Sunday, December 17, 2017 Detailed Auto Topics
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Anyone serious about repairing an automobile soon realizes the importance of diagnosis. Testing allows us to eliminate possible causes, without changing parts. One of the first diagnostic tools we should add is a fuel pressure gauge. With this tool and a little knowledge, we can solve many fuel system problems.

A basic gauge and adapters

A basic fuel pressure tester is little more than a gauge with a hose attached. In this form, it is not very useful. We need an adapter, to connect the hose to the vehicle we wish to test. Vehicle manufacturers do not provide a standardized means to attach a fuel pressure gauge on most vehicles. They use many thread types and sizes and some provide no attachment means. The technician uses many adapters to connect their gauge to the various models.

We check service data to show the means of connecting to the fuel system and the adaptor type that we need. Often this is as simple as a fitting to attach to a test port. Other times, we remove components and place adaptors in the fuel line to provide access.

a few fuel pressure test adapters

Buying the individual adapters may cost us far more than a fuel pressure gauge. Large assortments are available that provide many popular adapters. Buying an assortment is less expensive than buying each piece separately. It may also be a waste for an occasional user. More cost effective is buying a quality gauge and only the adapters needed to fit our vehicles. Eventually we may need additional adapters and these are added as required.

Fuel pressure gauge accessories

Fuel pressure gauges with and without a volume test valve

Many gauges come equipped with a valve, designed for fuel volume testing. This is a good feature and adds little to the cost. To test the fuel system for pressure bleed-down we will also need on/off valves. We may also need fittings and hoses to connect them to the system. This may also be added later or we can buy them in a set with our gauge.

Other things we need for fuel pressure testing

Along with our gauge and adapter, we need service data for the vehicle. We need the location of the test port, the fuel pressure specification and a volume specification. A wiring diagram for the fuel pump circuit is also helpful. With a wiring diagram, we can figure out how to run the fuel pump so we can test pressure on a vehicle that does not start.

Fuel pressure with the engine idling

A basic pressure test at engine idle

The most basic fuel pressure test is a measurement of pressure, with the engine idling. This tells us if the pump is working and if the fuel pressure meets the specification. Such information is extremely helpful. Fuel pressure below specifications results in a loss of power, a check engine light and a dying engine.

With this simple test, we can rule out many possible problems. An example could be an engine that dies at idle. We connect the fuel pressure gauge and start the engine. If the engine dies, yet the fuel pressure is unchanged, we know the fuel system is not a likely cause. When pressure starts to drop and then the engine dies, we know we need additional fuel system tests.

Many technicians never move beyond this basic test, but far more information is available. With a bit of knowledge, the fuel pressure tester can yield far more data.

Fuel pressure under acceleration

On acceleration, fuel pressure should rise

When we accelerate a normally aspired engine, the vacuum in the intake drops. This signals a need for more fuel pressure. The fuel pressure regulator opens and raises pressure, on vehicles so equipped. On other designs, a sensor informs the system and voltage at the fuel pump increases, to raise fuel pressure.

We can test for this by attaching our fuel pressure gauge outside the windshield. Heavy tape is useful to attach the gauge so we can read it through the windshield. We should never route fuel inside a vehicle.

During the test, we accelerate and watch the fuel pressure. On average, fuel pressure increases about 10 percent on acceleration. If the pressure remains the same, a faulty fuel pressure regulator or sensor is suspected. If fuel pressure drops, a restriction such as a plugged fuel filter could be the cause. A fuel volume test confirms this and we cover this in the next article.

Testing for fuel volatility

When we heat fuel, it may boil and cause problems. This can occur if we use winter grade fuel in hot weather. Other causes are too much ethanol or fuel system additives in the fuel. Volatile fuel can make an engine hard to start, after it has run.

We attach the fuel pressure gauge as above, but after noting the idle pressure, we switch the engine off. Fuel pressure should remain the same or drop slightly. Fuel pressure that begins to rise can show fuel boiling in the fuel rail. This sometimes occurs when someone uses fuel system additives. The easiest repair is to add fresh gasoline as we use the fuel out of the tank. Use one-quarter tank and refill. Continuing to refill should dilute the volatile fuel and eliminate the need to drain the tank.

Dead head testing

We can use a dead head test to check the full capacity of a fuel pump in a regulated system. With a dead head test we connect the fuel line directly to the gauge and block the remainder of the system. This is easiest with valve adapters.

Dead head pressure is higher than open pressure

We should only run the test briefly as dead heading can damage the fuel pump. The dead head test measures the full pressure that a pump can produce. Checking the pump without the regulator in the circuit shows a pressure rise of at least 10 percent over the idle specification. Pressure that does not rise, on a dead head test, shows a weak fuel pump.

In next week’s article, we cover additional fuel pressure and volume tests.





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