Symptoms of a failing fuel pressure regulator include blackened spark plugs, a rough running engine and black smoke coming out of the tailpipe. This is because a failing regulator causes inconsistent fuel pressure to the engine, leading to a rich running condition and excessive fuel use.
A fuel pressure regulator maintains fuel pressure by using a spring that pushes against a diaphragm at a pre-set pressure. The fuel pumped by the fuel pump must be at enough volume and pressure to overcome the spring pressure; the excess is sent to the return line back to the fuel tank. At engine idle, a vacuum hose attached to the regulator causes the diaphragm to push against the spring with extra pressure, reducing the fuel pressure in the system. When the accelerator is pressed, the amount of vacuum lowers, causing the fuel pressure to increase the fuel flow to a rate necessary for the engine's performance.
A failing regulator is due to damage to this diaphragm, causing excess fuel pressure or a delay in creating fuel pressure. Excess fuel pressure causes extra fuel to be injected, which ends up blackening the spark plugs. Once fouled, the spark plugs have trouble igniting the air-fuel mix to create power, leading to poor performance.The unburned air-fuel mix leaves the engine and blackens the exhaust stream. A delay in creating fuel pressure also results in an engine that is hard to start.