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

In this article, I'm gonna' show you the most effective way to test the fuel pump on GM 4.3L, 5.0L, 5.7L engines with the 'Spider' fuel injector assembly and it involves using a fuel pressure gauge. I've also included how to check a no fuel condition with starting fluid (although this is not the most effective way to test the fuel pump).

It's been my experience that misdiagnosing a bad fuel pump is a common occurrence on the 4.3L, 5.0L, 5.7L GM vehicles with the 'Spider' fuel injector assembly. Why? This is because in about 50% of the time, the fuel pump delivers enough fuel to keep the engine running but not enough to start the engine (instead of just frying completely and not delivering any fuel at all). When this happens, you're able to spray starting fluid (or carb spray) down the engine and it'll start and run (once you turn it off, you have to spray again to get it started).

Yes, let me repeat that one more time: As crazy as this sounds, the fuel pump will deliver enough fuel to keep the engine running, but not enough to start it when you're cranking the engine and this makes a lot of folks think that the problem lies anywhere else but in the fuel pump!

NOTE: This tutorial only covers GM pickups and SUVs equipped with the 'Spider' fuel injection system. For the fuel pump test of throttle body fuel-injected (TBI) pickups and SUVs, the following tutorial will help:

• How To Test The Fuel Pump On GM TBI Fuel Systems.

IMPORTANT: Since you'll be working around gasoline and/or starting fluid, be careful, take all necessary safety precautions and think safety all of the time.

You can find this tutorial in Spanish here: Cómo Probar La Bomba de Combustible (GM 4.3L, 5.0L, 5.7L) (at: autotecnico-online.com).

Where To Buy A Fuel Pressure Gauge Kit

Owning a fuel pressure gauge is now becoming a must. All vehicles now use an electrical fuel pump to deliver fuel (under high pressure) to the fuel injectors and the best way to diagnose this fuel pump is with a fuel pressure gauge.

In all of the GM 4.3L, 5.0L, 5.7L equipped vehicles, GM was kind enough to put a Schrader valve on the fuel injector rail where you can tap into the fuel system and check its performance.

A fuel pressure gauge can cost you as little as $40 (US) or as much as $200+ (US). The price difference depends on what type of fuel pressure gauge set you buy, this means either buying a non-professional technician grade tool or a professional technician grade tool. Whichever one you buy, it's an investment that will pay for itself many times over.

Your local parts house will have a fuel pressure gauge set to sell you, although you'll pay more for it and it won't be a professional grade tool (if that's what you're looking for).

The one I recommend is the: BETOOLL 0-140 PSI Fuel Pressure Tester (Amazon affiliate link).

You'll be able to test most of the GM vehicles, most Ford/Chrysler vehicles, among several makes. Although this is not that important if you're only worried about testing your GM vehicle.

TEST 1: Checking The Fuel Pressure Specification

The fuel system in your 4.3L, 5.0L, or 5.7L Vortec engine needs at least 50 PSI of fuel pressure to start.

The fuel pump produces much more than just 50 PSI of course but the bare minimum the engine needs to start is 50 PSI. Anything less and it will crank for ever before it starts or not start at all.

So, the very first thing you need to do is to verify that the fuel pump is delivering at least 50 PSI of fuel pressure while the engine is cranking. Yes, while the engine is cranking.

Locate the Schrader valve on the fuel line that connects to the 'Spider' assembly. This is where you'll connect your fuel pressure gauge (see photo above).

NOTE: In case you're wondering, the factory manual specifies that the fuel pressure for the CSFI system should be between 56-62 PSI. The CPI system's fuel pressure should be between 60-66 PSI. The manual doesn't specify if this pressure is key on engine off, only that this pressure should be maintained 'under all operating conditions'.

If you don't know whether you have a CPI or CSFI system, you can find out the difference between them by checking out the info in this subheading: 'Spider Fuel Injection Basics' found in this tutorial: How To Test The 'Spider' Fuel Injector Assembly (4.3L, 5.0L, 5.7L).

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

1. 1

   Place a shop towel underneath the area of the Schrader valve.
   
   The shop towel's job is to absorb any fuel that may leak when doing step 2.

2. 2

   Connect your fuel pressure gauge to the Schrader valve.

3. 3

   Pressurize the fuel system by activating the fuel pump.
   
   You can do this by simply turning the key ON and OFF several times. Once the fuel pressure gauge registers pressure, check for fuel leaks around the Schrader valve.

4. 4

   Check for fuel leaks around the area where you connected the gauge to the Schrader valve. If any are present, resolve them.
   
