How To Test The Fuel Pump (GM 3.8L)

When the fuel pump goes bad on your GM 3.8L equipped car or mini-van, it's going to crank but not start. Although it stinks that your vehicle is not starting, the cool thing is that testing the fuel pump is not hard.

I'm going to show you two different methods that you can use to verify if the no-start condition on your 3.8L GM vehicle is due to a lack of fuel.

Now, in case you're wondering, in this tutorial I'm gonna' show you 2 specific tests. One fuel pump test involves using just starting fluid. The other involves using a fuel pressure gauge (which is the most accurate of the two).

You can find this tutorial in Spanish here: Cómo Probar La Bomba de Combustible (GM 3.8L) (at: autotecnico-online.com).

IMPORTANT: Gasoline and starting fluid are extremely flammable! Be very careful. Take all necessary safety precautions and stay alert. You'll be able to accomplish both tests, indicated in this article, without any unhappy consequences, if you're careful and use common sense.

Symptoms Of A Bad Fuel Pump

I mentioned at the beginning of this tutorial that a bad fuel pump will cause your 3.8L equipped car or mini-van to ‘crank but not start’. Well, there are a few other specific symptoms you'll see. These are:

1. All of the coil packs, sitting on top of the ignition control module (ICM), are creating and delivering spark to the spark plugs. So you know that the ignition system is not at fault.
2. You've done a fuel injector Noid Light test and all of the fuel injectors you tested are getting their activation signal.
3. You've sprayed starting fluid (or carburetor spray) into the throttle body and your car (or mini-van) started, even if only momentarily.

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

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 3.8L V6 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 with a fuel pressure test gauge.

A fuel pressure gauge can cost you as little as $40 (US) or as much as $200 (US). The price difference depends on whether you're 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).

All of the fuel pump pressure test kits above have the fitting that will connect to your vehicle's Schrader valve.

FUEL PUMP TEST 1: Fuel Pressure Test With Gauge

GM has made it super easy for you and me to test the fuel pump (with a fuel pressure gauge), since they have installed a fitting (called the Schrader valve) on the fuel pressure regulator to which you and I can connect the fuel pressure gauge to (see photo above).

Using a fuel pressure gauge is one of the most accurate ways to make sure that enough fuel is reaching the fuel injectors. If you do own a fuel pressure gauge, this is the test for you.

If you don't own one, I'm gonna' make some recommendations to you as to which one to buy and where to buy it in the section Which And Where To Buy A Fuel Pressure Gauge.

As mentioned in the previous page, you can run down to your local AutoZone or O'Reilly auto parts and rent one too.

OK, let's get started with this test:

1. 1

   Connect the fuel pressure gauge to the Schrader valve on the fuel rail.

2. 2

   When ready, ask your helper to crank the engine while you observe the fuel pressure tester's gauge.

3. 3

   Your fuel pressure gauge will register one of the two following results:
   
   1.) The fuel pressure gauge will register 35 PSI, or
   
   2.) The fuel pressure gauge will register 0 PSI.

OK, let's analyze your test results:

CASE 1: If the fuel pressure gauge registered 0 PSI. This confirms that the cause of your 3.8L equipped GM car or mini-van is caused by a lack of fuel.

Now, I'm going to make one more recommendation to you: Before you condemn the fuel pump as bad, 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: If the fuel pressure gauge registered 35 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 3.8L GM vehicle is not starting is due to another reason. The fuel pump is OK.

FUEL PUMP TEST 2: Using Starting Fluid

Testing a no-start condition, to see if it's being caused by a lack of fuel can be very rapidly done with starting fluid. 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.

I personally have used this starting fluid test with a lot of success, along with a few other tests, to verify if the vehicle I'm working on is not starting due to a lack of fuel.

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, manually, and spray starting fluid down the bore. 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 hose clamp).
   
   Reconnecting the intake air duct is important as a safety precaution.

3. 3

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

4. 4

   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: If the engine started and ran for a few seconds. This means that the reason your 3.8L equipped car or minivan is not starting is due to a lack of fuel.

Usually, this also means that the fuel pump is bad. But since we don't live in a perfect world, this could also mean a few other things like: the fuel pump fuse is blown, or the fuel pump relay is bad or the factory anti-theft system has activated and is disabling the fuel injectors. Now, don't worry about this too much, because you can test this by:

1. Checking fuel pressure with a fuel pressure gauge. Go to: Your next step is to go to: FUEL PUMP TEST 1: Fuel Pressure Test With Gauge.
2. Making sure that the fuel pump is getting power (12 Volts) when you crank the engine.
   • You can accomplish this by locating the wire (circuit) that feeds power (12 Volts) to the fuel pump and with a multimeter in Volts DC Mode, verifying that 12 Volts are reaching the fuel pump when you crank the engine.

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 get yourself a fuel pressure gauge at your local auto parts store and check fuel pressure with it. If you have an AutoZone or an O'Reilly auto parts store nearby, you can rent one for free (after leaving a cash deposit which you'll get back when you return the tool). Your next step is to go to: FUEL PUMP TEST 1: Fuel Pressure Test With Gauge.

More 3.8L V6 GM Tutorials

If you enjoyed and/or found this ‘how to’ article helpful, I've written several more 3.8L specific test articles. You can find a complete list of 3.8L tutorials here: GM 3.8L Index Of Articles.

• How To Test The Fuel Injectors (GM 3.8L).
• How To Do A Fuel Injector Noid Light Test (GM 3.8L).
• How To Test The 3.8L GM Crank Sensor With a Multimeter.
• How To Test The Camshaft Position Sensor -P0341 (GM 3.8L).
• How To Troubleshoot A No Start (GM 3.8L).
• GM 3.8L Ignition Control Module and Crank (3X, 18X) Sensor Test (at: easyautodiagnostics.com).