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Troubleshooting Symptoms That May Mean a Bad Fuel Pump

Updated on September 4, 2016
Automotive fuel pump
Automotive fuel pump | Source

Fuel pump problems are hard to diagnose sometimes. So you may not be able to tackle every fuel system issue on your car without some experience in car repair. Still, you can take on some common pump problems using one or two inexpensive, special diagnostic tools: fuel pressure gauge and hand-held vacuum pump.

This guide helps you find out what type of issue you're dealing with, and whether you'll be able to fix it yourself. With the help of some simple tests, you'll troubleshoot the system and speed up the repair process to get your engine running good again and save some money as well.

Symptoms That May—But Don't Necessarily—Mean a Bad Fuel Pump

*Noisy fuel pump

*Engine dies every few miles of driving (see Tip below)

*Crank-no-start condition

*Hard-to-start engine

*Lack of engine power at highway speeds

A clogged fuel filter can also prevent your engine from starting.
A clogged fuel filter can also prevent your engine from starting. | Source

Index

I. Doing Your Homework: Preliminary Checks

II. How to Test Fuel System Pressure

III. How to Check the FPR with a Vacuum Pump

IV. Testing for Fuel Pressure on a TBI System

I. Doing Your Homework: Preliminary Checks

Before testing the fuel pump, go over this checklist. This includes some obvious items and often forgotten common maintenance issues that may turn into a false "bad fuel pump" diagnostic.

1. Check for fuel in the tank. If the engine cranks but refuses to start, make sure that you actually have fuel in the tank.

2. Listen for fuel pump noise. Usually, you can do this test yourself from inside the vehicle. By turning the ignition key to the ON position (engine off), you should hear the fuel pump come alive with a swishing sound for about 2 seconds. If you can't hear the pump working, enlist the help of an assistant and follow the next steps:

  • Remove the fuel filler cap.
  • Put your ear close to the fuel filler opening.
  • Ask an assistant to turn the ignition key to the ON position, but don't start the engine.
  • You should hear a whirring sound coming from inside the fuel tank for about two seconds. That's the sound of the fuel pump being energized. That means the pump is receiving power and it's responding.
  • If you don't hear this sound but the engine cranks when you try to start it, you may have a problem in the fuel pump electrical circuit. Check the fuel pump fuse, fuel pump relay, and, if necessary, the wiring and other related sensors that the engine control module (ECM-car computer) relies on to activate the fuel pump, like the camshaft position sensor.
  • Check the car computer for any stored diagnostic trouble codes (DTC) in the computer that may point in the direction of the problem. If the car moves, you can go to a local auto parts store that can pull the DTC for you.
  • If you think there's a problem in the electrical circuit and the fuel pump fuse is okay, try switching the fuel pump relay with another similar relay in your car and try starting the engine.
  • On some vehicle models, insufficient engine oil pressure will keep the engine from starting to prevent engine damage. Check for any stored DTCs. If necessary, the vehicle repair manual for your particular car make and model will guide you to make this checks.

3. Make sure the timing belt is okay. If the engine cranks but doesn't start and the engine uses a timing belt, make sure the belt is still in place or not loose. On average, a timing belt has a service life of about five years. On some models, checking the timing belt is a simple procedure. After removing the cover or being able to pull the cover a bit, confirm that the belt is in place. If so, have an assistant crank the engine while you observe the belt. Make sure the belt runs smoothly.

4. Consider whether the fuel filter is clogged. Have you changed the fuel filter according to your car manufacturer service schedule? Look up the service interval for the fuel filter in your car owner's manual or vehicle repair manual. If necessary, replace the filter to make sure you are not dealing with a restricted or clogged fuel filter.

5. Check the vacuum line to the fuel pressure regulator (FPR). Disconnect the vacuum line from the fuel pressure regulator (FPR). This is a small metallic cylinder connected to the fuel rail. Check that the vacuum line has no traces of fuel and is completely dry (if possible, do this check with the engine idling). If the inside of the vacuum hose is wet, it's sucking fuel off the pressure regulator because the diaphragm is torn. Replace the fuel pressure regulator.

