The fuel pressure regulator in the Fuel Pump system can be tested by checking the overall fuel pressure in most modern gasoline engines. Typically, it falls in the range of 40 to 60 psi. Outside of these limits, the fuel pressure regulator may malfunction. About 30% of all performance problems in the fuel system are due to improper fuel pressure, according to ASE.
Attach a fuel pressure gauge to the Schrader valve or fuel rail test port. After attaching the gauge, turn the ignition on but do not start the engine. This will turn the fuel pump on without cranking the engine, and you can read the fuel pressure. It should stabilize within the manufacturer's specified range in 2 to 3 seconds. If the pressure is low, less than what is recommended, it means the fuel pump may be malfunctioning or the regulator has an issue.
Then comes the most important part of testing, which is to test the functionality of the regulator using the vacuum line connected to the regulator. Usually, a fuel pressure regulator depends on engine vacuum for fuel pressure adjustment at idle and load. At idle, there should be a slight suction on the vacuum line. A decrease in fuel pressure when this line is disconnected or a vacuum leak around the regulator could prove to be an indication of failure. This procedure is important because a drop of 5 to 10 psi in fuel pressure due to a malfunctioning regulator may highly impact the working of the engine, which will decrease fuel economy and lead to misfires or hesitation.
Other procedures involve a key-on, engine-off test. With the ignition turned on but the engine off, fuel pressure should increase to a stable reading and hold steady. If the pressure drops sharply after the key is turned off, then this is an indication of an internal leak in the fuel pressure regulator or check valve. A pressure drop greater than 10 psi after shutdown often means the regulator isn't holding the necessary pressure, possibly causing poor start-up and rough idling.
The diaphragm of a fuel pressure regulator is supposed to control fuel flow, hence proper engine functioning. Upon failure, the fuel will gain access to the vacuum line, contaminating the fuel and causing irregular engine behaviors. To verify this, check the vacuum line for any signs of fuel inside. If fuel is present in the vacuum line, replace the fuel pressure regulator immediately to avoid further damage. A bad diaphragm can cause fuel system pressure to be off by as much as 15% and result in higher emissions and poor fuel economy.
Another symptom of a bad Fuel Pump is incorrect fuel pressure. If the fuel pressure is low constantly and testing the regulator does not change that, it may mean that the pump has failed. Replacement of the fuel pump may be required to return the proper system pressure. The replacement cost of a Fuel Pump usually ranges from $150 to $500, depending on the model and brand of the vehicle. Neglecting to have your car regularly serviced or failure to inspect fuel pressure regulators may lead to more costly repairs later on; owners of some vehicles have reportedly increased their repair costs by as much as 20% due to undiagnosed fuel system problems.
The test for fuel pressure regulators is an overall process that includes various careful checks on the functioning of the Fuel Pump system. Regular diagnostics, along with timely replacement of faulty components, prevent the risk of engine misfires, poor performance, or even complete failure of the engine due to a lack of sufficient fuel delivery.