Fuel Pressure Tests
A fuel injector will be unable to inject the correct quantity of fuel if the fuel pressure is not within the specifications for that vehicle. A vehicle with a computer may be able to compensate for an incorrect fuel pressure to some extent by altering the fuel/air mixture. However, this is still likely to cause driveability problems while the engine is cold. Fuel pressure tests can generally be categorized into low-pressure tests and high-pressure tests.
The vehicle may need to be road-tested if the problems occur only at high speeds. This may require the fuel pressure gauge to be attached to the fuel system with a long hose, allowing a passenger to read the gauge while the vehicle is moving.
Most problems with fuel pressure are due to the pressure being too low. The inability of the fuel pump to increase the fuel pressure up to the vehicle’s specifications may have several causes.
- A fuel leak is a cause of low fuel pressure, although this problem can often be identified without the need for installing a fuel pressure gauge.
- Other problems that can cause low fuel pressure include a kinked fuel inlet line or a clogged fuel filter or strainer.
- A bad fuel pump in the fuel tank can also prevent the fuel pressure from reaching the required level.
Fuel pressure that is too high can cause the engine to run too richly. Common causes of high fuel pressure include a bad fuel regulator or a clogged return line. You will need to perform two different tests to identify the reason for excessive fuel pressure.
- The first test requires you to relieve the system fuel pressure and connect a relief hose to the fuel return line. Place the free end of the hose in a container and activate the fuel pump. The fuel return line is blocked if the fuel pressure now meets specifications. Otherwise, the fuel pressure regulator may be faulty.
- The second test requires you to allow the engine to idle while disconnecting the vacuum hose on the fuel pressure regulator for vehicles that are so equipped. The fuel pressure regulator may be faulty if the fuel pressure does not increase by about 5-10 psi.