Obviously, a completely dead pump will keep the engine from starting. But how will a pump that's on its last legs affect performance? Any fuel injected engine that's difficult to start, idles rough, hesitates, lacks power under load or runs lean probably isn't getting enough fuel, and a weak pump may be to blame.

Install a fuel pressure gauge. Some systems provide a convenient pressure tap; it may be necessary to tee into the supply line on others. Start the engine to determine whether the regulated and unregulated fuel pressures are up to specs. Remove the vacuum hose leading to the pressure regulator. This unregulated fuel pressure indicates how much pressure the pump will deliver while the engine is under full-load, low-manifold-vacuum conditions.

Reinstall the pressure regulator vacuum hose. The fuel pressure should return to the manufacturer's idle specification. The regulator adjusts fuel pressure in response to changes in intake manifold vacuum and engine load. If the regulated fuel pressure is too high, the likely causes are a damaged pressure regulator or an obstructed return line. Low pressure can be caused by a clogged fuel filter or in-tank filter sock, a defective pressure regulator, a restricted fuel supply line or a weak pump. Eliminate all other possible causes of low pressure before installing a new fuel pump.

A deadhead fuel pressure test is similar to the stress test a cardiologist may one day ask you to take. It determines the fuel pump's reserve capacity by briefly asking it to deliver as much pressure as it can. To perform a deadhead fuel pressure test, pinch off the fuel return line while observing the rise in fuel pressure. This test should be brief.

A healthy pump may manage to jump from 60 to 90 psi. But if the pressure rise is minimal, either the fuel pump is weak or something is keeping it from delivering its all.

To prevent tuel percolation and aid hot- and cold-engine starting, fuel injection systems must maintain residual, or "rest," pressure when the engine is off. All systems lose residual pressure over time. Its just a question of how fast and how much before its a problem.

Since the entire supply side of the fuel system is normally pressurized, a loss of residual pressure could be caused by a leaking injector, pressure regulator or fuel pump check valve. Determining which involves isolating various parts of the fuel system, saving you the trouble of visually inspecting each component.

A ruptured regulator diaphragm will pass fuel to the vacuum hose and into the intake manifold. To isolate the regulator, pressurize the fuel system, then pinch off the fuel return line with pliers or clamps designed for this purpose. If the residual pressure continues to drop, the cause lies elsewhere. Repeat the same test to isolate the injector rail.

The fuel pumps internal check valve may be allowing residual pressure to leak into the fuel tank. To test, isolate the pump close to the fuel injector rail, then pressurize the system. No fuel pressure should reach the injectors or the pressure regulator during this test. If the fuel line pressure rapidly drops to zero, the residual fuel pressure is leaking past the fuel pump check valve, or through a fuel line fitting.

An often overlooked, final test is pump volume. The simplest volume test involves opening the fuel supply line at the injector rail, then measuring fuel volume during a timed, power-on fuel purrlp test. This involves pumping gasoline into an open container at a high rate, which poses obvious safety risks. Additionally, this test does not show what the pump will do when its under load. Dedicated equipment is available to safely and conveniently test fuel pressure and volume.

A final word about volume: Some systems use more than one fuel pump. On early Bosch fuel injection systems, one pump is responsible for volume, while another handles pressure. The low-pressure, high-volume pump is located in the tank, and a high-pressure pump is attached to the vehicle frame. The system may work with a worn or inoperative in-tank pump, although not well. This also puts a strain on the external pressure pump, so always check both before replacing either.