TEST 1: Fuel Pressure Leak Down Test

Fuel Pressure Test Schrader Valve Location (GM 4.3L, 5.0L, 5.7L)

The very first test (and the most important) to do is a fuel pressure leak down test. Why?...

...The reason why is that the fuel system in all of the CPI and CSFI equipped Vortec engines are designed to maintain fuel pressure for a few minutes after engine shut off.

So, when the fuel pressure regulator (or a fuel injector) is leaking, the fuel pressure will drop down to 0 PSI as soon as you turn the engine off (and if this happens... then you know that you have a problem).

Therefore, very first thing you need to do is a fuel pressure leak down test with a fuel pressure gauge.

The photo above shows the Schrader valve location on the fuel rail where you'll connect your fuel pressure gauge to.

Alright, here are the steps:

  1. 1

    Connect your fuel pressure gauge to the Schrader valve (see photo above).

    Before you do, place rags under and around the Schrader valve (as a safety precaution, since fuel will leak when you install the fuel pressure gauge).

  2. 2

    Activate the fuel pump to pressurize the line.

    You can easily accomplish this by having a helper cycle the Key On and Off a few times (without cranking the engine). Once the pressure rises on the gauge, the next step is to check for fuel leaks around the Schrader valve (it's critical that no leaks are present for the accuracy of this test).

    If no leaks, go on to the next step. If the gauge is leaking... you'll need to tighten it some more and recheck.

  3. 3

    Now that you have verified that the fuel pressure gauge is not leaking, crank the engine.

    If the engine starts, have him (or her) turn it off.

    Observe the fuel pressure gauge's needle, as soon as you turn off the engine. You'll see one of two results:

    1. The fuel pressure gauge's needle will drop 5 PSI only and remain steady for at least one full minute.
    2. Or the fuel pressure gauge's needle will immediately drop to 0 PSI.
  4. 4

    If the fuel pressure drops down to 0 PSI within seconds... then you've got a problem. In a system where the fuel pressure regulator or fuel Lines are not leaking.. the pressure should hold steady (not dropping to zero) for about 5 minutes.

Let's interpret the results of your fuel pressure leak down test:

CASE 1: The fuel pressure gauge's needle DID NOT drop down to 0 PSI immediately after the Key was turned to the Off position: This tells you that the fuel pressure regulator and the other components that are part of the fuel injector assembly are NOT BAD.

This test is very accurate, so you don't need to remove the intake manifold plenum to visually confirm it's effectiveness and/or truthfullness! If your specific vehicle is having a rough idle problem, take a look at the following case study, since it might help your troubleshooting:

  1. How to Diagnose a Hard to Find Misfire Case Study (GM 4.3L, 5.0L, 5.7L)

CASE 2: The fuel pressure gauge's needle DID drop down to 0 PSI immediately after the Key was turned to the Off position: This tells you that there is something leaking inside the plenum. More than likely, it'll be the fuel pressure regulator, to make sure... go to TEST 2.