we should check the voltage available to the pump. Checking the amperage that a pump requires, gives us even more information. We check voltage by back-probing the fuel pump connector. Checking between the power lead and the ground, we should have 12.5 volts. Lower voltage causes the fuel pump to run slower and it will produce less pressure.

Checking fuel pump amperage at the relay socket

We can check fuel pump amperage with an ammeter in series with the power supply. This is easiest at the fuel-pump relay socket. We remove the relay and use an ammeter to jump across the terminals. Usually, a fuel pump draws about one amp for every 10 pounds of fuel pressure. Excessive amperage shows a bad fuel pump.

Residual pressure and hard starting

For the engine to start, gasoline must be available at the fuel injectors. We turn the ignition off and the fuel pump stops running. With no fuel pressure, gravity causes the fuel to run back to the fuel tank. This leaves no fuel available to start the engine. When we turn the ignition on, the pump runs for a few seconds. This is not sufficient to fill the lines and build pressure.

To prevent hard starting, a check valve in the fuel pump closes and keeps fuel in the lines. When we turn the ignition on, the fuel pump can quickly raise pressure and the engine starts. A leading cause of hard starting is fuel pressure that bleeds down too quickly. Residual-pressure should remain in the system after we turn the ignition off. With a fuel pressure gauge, we can measure the loss of fuel pressure.

To test residual pressure, we start the vehicle and note the fuel pressure as in the idle pressure test. Switching the ignition off will cause fuel pressure to drop slightly. The pressure should remain at this point and not return to zero. Fuel pressure that drops quickly shows a leak in the residual pressure system.

Residual pressure bleed down test 

Leaking injectors, a bad fuel pressure regulator or a bad fuel pump may all cause the pressure to drop. With on/off adapters we can isolate the components. First we close the valve between the gauge and the fuel rail inlet (a), as with the dead head test. We turn the ignition on to build pressure and then switch the ignition off. Should the fuel pressure continue to drop, it shows a leaking check valve in the fuel pump.

Closing the valve to the fuel line (b) and opening the valve to the fuel rail (a), allows us to check the injectors for bleed-down. We may run this test if pressure does not drop with the previous check.

By thoroughly testing fuel pressure and volume we can solve many difficult running problems. Sometimes having these tests run by a professional is more cost-effective. In either case, an understanding of fuel pressure and volume gives great insight into fuel system maladies.

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