Fuel pressure regulator on fuel rail (bottom of the assembly).
Fuel pressure regulator on fuel rail (bottom of the assembly). | Source

6. Make sure there is fuel in the fuel lines. On EFI models you can do this) by depressing the Schrader valve or "test port." You'll find the valve somewhere along the fuel rail, which holds the fuel injectors in place. Cover the valve with a shop rag and use a small screwdriver to depress the valve. Use the rag to catch the squirt of fuel. If there's fuel in the line, it just confirms that the fuel pump is delivering fuel, not necessarily the right amount, though.

  • On throttle body injection (TBI) models, you can actually see the fuel spraying into the throttle body by removing the air cleaner housing and watching the injector while an assistant cranks the engine.
  • If your system doesn't have a Schrader valve and the engine doesn't start, remove the fuel pump fuse, crank the engine for a few seconds. Disconnect the fuel line from the fuel rail and point the end of the line into a fuel approved container (if it is a steel line, you can connect a small length of hose to the line). Turn the key to the ON position or crank the engine. If the pump delivers fuel, it is working; but it doesn't confirm that the pump is delivering enough fuel, though. If the pump doesn't deliver fuel, you may be dealing with a clogged, restricted fuel line, fuel pump electrical circuit problem or failed pump.

Tip 1: When the Engine Dies After Running a Few Minutes

If the engine dies within minutes of driving and it won't start until the engine cools down, do the next test as soon as the engine dies:

  • Open the hood and pull out a spark plug wire.
  • Grab the wire with a pair of pliers with insulated handles.
  • Position the tip of the wire about half an inch to an inch from bare metal engine or a metal bracket (you want a grounded spot).
  • Ask an assistant to crank the engine, or use a remote starter switch.
  • Is there a spark coming out from the end of the wire?
  • If there's no spark, you may have a bad ignition coil. If it sparks, you may have a bad fuel pump.

Tip 2: Check the Inertia Switch

Ford vehicles—and some other makes—use an inertia switch to shut off the fuel pump when the car is involved in a collision. But other events may trigger the switch:

* Hitting a rock

* Driving over a pothole

* Even a little bumper push in a parking lot

So check your car owner's manual or vehicle repair manual for the location of the switch and how to reset it, if necessary.

II. How to Test Fuel System Pressure

When you know the fuel pump is delivering fuel, a fuel system pressure test can sometimes identify the source of the fault. With the next series of tests, you'll use a fuel pressure gauge to test system pressure.

1. Relieve fuel system pressure.

  • If the fuel system on your car comes equipped with a Schrader or test port valve (it's similar to the air valve on your tires) cover the valve with a shop rag and carefully depress the valve with a small screwdriver. Use the rag to catch the squirt of fuel. This valve is similar to the air valve on your tires.
  • If your fuel system isn't fitted with a test port, you can remove the fuel pump fuse and start the engine. It'll die after a few seconds for lack of fuel. Plug back in the fuel pump fuse.

2. Connect the fuel pressure gauge to the test port. If you don't have the correct adapter to connect the gauge to the Schrader valve, try removing the valve and connecting the gauge to the fitting; however, if there's no test port, disconnect the fuel supply line from the fuel rail (this is the line coming from the fuel filter), and connect a T-adapter to the line so that you can reconnect the line to the fuel rail and connect the gauge as well.

3. Start the engine and let it idle. NOTE: The repair manual for your particular vehicle may include a Key On-Engine Off test. Check your manual if necessary.

4. Compare fuel pressure to specs. Check your fuel gauge pressure reading and compare it to the specification in your vehicle repair manual (if the vehicle doesn't start, turn the ignition key to the ON position and read fuel pressure. Compare your reading to the specification in your manual for key ON pressure). An injection fuel system usually produces between 15 and 40 psi (pounds per square inch)(100-280 kPa) or more, depending on the application. The reading includes pressure from the fuel pressure regulator (FPR). So, if your reading is out of specs, you may have a problem with the FPR, so keep reading. NOTE: Newer modern fuel systems don't use an FPR on the injection system; rather, they vary voltage to the pump or use an internal pump valve to handle system pressure.

