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!
OK, to make it easier to navigate this article, here are the contents of this article at a quick glance:
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:
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).
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... so 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 (the first photo, in the image viewer, is of a 4.3L, and the second of the V8 engine).
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:
Connect your fuel pressure gauge to the Schrader valve.
Before you install the fuel pressure gauge, place some rags around it and underneath the fuel line (in case some fuel spills out when you connect the gauge to it).
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.
It's very 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.
Have your assistant crank the engine as you eyeball the fuel pressure gauge. It's important that you have someone else crank the vehicle so that you can eye-ball the fuel pressure gauge.
You'll get one of two results with this test:
1.) The fuel pressure gauge will register at least 50 PSI and the engine will start or.
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: If 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 pick-up 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: If 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).
“When I was born I was so ugly the doctor slapped my mother.”