How To Test The Fuel Pump (1989-1991 3.9L V6 Dodge Ram Pickup)

You can accurately check if the fuel pump on your 1989-1991 3.9L V6 Dodge Ram pickup with a fuel pressure gauge.

In this tutorial, I'll show you how to do that. With your test result, you'll know if the fuel pump is working or shot.

I'm also gonna cover how to use starting fluid to help you figure out if your engine isn't starting because it's not getting fuel.

APPLIES TO: This tutorial applies to the following Dodge Ram pickups:

• 3.9L V6 Dodge Ram 100 Pickup: 1989, 1990, 1991.
• 3.9L V6 Dodge Ram 150 Pickup: 1989, 1990, 1991.
• 3.9L V6 Dodge Ram 250 Pickup: 1989, 1990, 1991.

Fuel Pump Wiring Diagrams:

• Fuel Pump Circuit Wiring Diagram (1989 3.9L V6 Dodge Ram Pickup).
• Fuel Pump Circuit Wiring Diagram (1990-1991 3.9L V6 Dodge Ram Pickup).

Symptoms Of A Bad Fuel Pump

Most of the time, fuel pumps quit in one of two ways:

• Complete failure: The pump gives out completely and stops sending fuel to the injectors. The engine cranks but just won't start.
• Degraded performance: The pump still works —it sends enough fuel to get the engine started, but not enough to keep it going, especially when you're driving and putting load on it.

If the pump's weak but not totally dead, you might notice one or more of the following:

• Rough idle: The engine runs rough when it's idling.
• Hard starting: It seems you've got to crank the engine forever for it to start.
• No power: You'll feel it most when you shift into Drive and try to speed up —truck just won't move like it should.
• Backfires: These are the classic "popping" sounds coming from the intake when you hit the gas and the engine's is running with a lean air/fuel mixture.
• Running lean: Not enough fuel messes with the air/fuel mix and can trigger oxygen sensor or fuel system codes.

Doesn't matter if your pickup won't start at all or just runs like crap —a quick fuel pressure test can tell you if the fuel pump is the problem.

What does "lean" mean? Your engine needs the right mix of air and fuel to perform properly. The fuel injection system adjusts how much fuel is added depending on the airflow entering the engine. If not enough fuel is added for the amount of air the engine is breathing —like when the fuel pump is going bad — the engine runs "lean" and the air/fuel mixture is said to be "lean".

If the system adds too much fuel for the air coming in, the mixture is considered "rich".

Fuel Pressure Specifications

1989-1991
3.9L V6	14.5 PSI (Throttle Body Injection -TBI- System)

Where To Buy A Fuel Pressure Test Gauge

A fuel pressure test gauge is a must-have for any serious DIY mechanic and thankfully, you can buy them just about anywhere. If you don't already own one, it's definitely worth adding to your toolbox.

Here are a few good kits that will work with your Dodge Ram Pickup:

TEST 1: Checking Fuel Pressure With A Fuel Pressure Gauge

Your throttle body's got two rubber fuel hoses hooked up to it and secured with hose clamps. One's the fuel pressure inlet hose (5/16") and the other's the return hose (1/4").

To test the fuel pump's pressure, we're gonna connect a fuel pressure test adapter between the 5/16" inlet hose and the throttle body's metal inlet port (see the illustration above).

You'll find that inlet port at the bottom rear of the throttle body —that's where the 5/16" hose plugs in. The smaller hose on the top port is the 1/4" fuel return hose.

If you don't have a pressure test kit with the right adapter, check out my picks here: Where To Buy A Fuel Pressure Test Gauge.

Alright, here's how we're gonna do it:

1. 1

   Remove the air filter cleaner assembly from the throttle body.

2. 2

   Disconnect the vacuum hose from the fuel pressure regulator.

3. 3

   Disable the ignition system. The engine should not start during the test.
   
   NOTE: You can disable the ignition system by disconnecting the one of the two wires from the ignition coil. DO NOT disconnect the distributor from its 3-wire connector.

4. 4

   Disconnect the fuel injectors' connector. The fuel injectors should not spray fuel during the fuel pressure test.

5. 5

   Place a shop towel under the fuel inlet hose.
   
