How to troubleshoot a fuel pump
Fuel pumps can fail in four ways: no pressure (dead pump, low pressure, low volume or pressure leak down. The best way to troubleshoot a fuel pump is with a fuel pressure gauge, but I know most DIYers don’t have that kind of expensive gear. So here are some DIY tips for alternative methods to troubleshoot your fuel pump.
Tricks for troubleshooting a dead fuel pump
A fuel pump for a fuel-injected engine is an electric motor with a centrifugal pump. Just like any other electric motor, it can develop wear spots on the armature that prevent it from starting when the armature lands in a worn area. In addition to armature problems, fuel pumps require power and ground. Since they’re located inside the fuel tank, the electrical connections at the top of the tank can develop corrosion that prevent the pump from working.
Always start troubleshooting by checking the fuse for the fuel pump. If the fuse is good, move on to check for voltage and good ground at the electrical connector for the fuel pump. If your meter shows power and ground, move on to the tips below. However, if you don’t see power at the fuel pump connector try swapping the fuel pump relay with another relay of the same size from another component in the fuse box. If you then see power, replace the bad fuel pump relay.
Smack the fuel tank to jump-start the fuel pump
One way to move the armature off the worn/dead spot is to vibrate the pump. To do that, remove a shoe and smack the bottom of the fuel tank. Sometimes the vibration from the hell of your shoe is enough to get your going. If this trick works, don’t think you’ve avoided the costly repair. It won’t work for long. Get the pump replaced as soon as possible.
Tips for troubleshooting fuel pressure leak down
A “cranks but won’t start” condition first thing in the morning or after the vehicle sits for a long time may be caused by a leaking check valve in the fuel pump. The check valve is designed to prevent all the fuel in the line from leaking back down into the tank when you shut off the engine.
When a check valve fails, the fuel lines are empty and it can take up to 10 seconds of cranking to refill the lines. Here’s the procedure to test for a failed check valve
Turn the key to RUN but do not crank the engine. Leave it in the RUN position for 2-3 seconds. Then turn to OFF. Turn the key back to the RUN position for another 2-3 seconds but do not crank the engine. Repeat this procedure 2 more times. After four “prime” attempts, try starting the engine. If it starts right up, that proves the check valve is bad.
Normally you have to replace the entire fuel pump to fix a bad check valve. But there are some fuel system designs that can be repaired using an add-on check valve. See this post.
Troubleshoot low fuel pump pressure
Low fuel pump pressure can cause a “cranks but won’t fire up” condition. That’s because a fuel injector doesn’t really “inject” fuel. It’s just a valve that opens and closes. It relies on fuel pump pressure to atomize the fuel once the injector opens. However, fuel injector operation isn’t just a fully-open or fully-closed operation. The ECM pulses the fuel injector to regulate on/off time which meters how much fuel enters the cylinder.
When fuel pump pressure is too low, not enough fuel enters the combustion chamber, so the engine can’t fire up. One way to trick the system is to press the gas pedal about halfway during cranking. That overrides the computer’s factory programming and tells the computer to add more fuel. So the computer will tell the fuel injectors to remain open longer, which feeds more fuel to the cylinders. If that trick works to get you started, don’t rely on it forever. The pump is still failing and it must be replaced.
Troubleshoot low volume fuel pump issue
This one is much tougher to troubleshoot without the proper tools. A fuel pump can output the proper pressure and enough fuel to start the engine, but fail to pump enough volume of fuel to keep the engine running at higher speeds.
The symptom of a low volume fuel pump issue is lack of acceleration, especially at higher speeds. In other words, the pump can’t output enough fuel to satisfy the engine requirements. This can be a confusing symptom because a clogged fuel filter can also cause this condition. So always start your troubleshooting by replacing the fuel filter. That’s easy to do on many vehicles where the fuel filter is located outside of the tank. But on many late-model vehicles, the filter is inside the tank.
In that case, you should get this condition checked by a professional shop before repalcing any parts.
©, 2019 Rick Muscoplat
Posted on by Rick Muscoplat