TEST 2: Spraying Starting Fluid (Quick Test)

Troubleshooting the Fuel Pump (GM 4.3L, 5.0L, 5.7L)

In this fuel pump test, you'll use starting fluid (or carb clean spray) to see if the fuel pump is not delivering enough or any fuel to the engine.

Now, let me tell you that this is not the most accurate way of testing the fuel pump... but it doesn't mean it's not effective.

The main purpose of this test step is to complement the results of your fuel pressure gauge test.

OK, to get this show on the road, this is what you'll need to do:

  1. 1

    Remove the intake air duct from the throttle body. You don't have to completely remove it, since you'll have to reconnect it in one of the next steps.

  2. 2

    Open the throttle plate, manually, and spray starting fluid down the bore. When you have sprayed a good squirt of starting fluid, quickly reconnect the air duct to the throttle body (you don't have to tighten the fastener).

    Reconnecting the intake air duct is important because your 4.3L, 5.0L or 5.7L vehicle is equipped with a MAF Sensor that it needs to Start. Also, it's a very important safety precaution in case you get a back-fire thru' the intake.

  3. 3

    Once the intake air duct is back on the throttle body, have your assistant, inside the vehicle, crank the engine.

  4. 4

    You'll get one of three possible results with this test:

    1.) The engine will start momentarily and after a few seconds will die or...

    2.) The engine will only crank but not start at all or...

    3.) The engine will start and stay running.

OK, let's find out what your results mean:

CASE 1: If the engine started and ran for a few seconds. This means that the reason your vehicle is not starting is due to a lack of fuel.

If you haven't done so already, the next step is to confirm fuel pressure with a fuel pressure gauge. Since there's a good chance that the PassLock anti-theft system may have armed itself, take a look at this section: Difference Between a BAD Fuel Pump and an Anti-Theft System (PassLock) Failure.

CASE 2: The engine did not Start, not even momentarily. This usually means that a lack of fuel IS NOT the reason your car is not starting.

To be absolutely sure you should:

  1. Confirm fuel pressure with a fuel pressure gauge (if you haven't done so).
  2. Verify that the ignition system is creating and delivering spark to all cylinders.
  3. The following tutorial may be of help: How To Troubleshoot A No Start (GM 4.3L, 5.0L, 5.7L).

CASE 3: The engine started and stayed running. This confirms that the fuel pump is starting to fail. The next step is to confirm the exact fuel pressure with a fuel pressure gauge to be absolutely sure.

Difference Between A BAD Fuel Pump And An Anti-Theft System (PassLock) Failure

One of the things that can have you replacing a good fuel pump is an anti-theft system (PassLock) failure. How?...

...This is due to the fact that when the Passlock system arms itself on your GM pickup or SUV... the engine will crank but won't start, since the PCM has disabled the fuel injectors.

If you then spray carburetor cleaner or starting fluid into the intake... the engine starts (although momentarily because the engine will stall again as soon as the starting fluid -or carb spray- you sprayed into the throttle body gets consumed).

Because this behavior mimics a failed fuel pump, it unfortunately leads a lot of folks to believe that the fuel pump has failed (when it hasn't).

I've created the following chart to help you tell the difference between the two so that you can find out which one is the one causing your pickup (or SUV) not to start (since PassLock failures (activations) and bad fuel pumps are a very common problem):

Bad Fuel Pump to PassLock Activation Relationship
Symptom PassLock Fuel Pump
Engine cranks but won't start YES YES
Engine starts with starting fluid (momentarily) YES YES
Ignition system delivers spark YES YES
Passlock MIL flashes on instrument cluster YES
Engine starts momentarily then stalls YES
Fuel pressure at spec (as checked
w/ fuel pressure gauge)
PCM disables fuel injectors a few
seconds after the engine starts