How To Test The Fuel Pump On GM TBI Fuel Systems

As you're already aware, the throttle body fuel injection system on your 1987 to 1995 4.3L, 5.0L, or 5.7L pickup (or SUV) uses a low pressure (12 PSI) electric fuel pump located in the gas tank.

And although GM did not include a Schrader valve to which you could connect a fuel pressure gauge, it can be tested with one to find out if the fuel pump is producing the correct pressure (or not).

In this tutorial, I'm going to show you two different methods to verify if the no-start condition on your 4.3L, 5.0L, or 5.7L throttle body fuel injection (TBI) GM vehicle is due to a lack of fuel, which usually means that the fuel pump has gone bad.

One method involves using just starting fluid and the other involves using a fuel pressure gauge. One method is a very accurate way of diagnosing a lack of fuel condition causing a no-start, the other method is not that accurate.

You can find this tutorial in Spanish here: Cómo Verificar La Bomba De Combustible En Los Sistemas TBI (at: autotecnico-online.com).

IMPORTANT: Gasoline is extremely flammable. Be careful and take all necessary safety precautions. Use extreme care when using starting fluid since it ignites as easily as gasoline. Accomplishing both tests indicated in this article, without any unhappy consequences, is more than possible if you're careful and use common sense.

NOTE: This tutorial covers only the TBI equipped 4.3L, 5.0L, and 5.7L pickups (SUVs). If you need to test the 'spider' equipped GM pickups or SUVs, the following tutorial will help:

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

ENGINE NO-START DIAGNOSTICS:

• How To Troubleshoot A No Start (GM 4.3L, 5.0L, 5.7L).

Symptoms Of A Bad Fuel Pump

A bad fuel pump will make your 4.3L, 5.0L, or 5.7L throttle body fuel-injected pickup or SUV crank but not start. Here are some more specific symptoms:

1. The ignition system is creating and delivering spark to the spark plugs. So you know that the ignition system is not at fault (in other words: the ignition control module is OK, the ignition coil is OK, etc.).
2. You've sprayed starting fluid (or carburetor spray) into the throttle body and your vehicle started, even if only momentarily.
3. You don't hear the fuel pump making a buzzing sound as you turn the key to the ON position or crank the engine.
4. You don't see the fuel injectors spraying gasoline.

Although the above list is a not a very complete list of symptoms, the theme that runs thru' them, and any other related symptom, is that the engine will crank and crank, but not start.

Which And Where To Buy A Fuel Pressure Gauge

There are two ways to tap into the fuel line to see how much pressure the fuel pump in producing.

One method involves removing the fuel filter and using an adapter, that has a Schrader valve on it, in its place. You then connect your fuel pressure test gauge to this adapter.

The other method is to use an adapter between the throttle body assembly and the pressure line. Once this adapter is installed, you then connect your fuel pressure gauge to it.

Below you'll find the adapter (that's use in the fuel filter's place) and the fuel pressure gauge kit that comes with the adapter that fits between the throttle body assembly and the fuel pressure line.

TEST 1: Fuel Pressure Test With Gauge

Testing the fuel pump's fuel pressure may seem impossible on the 4.3L, 5.0L, or 5.7L TBI equipped pickup or SUV since they don't come equipped with a Schrader valve.

But the fuel pressure can be tested using a special adapter that either fits between the throttle body and the pressure line or fits in place of the fuel filter.

If you don't own a fuel pressure gauge, you can rent one from your local auto parts store (AutoZone, O'Reilly Auto Parts). You just have to make sure that they have the TBI system adapter.

If you're interested in buying your own, check out my recommendations here: Which And Where To Buy A Fuel Pressure Gauge.

NOTE: Before removing the fuel filter, relieve the fuel system pressure to prevent fuel from spraying when disconnecting the lines. This can typically be done by locating the fuel pump fuse or relay in the fuse box and removing it, then starting the engine and letting it run until it stalls, indicating that the fuel pressure has been relieved.

OK, let's get started with this test:

1. 1

   Disconnect the fuel line where it exits the fuel filter.
   
   The fuel line (at the fuel filter) that needs to be disconnected is the one that delivers fuel to the throttle body.
   
   NOTE: With a shop towel, mop up any fuel that was released during the process of disconnecting the fuel line.

2. 2

   Connect the adapter to the fuel filter.

3. 3

   Connect the fuel line to the adapter.

4. 4

   When ready, ask your helper to crank the engine.

5. 5

   Check for leaks around the adapter.
   
   If any leaks are present, resolve them before continuing to the next step.

6. 6

   Have your helper crank the engine while you observe the fuel pressure tester's gauge.

7. 7

   Your fuel pressure gauge will register one of the two following results:
   
   1.) The fuel pressure gauge will register 12 PSI, or
   
   2.) The fuel pressure gauge will register 0 PSI.

