Fuel pressure check can identify a number of problems

When you bring your vehicle in for a tune-up, a mechanic will typically check the fuel pressure to make sure there aren't any problems with the fuel system. You can also take this diagnostic step on your own by picking up a simple gauge at an auto parts store.

When the fuel is delivered to the engine, it must have a sufficient volume and pressure. Mityvac, a company which manufactures fuel systems testers and other automotive equipment, says that if the fuel doesn't have the proper volume or pressure, the engine will not run properly and may even suffer damage.

If there is a problem with your fuel system, you may notice a number of symptoms. These could include poor fuel economy, sluggish performance when traveling uphill or pulling a heavy load, or backfiring during acceleration. Karl Seyfert, writing for the magazine Motor, says the engine may also be difficult to start or have a rough idle.

A fuel pressure gauge will connect to your fuel system and let you know if the fuel doesn't have enough pressure or is too pressurized. Some vehicles have a pressure test port to connect a gauge; check your owner's manual to see if you have one and where it is located. The AGCO Automotive Corporation, a business in Baton Rouge, La., says it may also be necessary to use an adapter to get the best connection.

Before working on your fuel system, make sure there are no obvious hazards. Delphi Technologies, an automotive products company based in the United Kingdom, says you should work in a well-ventilated area and keep your workspace free of any sources of ignition. Wear gloves and safety glasses while working on the engine.

Once the gauge is connected to the test port, start the engine and let it idle. The resulting pressure reading should fall within the specifications recommended by the manufacturer.

The test is a simple way to determine whether fuel pressure or volume is the likely source of engine trouble. If the gauge doesn't seem to be working, you'll want to make sure the hose and other connections are securely fastened and not leaking. Mityvac says you'll also want to make sure the fuel pump is activated, since a problem with the fuses and relays could prevent this from occurring.

An inadequate fuel pressure reading can suggest a number of different problems in the fuel system. These could include a clogged fuel filter, restricted or damaged fuel line, or a problem with the fuel pump or fuel pressure regulator. Seyfert says that if the pressure reading is too high, it is likely due to damage to the regulator or a blocked return line.

Low pressure can be a result of a number of other causes, including a clogged filter, defective regulator, or blocked fuel line. A weak fuel pump can also be to blame, but it is best to rule out the other sources before going with the potentially expensive repair of putting in a new pump.

If the pressure is adequate, you'll need to find out how much gas is being delivered to the fuel injectors. The AGCO Automotive Corporation says some fuel pressure gauges will have a volume test button and hose which will allow you to collect a sample of gasoline.

Naturally, you'll want to be very careful when collecting the flammable fluid in an open container, and keep it away from any hot surfaces. Hold the test button for 15 seconds and see how much fuel is produced; most fuel pumps put out at least one pint of fuel in this time, but you'll want to do any necessary conversions and check them against the manufacturer's specifications.

A "dead head" test can also be useful. In this test, the gauge is connected directly to the fuel line while the rest of the system is blocked off. This shows you how much pressure the fuel pump can produce when it is not regulated. The test should only be run briefly, since a prolonged test can damage the fuel pump.

Ideally, the pressure produced in a dead head test should be at least 10 percent higher than the reading in the idle test. If the pressure does not rise sufficiently, a weak fuel pump is likely the cause of the trouble.

A similar test can be done during a fuel volume test. If the fuel pump is producing adequate volume with a blocked system, there is likely a leak in the fuel system. If the volume remains insufficient, the problem is more likely to lie with a bad filter, fuel line, or fuel pump.

Another fuel pressure test is to rev the engine while the gauge is attached. Normally, this will result in a pressure increase of about 10 percent. When the pressure doesn't change, the regulator or a sensor might be causing the problem. If the pressure drops instead of increasing, the fuel filter is likely clogged.

If you are experiencing hard starts, check the fuel pressure during an idle and then turn off the engine. While the pressure should drop slightly, it should leave enough residual pressure to start the engine the next time. Poor residual pressure is often a cause of hard starting and may indicate a leaking fuel injector, a bad regulator, or a faulty fuel pump.

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