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!
Contents of this tutorial:
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).
Which And Where To Buy A Fuel Pressure Gauge
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.
I'm going to review two different fuel pressure gauge sets that will shed some light into which one is the one you may want/need to buy.
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 Actron CP7838 Professional Fuel Pressure Tester is a good tool but not a professional technician level fuel pressure gauge. Although the name used by Actron to describe it has the word ‘professional’ as part of its name.
Why do I know this is a good tool? Well, I own one (I also have a professional level grade fuel pressure gauge set) and I have used it in a regular manner and I can tell you that it works.
The only things I don't like about it is 1.) No molded case to put the fuel pressure gauge and the fittings that go with it and 2.) You can't buy the fittings, that you may lose or break, separately. But for the price, it's a great deal.
You'll be able to test most of the GM vehicles, most Ford vehicles, among several makes. Although this is not that important if you're only worried about testing your GM vehicle.
The OTC 6550 Master Fuel Injection Kit will have the adaptors for Asian and Domestic (USA) vehicles. So, if you work on cars for a living and you work on a variety of Makes and Models, this is the fuel pressure gauge set that you need to have in your tool box.
Since this fuel pressure gauge set is a professional grade technician tool, you can buy most of the components, that make up the set, separately. This comes in handy, because you're able to buy whatever fitting you lose or gets damaged.
Also, and probably the last consideration, is that this is an OTC tool, and OTC has been around for a long, long time and makes a ton of automotive specialty tools known for quality.
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 (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).