There are many forum & blog posts and advice telling folks that a high pressure fuel system, such as found on many Chevrolets from 1996 and up, can be tested for pressure loss by pinching off the return line while the system is at full pressure. As a matter of fact, several tool manufactures make a tool to pinch off lines, but pinching off the return fuel line is not an effective way to check for pressure loss in a high pressure fuel system except to possibly check for fuel pump pressure loses if done right.

Since the fuel line to the engine begins at the fuel pump and ends at the fuel pressure regulator/injector(s) and the fuel gauge attaching port is located in that line, a leak either at the fuel pump or at the pressure regulator will drop the pressure on the gauge. Pinching off the return fuel line alone will show you absolutely nothing in regards to a faulty fuel pressure regulator!

Note: Gasoline is dangerous, especially pressurized gasoline. Fire or an explosion could result from contact with a heat source. A very capable person or service technician only, should work around or with any open or pressurized containers of gasoline including fuel systems.

The first place to start checking is at the fuel pressure gauge port on the supply gas line located in the engine compartment, and with a properly calibrated pressure gauge. Turn the key on and let the fuel pump run until it stops (about 2 seconds). The pressure on the gauge should settle and read somewhere inside the range of pressures that a service manual for your vehicle/engine states. This pressure should also hold for an extended period of time. If the pressure builds and holds at the proper pressure by only turning the key on, no problem is indicated as far as the fuel system's fuel delivery capability is concerned.

The best place to check for a leaky fuel pump is a toss up between either blocking the return line and building up the pressure to above fuel pressure regulator relief pressures, or by installing a hand pump and a pressure gauge at the inline filter or another place the fuel line can be separated.

To adequately check for fuel pump leakback by pinching off the return fuel line and because the fuel pressure regulator normally holds pressure on the supply fuel line, the pressure must be built up to above the regulator's preset pressure holding capability with the return line blocked to adequately check for fuel pump leakback. Otherwise, faulty diagnosis of the fuel pump can result from the regulator's normal action.

To test for fuel pump leakback at the filter or another place the line can be separated, on the section of line going back to the fuel pump, rig up both a pressure gauge and a hand pump that will hold pressure when not pumping. Pump the hand pump filled with a little gasoline until the gauge reads something like 20-30 lbs. See if the pressure will hold there for several minutes. The vehicle's fuel pump is not the fuel system leak problem if the pressure holds, and is the problem or a problem if the pressure does not hold.

If there are no problems with fuel pressure loss at the fuel pump or in the line from the pump to the engine, and the fuel pump builds adequate pressure, the fuel pressure regulator operation can be observed by attaching an appropriate pressure gauge to the fuel pressure testing port.

With the help of a helper, simply turn the key on and the fuel pump will run for two seconds. The pressure should read and hold (after a few seconds) about 58-61 lbs on many of these vehicles (check an appropriate manual for the specific minimum/maximum fuel pressure on your model car/truck and engine) by the time the fuel pump stops. Note this pressure reading and then turn the key off while observing the pressure gauge, the pump should again run for two seconds and the fuel pressure should momentarily climb above the pressure observed on the first run of the fuel pump. If the fuel pressure regulator is working as it should, the pressure should settle back to the correct fuel pressure for your particular engine and hold there (should be the same pressure as noted before).

If the fuel pressure falls below the minimum for your particular vehicle/engine, or continues to fall completely out, the fuel pressure regulator is probably defective.

Tip: An old R12 type set of AC gauges with at least one good gauge makes a perfect fuel pressure testing gauge. They are easily calibrated and the hose will fit the pressure testing port as well.