   IMPORTANT: It's important that NO fuel leaks are present for the accuracy of the test. Once you have verified no fuel leaks, continue to the next step.

5. 5

   Have your assistant crank the engine as you eyeball the fuel pressure gauge.
   
   NOTE: For the accuracy of your test results, it's important that you have someone else crank the vehicle so that you can eye-ball the fuel pressure gauge.

6. 6

   You'll get one of two results:
   
   1.) The fuel pressure gauge will register at least 50 PSI and the engine will start.
   
   2.) The fuel pressure gauge will register 49 PSI or less and the engine will not start.

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

CASE 1: Your fuel pressure gauge read 50 PSI or more as the engine cranked. This means that the fuel pump on your 4.3L, 5.0L, or 5.7L is OK and not the cause of your no-start condition.

The next step is to observe the fuel pressure gauge's needle (after your helper stopped cranking the engine). The needle should descend to about 40 to 45 PSI and stay there for at least 4 to 5 minutes. If the needle DOES NOT stay at this fuel pressure (more specifically: if the needle drops to 0 PSI immediately -by immediately, I mean within the first 15 seconds), you have either the fuel pressure regulator or a fuel injector leaking fuel inside the plastic intake plenum.

You can find the fuel pressure regulator test here: How To Test The 'Spider' Fuel Injector Assembly (4.3L, 5.0L, 5.7L).

IMPORTANT: This test result could be telling you that the PassLock anti-theft system has armed itself and is keeping your pickup or SUV from starting. I recommend taking a look at the following section (in the next page): Difference Between A Bad Fuel Pump And An Anti-Theft System (PassLock) Failure.

The following tutorial may also be a good starting point in diagnosing the cranks but does not start condition your GM pickup or SUV is experiencing (if it's not fuel pump related): How To Troubleshoot A No Start (GM 4.3L, 5.0L, 5.7L).

CASE 2: Your fuel pressure gauge DID NOT read 50 PSI or more as the engine cranked. This means that the fuel pump is bad, even if you spray starting fluid down the throttle body and your vehicle Starts.

If the engine does not get at least 50 PSI when cranking, it won't start (and is the surest indicator that the fuel pump will go completely bad in short). If in your case the gauge read 45 to 49 PSI, the engine will eventually start after several attempts as the fuel starts to accumulate on the Pistons or if you spray starting fluid (or carb spray) into the intake.

If the fuel pressure reading was 0 PSI, then the fuel pump is probably fried. Why 'probably'? Well, you still need to make sure: 1.) That the fuel pump fuse and relay are OK. 2.) That you have enough fuel in the tank (don't laugh, it happens).

TEST 2: Spraying Starting Fluid (Quick Test)

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.

3. 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.
   
   NOTE: 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.

4. 3

   Have your assistant crank the engine once the intake air duct is back on the throttle body.

5. 4

   You'll get one of three possible results:
   
   1.) The engine will start momentarily and after a few seconds will die.
   
   2.) The engine will only crank but not start at all.
   
   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:

• Confirm fuel pressure with a fuel pressure gauge (if you haven't done so).
• Verify that the ignition system is creating and delivering spark to all cylinders.
• 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):

Where To Buy The Fuel Pump And Save $

The fuel pumps that are used in all of the GM Vortec equipped vehicles are expensive, especially if you buy them at your local auto parts store. There, they're really gonna' stick it to you at around $280 to $300 (US). My personal opinion is that this is highway robbery since there's no logical reason why they should be selling them at such an outrageous price (after all, they're made in freaking China!) !!!

Or you could buy them here (the Delphi fuel pump is the factory original):

Related Test Articles

If you enjoyed and/or found this 'How To' article helpful, I have written several more 4.3L, 5.0L, 5.7L specific test articles. Some you can find here, at troubleshootmyvehicle. Com and some you'll find at my other web site at easyautodiagnostics. Com.

Here, you can go to: GM 4.3L, 5.0L, 5.7L Index of Articles

Here's a small sample of the articles you'll find there:

• How To Troubleshoot A No Start (GM 4.3L, 5.0L, 5.7L).
• How To Test The 'Spider' Fuel Injector Assembly (4.3L, 5.0L, 5.7L).
• How To Test The Engine Compression (GM 4.3L, 5.0L, 5.7L).
• How To Test The Crank Sensor (GM 4.3L, 5.0L, 5.7L).
• How To Test The GM MAF Sensor 3.1L, 3.4L, 4.3L, 5.0L and 5.7L (at: easyautodiagnostics.com).
• How To Test A Misfire / No Spark-No Start Condition (4.3L, 5.0L, 5.7L 96-04) (at: easyautodiagnostics.com).