How To Test The Fuel Pump In 2 Tests (4.7L Jeep)

How To Test The Fuel Pump (4.7L Jeep)

There ar two methods to check to see if the fuel pump has failed. One is to use starting fluid and the other is to use a fuel pressure gauge.

In this tutorial I'll explain both methods in detail.

Symptoms Of A Bad Fuel Pump

Fuel pump failures fall into two categories: Total fuel pump failure that cause the engine to crank but not start. Or a fuel pump that still works and is able to start the engine, but doesn't send enough fuel volume to the fuel injectors (provoking a lean air/fuel mixture and a lack of power).

To delve into the specific symptoms of a total fuel pump failure, you'll see:

  1. The engine turns over but will not start.
  2. All 8 ignition coils are creating/delivering spark to the cylinders.
  3. The PCM will still pulse (activate) all 8 fuel injectors.

When a fuel pump still works, but doesn't send enough fuel to the fuel injectors, you'll see one or more of the following symptoms:

  1. Rough idle.
  2. Engine starts after extended cranking.
  3. Lack of power when accelerating the vehicle down the road.
  4. Back-fires thru' the intake manifold when accelerating your pickup or van down the road.

Both of these conditions can be tested with a fuel pressure gauge. Alright, with this info under our belts, let's get testing.

Where To Buy A Fuel Pressure Test Gauge

You can buy a fuel pressure test gauge just about anywhere and is one of the most important tools any serious DIY'er should have in his/her tool box.

The following fuel pressure test gauge kits are pretty good deals and also include the adapter to tests all Ford with a Ford-type Schrader valve:

All of the fuel pump pressure test kits above will connect to your 4.7L Jeep's Schrader valve.

TEST 1: Checking Fuel Pressure With A Fuel Pressure Gauge

How To Test The Fuel Pump (4.7L Jeep)

Whether the fuel pump has stopped pumping fuel completely or is pumping just enough to start the engine, checking it with a fuel pressure gauge is the most accurate way.

The engine, in your 4.7L Jeep, comes equipped with a fuel pressure test gauge test port called a Schrader valve. It's to this Schrader valve, located on the fuel injector rail, that we can connect the fuel pressure test gauge to.

NOTE: If you don't have a fuel pressure gauge, take a look at the section: Where To Buy A Fuel Pressure Test Gauge.

OK, let's get started with this test:

  1. 1

    Place a shop towel around the Schrader valve. The shop towel's job is to absorb any fuel that may leak when doing step 2.

  2. 2

    Connect the fuel pressure gauge to the Schrader valve on your 4.7L Jeep's fuel injector rail (it's located on the passenger side of the fuel rail).

  3. 3

    When ready, ask your helper to cycle the key on and off but don't crank the engine while you observe the fuel pressure tester's gauge.

    Check the connection at the Schrader valve for fuel leaks and if any tighten the fuel pressure a bit more (by hand only) to eliminate them.

  4. 4

    Your fuel pressure gauge should register: 30 to 45 PSI with the Key On Engine Off (KOEO) if the fuel pump is OK.

Let's take a look at what your results mean:

CASE 1: If the fuel pressure gauge registered 0 PSI. This confirms that the cause of your 4.7L Jeep's no-start problem 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: If the fuel pressure gauge registered 30 to 45 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 4.7L Jeep is not starting is due to another reason. The fuel pump is OK.

Jeep Vehicles:

  • Grand Cherokee 4.7L V8
    • 1999,