OK, now that the testing part is done, let's take a look at what your results mean:

CASE 1: The fuel pressure gauge registered 0 PSI. This confirms that the cause of your TBI equipped pickup's 'no-start' condition is caused by a lack of fuel.

Now, I usually take one more precaution, before condemning the fuel pump as bad and this is to check that the fuel pump is getting 12 Volts as the engine is cranking. This is just to make sure that the fuel pump relay and fuse are OK and doing their job.

What you'll have to do to accomplish this is to attach a multimeter in Volts DC mode to the wire that supplies this voltage to the fuel pump and while a helper cranks the engine, verify that the fuel pump is getting this power. If the 12 Volts are being supplied to the fuel pump, you have now 100% verified that the fuel pump is fried and needs to be replaced.

CASE 2: The fuel pressure gauge registered 12 PSI. This fuel pressure gauge result lets you know that the fuel pump is working and delivering enough fuel to the fuel injectors. The reason your 4.3L, 5.0L, or 5.7L GM vehicle is not starting is due to another reason. The fuel pump is OK.

The following tutorial may have some helpful testing suggestions: How To Troubleshoot A No Start (GM 4.3L, 5.0L, 5.7L).

TEST 2: Using Starting Fluid

The fastest/easiest way to find out if a lack of fuel is causing the engine to not start is to see if the fuel injectors are spraying fuel while a helper is cranking the engine.

If the injectors are not spraying fuel, then the next step is to coax the engine into starting using starting fluid.

In this test section, I'll explain how to use starting fluid to check if a lack of fuel is keeping the engine from not starting.

IMPORTANT: For the accuracy of your starting fluid test, you need to confirm that the all six or eight spark plug wires are delivering spark to the spark plugs. If you haven't done so already, check for spark with a spark tester.

CAUTION: You'll be working around a cranking engine, so take all necessary safety precautions and think safety all of the time.

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

1. 1

   Remove the top cover of the air filter assembly.
   
   You don't have to completely remove the assembly, just the cover.

2. 2

   Spray starting fluid down the bore of the throttle body.

3. 3

   Quickly place the air filter assembly's cover back on.
   
   IMPORTANT: Placing the cover back on the air filter assembly is important because it'll prevent any back-fire, that may occur, from spouting flames out of the throttle body.

4. 4

   Have your helper crank the engine once the intake cover is back on the air filter assembly.

5. 5

   You'll get one of two 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.

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

CASE 1: The engine started and ran for a few seconds. This means that the reason your pick up (or SUV) is not starting is due to a lack of fuel.

Now that you have confirmed that the 'no-start' problem is due to a lack of fuel, you now need to actually confirm how much pressure the fuel pump is producing with a fuel pressure gauge. For this test go to: TEST 1: Fuel Pressure Test With Gauge.

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.

Now, remember what I said about this test not being very accurate? Well, I suggest you do one more test.

This is to remove the fuel line that connects to the fuel injector rail and place it in a container. Then have a helper bump the starter while you check to see if fuel comes out of the Line and into the container. This way, you can visually see if there is or there isn't fuel coming out of the fuel line.

Quite a few things can cause a 'no-start' condition and if you have ruled out the fuel pump as the problem, this tutorial may be of help: How To Troubleshoot A No Start (GM 4.3L, 5.0L, 5.7L).

Where To Buy The Fuel Pump And Save

You can find the fuel pump and fuel pump strainer filter in any auto parts store. If you're wanting the buy the original AC Delco (Delphi) fuel pump and fuel pump strainer filter, you can buy it online for a whole lot cheaper than somewhere local.

The following links will help you comparison shop for the AC Delco (Delphi) and after market fuel pump and fuel pump strainer filters:

Will the above fuel pump and fuel pump strainer filter fit your particular GM pickup? Don't worry, once you get to the site they'll make sure it fits (by asking you the specifics of your particular vehicle). If it doesn't fit, they'll find you the right one.

More GM 4.3L, 5.0L, 5.7L Tutorials

You can find a complete list of tutorial in this index: GM 4.3L, 5.0L, 5.7L Index Of Articles.

Here's a sample of the tutorials you'll find in the index:

• How To Test The Blower Resistor (1988-1993 Pickups).
• How To Test The Blower Motor (GM 4.3L, 5.0L, 5.7L).
• How Can I Tell If My 4L60-E Transmission Is Bad?
• Shift Solenoid A And B Tests (GM 4.3L, 5.0L, 5.7L).
• How To Test The Engine Compression (GM 4.3L, 5.0L, 5.7L).
• How To Test The GM Distributor Mounted Ignition Module (at: easyautodiagnostics.com).