Proper fuel pressure is vital for engine operation and performance. Low fuel pressure results in a lack of horsepower, stalling, and a slow or no-start condition. A faulty fuel pump or a clogged fuel filter will result in lower than normal fuel pressure at the rail.
High fuel pressure results in a rich air fuel ratio, because the increase in pressure forces more fuel through the injectors than required. A rich air fuel ratio causes black exhaust and high emissions (HC's). These excessive hydrocarbons overheat and damage the catalytic converter. A kink in the fuel return line will result in higher than normal fuel pressure at the rail.
Test fuel pressure with a fuel pressure gauge. Check the service manual for the proper fuel pressure for the vehicle being tested. Find the fitting; it is typically on the fuel rail. If not, a special fitting from a fuel injection kit will be required. After properly installing the gauge, secure it and turn the ignition key to the on position; it should indicate fuel pressure. Start the engine; the fuel pressure should remain the same. Snap the throttle and make sure it increases. This an indication of a good fuel pump.
Test a vacuum operated fuel pressure regulator's operation by removing the vacuum line while monitoring fuel pressure. The pressure should increase (5-15 psi.). A ruptured fuel pressure diaphragm will leak fuel into this vacuum line. Check with the manufacturer's manual for special procedures. Some fuel pressure regulators are electric and controlled by a control module (PCM).