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.
Contents of this tutorial:
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:
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:
- 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.).
- You've sprayed starting fluid (or carburetor spray) into the throttle body and your vehicle started, even if only momentarily.
- You don't hear the fuel pump making a buzzing sound as you turn the key to the ON position or crank the engine.
- 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
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
Connect the adapter to the fuel filter.
- 3
Connect the fuel line to the adapter.
- 4
When ready, ask your helper to crank the engine.
- 5
Check for leaks around the adapter.
If any leaks are present, resolve them before continuing to the next step. - 6
Have your helper crank the engine while you observe the fuel pressure tester's gauge.
- 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).