   The towel's there to catch any gas that might drip out in the next step.

6. 6

   Disconnect the 5/16" fuel inlet hose from the throttle body's inlet port.
   
   NOTE: You've got two hoses—one's 5/16" and the other's 1/4". The bigger one (5/16") is the inlet hose you're after.

7. 7

   Connect the adapter's hose to the inlet port, then connect the pressure gauge adapter to it.

8. 8

   Now take the 5/16" inlet hose you pulled off earlier and connect it into the fuel pressure gauge adapter.

9. 9

   When you're set, have your buddy turn the key on and off rapidly several times —but don't crank the engine.
   
   Check the fuel pressure gauge adapter and hoses for leaks. If fuel's leaking, tighten those hose clamps until it stops.

10. 10

    Crank the engine and watch the pressure gauge.
    
    NOTE: The engine should not start and the fuel injectors and ignition system must be disabled.

11. 11

    The fuel pressure gauge should report 14.5 PSI.

Now let's break down what your pressure test means:

CASE 1: Fuel pressure gauge shows 0 PSI. This confirms that the reason your engine isn't starting is because it's not getting fuel at all.

In 99% of the cases, the fuel pump is bad and needs to be replaced. In the other 1%, the fuel pump isn't getting power. So, you'll still need to make sure the fuel pump is getting 10 to 12 Volts DC (with the engine cranking) to be sure the fuel pump is toast.

CASE 2: Gauge shows pressure, but it's lower than spec. That tells you the pump's weak and not keeping up —it's on its way out. Replace the fuel pump.

CASE 3: Gauge shows the correct PSI. That means the pump's doing its job.

If your Dodge Ram pickup's engine is not starting, the fuel pump's not the problem. It's working and sending enough fuel to the injectors. You'll need to dig deeper —something else is keeping your pickup from starting.

TEST 2: Using Starting Fluid To Confirm Lack Of Fuel

A quick check to see if your truck's engine no-start is due to "no-fuel condition" is using starting fluid to see if it can get it to start.

This test simply involves spraying starting fluid into the throttle body, then cranking the engine to see if it fires up.

If a lack of fuel is the issue (behind the no-start), the engine will start and run for a few seconds —just long enough to burn through the starting fluid.

Now, this method isn't the most exact way to test the fuel pump, but it's a solid starting point when you're trying to figure out what's wrong.

NOTE: The accuracy of your test result depends on having all six spark plug wires firing spark. If you haven't checked for spark yet, do that first.

IMPORTANT: This test's quick and simple, but don't skip this key safety step: After spraying the starting fluid, place the air cleaner assembly back onto the throttle body before cranking the engine. You don't need to clamp it down —just set it in place. It'll keep any backfires from spewing flames onto the engine while cranking.

Let's get going:

1. 1

   Take the top cover off of the air cleaner assembly.
   
   You don't have to remove the whole thing from the throttle body —just its top cover.

2. 2

   Hold the throttle plate open and spray some starting fluid into the bore.

3. 3

   After you spray, go ahead and place the cover back on the air cleaner assembly.
   
   No need to tighten it.

4. 4

   Crank the engine once the top cover is back on the air cleaner assembly.

5. 5

   One of two things will happen:
   
   1.) The engine will fire up briefly and then stall.
   
   2.) The engine will just crank and not start at all.

Now let's break down what that result tells you:

CASE 1: Engine started and ran for a few seconds. That means your no-start issue's coming from a lack of fuel —most likely the pump's not delivering.

Your next move is to test the actual fuel pressure using a gauge. Head over to: TEST 2: Checking Fuel Pressure With A Fuel Pressure Gauge.

CASE 2: Engine didn't start at all —not even for a second. That usually means fuel isn't the issue behind the no-start.

But remember how I said this test isn't super precise? So even if it doesn't start, it's still worth checking the fuel pressure with a proper gauge just to be sure. Go here: TEST 2: Checking Fuel Pressure With A Fuel Pressure Gauge.

More 3.9L V6 Dodge Ram Pickup Tutorials

I've written quite a few 3.9L V6 Dodge Ram pickup ‘how to test’ tutorials. You can find them in this index:

• 3.9L V6 Dodge Ram Pickup Index Of Articles.