A. What to Do if Pressure is Correct

  • With the fuel pressure gauge still connected and engine idling, unplug the vacuum line from the fuel pressure regulator (FPR—the small metal cylinder that connects to the fuel rail).
  • Check that the vacuum line is completely dry. If you find traces of fuel inside the hose, it's sucking fuel off the pressure regulator because the diaphragm is torn. Replace the fuel pressure regulator.
  • Fuel pressure should increase anywhere from 3 to 10 psi. Otherwise, check the vacuum line: If the line has no vacuum, check that the hose is properly connected and not damaged and that the vacuum port is not restricted; if the line has vacuum but there's no increase in pressure when you disconnect the vacuum hose, replace the fuel pressure regulator. For best results, conduct the test described in the following section, Checking the FPR with a Vacuum Pump.
  • Turn off the engine and wait for at least 5 minutes. Compare your pressure reading to specification (for hold pressure) in your repair manual. If pressure drops below specification before the five minutes, you may have a leak somewhere: check the fuel lines, fuel pump, injector or pressure regulator for a leak.

B. What to Do if Pressure is Low

  • Place a shop rag over the fuel return line.
  • Pinch the line using a suitable pair of pliers to block fuel flow.
  • Watch the pressure gauge; if pressure remains low, either the fuel line is restricted, the fuel pump is receiving insufficient voltage, or the pump is worn out. However, if pressure rises, replace the FPR.

C. What to Do if Pressure is High

  • First, conduct the test described in the next section, 'How to Check the FPR with a Vacuum Pump."
  • Then, turn off the engine.
  • Disconnect the fuel return line.
  • Blow through the fuel return line to check that it's not blocked. If the return line is blocked, locate the problem.

NOTE: The video below gives you an idea how to test for power and ground on your fuel pump electrical connector.

III. How to Check the FPR with a Vacuum Pump

1. Disconnect the vacuum line from the fuel pressure regulator (FPR).

2. Connect a vacuum gauge—or vacuum pump—to the vacuum line,

3. Start the engine and let it idle.

4. Make sure that you have enough vacuum; depending on elevation, around 12 in-Hg (inches of Mercury). Consult the vehicle repair manual for your particular vehicle model, if necessary.

If there's not enough vacuum, there may be a leak or a restriction in the vacuum line or port. Check for vacuum hose damage, a loose connection or an obstruction in the line or the connection port.

5. With the engine idling and the fuel pressure gauge connected, plug in the hand-held vacuum pump to the FPR in place of the vacuum line.

6. Gradually apply vacuum to the FPR (up to 15 InHg). You should see the fuel system pressure decreasing as you increase vacuum to the FPR; you should see fuel system pressure increase as you decrease vacuum to the FPR.

If the FPR is receiving the necessary vacuum from the engine but pressure doesn't seem to increase or decrease when you apply vacuum to the pressure regulator, replace the FPR. For a more detailed test of the fuel pressure regulator, check the article How to Test a Fuel Pressure Regulator.

IV. Testing for Fuel Pressure on a TBI System

On throttle body injection (TBI) systems, the fuel injector is located on above the throttle plates when assembled.
On throttle body injection (TBI) systems, the fuel injector is located on above the throttle plates when assembled. | Source

Testing fuel pressure on a throttle body injection system is similar to testing fuel pressure on a EFI multiport system. You still use a fuel pressure gauge, but you may need a special adapter to connect the gauge to the fuel input line at the throttle body (check your repair manual for the correct adapter for your model).

1. Relieve the fuel system pressure (check your repair manual for the best procedure for your model). Usually, you can disconnect the fuel pump fuse or unplug the electrical connector from the ignition coil; start or crank the engine to use up the fuel in the lines).

2. Connect the fuel pressure gauge.

3. Start the engine and let it idle.

4. Read the system pressure and compare it to the specification in your repair manual.

  • If pressure is low, you may have a leak in the fuel line (check the line all the way back to the fuel tank) or throttle body injector (place a piece of paper under the injector and check it later for traces of fuel), or a worn out fuel pump.
  • If pressure is higher, check for a restricted fuel line or stuck fuel pressure regulator (consult your manual for the best way to check the FPR in your car).

Conclusion

Fuel pump problems are not easy to diagnose, specially because some symptoms are similar to those produced by problems in other systems. Also, troubles with a sensor can hinder the fuel system, making it hard to diagnose without some repair experience and special diagnostic tools. But this guide can help you with some common problems you're likely to encounter when you think the fuel pump has gone bad.

Test Your Knowledge of Fuel Pump Problems


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