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.

TEST 1: Using Starting Fluid To Confirm Lack Of Fuel

How To Test The Fuel Pump (4.7L Jeep)

Using starting fluid, to check for a lack of fuel condition, is probably one of the most common tests. It's a fast and easy test and a good starting point when checking for a bad fuel pump. Having said that, let me tell you that it's not the most accurate way to test for a bad fuel pump.

The most important thing to keep in mind is that the starting fluid test only works if you have confirmed that all the ignition coils are creating and delivering spark.

The rule of thumb is that if by spraying starting fluid your 4.7L Jeep starts, even if momentarily, then this tells you that fuel is missing from the equation (engine start equation: fuel+spark+air=combustion).

IMPORTANT: This is a very fast and easy test but you do have to take one very important safety precaution and this is to reconnect the air intake duct after spraying starting fluid down the throttle bore (although you don't have to fasten it). This will prevent any backfire, that might occur, from scaring the heck out of you when cranking the engine.

This is what you have 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 plate and spray starting fluid down the bore.

    As a safety precaution reconnect the air duct after you have sprayed a good squirt of starting fluid (but you don't have to tighten the air duct's hose clamp).

  3. 3

    Crank the engine once the air duct is back on and you're clear of the engine compartment.

  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 test result tells you that the no start problem is due to a lack of fuel.

Your next step is to check to see what the fuel pressure is with a fuel pressure test gauge. Go to: TEST 2: Checking Fuel Pressure With A Fuel Pressure 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 and this is to check the fuel pressure with a fuel pressure test gauge. Go to: TEST 2: Checking Fuel Pressure With A Fuel Pressure Gauge.

Jeep Vehicles:

  • Grand Cherokee 4.7L V8
    • 1999,