TEST 2: Fuel Pressure Test (Engine Starts)
If the engine starts and runs but isn't running optimally, there's a good chance the fuel pump isn't supplying enough fuel to the injectors.
We can easily verify that the fuel pressure is within specification by hooking up a fuel pressure gauge to the Schrader valve (see photo above).
The fuel pressure specification with the engine running is: 25-30 PSI (with the fuel pressure regulator connected to its vacuum hose).
Any fuel pressure less than 25 PSI with the engine running lets you know the fuel pump is failing and causing an engine performance problem.
NOTE: If you don't have a fuel pressure gauge, take a look at the section: Where To Buy A Fuel Pressure Test Gauge.
OK, let's get started with this test:
- 1
Remove the plastic dust cap from the Schrader valve.
- 2
Place a shop towel around the Schrader valve and fuel line. The shop towel's job is to absorb any fuel that may leak when doing step 3.
- 3
Connect the fuel pressure gauge to the Schrader valve.
- 4
When ready, ask your helper to cycle the key ON and OFF but don't crank the engine.
- 5
Check your connections for fuel leaks. Resolve any fuel leaks before continuing to the next step.
- 6
Crank the engine and check the fuel pressure gauge.
- 7
Your fuel pressure gauge should register the specified fuel pressure.
Let's take a look at what your results mean:
CASE 1: The fuel pressure gauge registered 25 to 30 PSI. This is the correct and expected test result.
If the engine is having an engine performance issue, this test result tells you that the fuel pump is not the culprit.
CASE 2: The fuel pressure gauge registered a pressure less than 25 PSI. This test result tells you that the fuel pump is failing.
TEST 3: Checking The Fuel Pump Fuse
If in TEST 1 the fuel pressure gauge read 0 PSI fuel pressure, we need to make sure the fuel pump fuse is not blown.
On the 1993, 1994 and early 1995 3.2L Isuzu Rodeo, this is fuse #9 in the underhood fuse/relay box (see illustration 1 of 2).
On the mid 1995 and later 3.2L Isuzu Rodeo, the fuel pump fuse is fuse EB-11 of the underhood fuse/relay box (see illustration 2 of 2).
These are the test steps:
- 1
Remove the plastic dust cover from the underhood fuse box.
- 2
Locate the fuel pump fuse.
1993, 1994, and early 1995 3.2L Isuzu Rodeo: This is fuse #9 of the underhood fuse/relay box.
Mid 1995 and later 3.2L Isuzu Rodeo: This is fuse EB-11 of the underhood fuse/relay box. - 3
Remove the fuse and make sure that it's not blown.
Let's take a look at what your results mean:
CASE 1: The fuel pump fuse IS NOT blown. This is the correct and expected test result.
If in TEST 1 you got a fuel pressure reading of 0 PSI, then you can conclude that the fuel pump is bad and needs to be replaced.
CASE 2: The fuel pump fuse is blown. Replace the fuse and retest.
If the fuse gets blown again, then this usually means one of two things: The fuel pump has suffered an internal short-circuit problem or the wiring between the fuse and the fuel pump has suffered an open-circuit problem.
