So, you think you may have a faulty fuel pump? Troubleshooting the problem is usually pretty straightforward. But occasionally, a tricky diagnosis will have you pulling out your hair and throwing wrenches. That’s why it’s helpful to have a few extra diagnostic tricks up your sleeve.
Before beginning, you’ll want to have the vehicle’s repair information handy. Manuals, such as those from Chilton, are good but an ALLDATA subscription is even better. ALLDATA has single-vehicle subscriptions for DIYers that provide detailed factory repair information.
Also, keep in mind, other issues can mimic a faulty fuel pump. You’ll want to perform a thorough diagnosis to ensure the pump is the problem before replacing it. And consult the previous article in this series regarding fuel pump symptoms to understand how both continuous and returnless fuel systems work.
Note: The following is for informational purposes only. Consult the factory information for repair instructions and recommended safety procedures.
The first step is to verify the problem is fuel related. Then, you can do additional testing to find out if the pump is to blame.
If the engine won’t start:
- Listen for the fuel pump: Put your ear near the fuel tank and have an assistant turn the ignition key to the “on” position. The fuel pump should make an audible noise if it’s working properly.
- Whack the fuel tank: Have an assistant crank the engine while you hit the fuel tank with a rubber mallet. If the vehicle starts during this procedure, it’s a pretty clear sign the electric motor inside the pump is bad. Whacking the tank jars the motor enough to get things going – but only temporarily.
- Use starter fluid: Be warned: this procedure can cause personal injury, as well as damage to your engine. Try it at your own risk with the proper safety equipment, such as a fire extinguisher, on hand. Definitely wear your safety glasses. Do not use starter fluid on a diesel engine.
If you decide to attempt the test, try to start the vehicle by removing the air intake and spraying starter fluid into the throttle body. Have an assistant crank the engine while you spray – if it starts and runs momentarily, the problem is fuel-related and may indicate a bad pump. You’ll want to check fuel pressure as your next step.
If the engine won’t start OR it exhibits performance problems:
- Check for diagnostic trouble codes (DTCs) with a scanner or code reader: In some cases, a faulty pump may set DTCs in your car’s computer. The codes may either be directly related to the fuel pump or make reference to an air/fuel ratio problem. A scan tool or a simple code reader can be used to retrieve the information. Many auto parts stores will pull codes for free. Plus, there are inexpensive code reading tools and apps for your smartphone. Even if there are codes, you’ll want to check fuel pressure as your next step.
- Check fuel trim with a scan tool: This test only applies if the engine is running. All scan tools (not code readers; there’s a difference) will display a data parameter called short term fuel trim (STFT). As a general rule of thumb, fuel trim readings should be between -10 and 10, though some vehicles are different.
If fuel trim is above 10, that means the engine is running lean and the fuel pump may not be delivering. Keep in mind, however, other factors, such as vacuum leaks, can cause a lead running condition as well. So, you’ll want to check fuel pressure as your next step.
Check fuel pressure and/or volume
Checking fuel pressure and/or volume will tell you whether or not enough fuel is making it from the tank to the engine. But that doesn’t necessarily mean the pump is bad. It’s wise to do further testing to rule out fuel pump circuit issues and other problems, such as a clogged fuel filter or bad pressure regulator, that could limit the fuel supply.
Caution: Before hooking up a fuel pressure gauge, be sure to relieve fuel system pressure as outlined in your repair manual. Have your safety glasses on and a fire extinguisher nearby.
- Check fuel pressure: Mechanical fuel pressure gauges are inexpensive and readily available.
Performing a static fuel pressure test is pretty straightforward. Simply connect the gauge as outlined in your repair manual. Most modern vehicles have a handy test port located on the fuel rail.
Next, turn the ignition to the on position (key on, engine off), note the reading on the gauge and compare it to the specification listed in your repair manual. If the reading varies significantly from spec, you may be dealing with a bad fuel pump.
Also, when performing a static test, you’ll want to note whether the pressure holds. The factory repair information will tell you how long (usually several minutes) fuel pressure should hold once the pump is turned off. If pressure bleeds off too fast, there is a fuel-related problem such as a bad pump check valve or faulty regulator.
Those looking to perform a more in-depth fuel pressure assessment may choose to check fuel pressure at idle or under load. Also, continuous fuel systems can be checked for “dead head” pressure (pressure with the return line pinched off). But that test does not work on newer returnless systems that have the regulator in the tank.
- Check fuel volume: A pump may produce adequate pressure yet not create enough volume. And that’s where a fuel pump volume test comes in handy. In many cases, you can use a fuel pressure gauge to measure volume as well.
Start by connecting the gauge and placing the bleed end of its hose into a clean container. Without starting the engine, activate the pump for 15 seconds while you hold down the relief valve on the tool. Typically, the pump should deliver about a pint of fuel during that time. Consult the repair information for the exact procedure and specifications for your vehicle.
Part 2: Before condemning the pump…
So, you’ve verified a lack of fuel pressure and/or volume and now, you think you can condemn the pump. But wait – not so fast. In many cases, you’ll want to check the following beforehand:
- Check the fuel pump electrical circuit: In most older continuous-style fuel systems, the engine control module (ECM) controls the fuel pump via a relay. You’ll want to check that relay before condemning the fuel pump.
In many newer, returnless fuel systems, the fuel pump control module (FPCM) directly controls the fuel pump. The device uses information from sensors, such as the fuel pressure sensor and fuel temperature sensor, to determine control of the pump’s ground circuit and vary the pump’s speed. It’s a good idea to make sure everything within the control system is working properly before blaming the pump.
With both continuous and returnless systems, you’ll also want to check the integrity of the wiring in the pump circuit before jumping to conclusions. A digital multimeter (DMM) can be used to check the circuit for continuity, power and ground, as well as any unwanted excessive resistance.
- Check the fuel pressure regulator: Returnless fuel systems do not have an external pressure regulator. But on continuous systems, it’s a good idea to check the regulator by disconnecting the attached vacuum hose and seeing whether pressure increases. If the pressure does not increase, either the regulator is faulty or it does not have an adequate vacuum supply.
- Check the fuel filter: Continuous fuel systems have an external filter that’s relatively easy to install. Rather than attempting to test the filter, it’s usually easier to just install a new one and see if that helps. Remember to relieve fuel system pressure before replacing the filter. Have your safety glasses on and a fire extinguisher nearby.
Part 3: Advanced tests
There are a couple of advanced tests available for those who have the interest – and the money – to try them out.
- Measure current draw with an oscilloscope: One of the best ways to test a fuel pump is with an oscilloscope and low amp clamp probe. Connecting the probe to the pump’s power wire provides a waveform pattern that, if you know what you’re looking for, exposes internal pump problems. Generally, a good pump should produce a pattern of even “humps” if it’s working properly. If the pump is bad, the humps will be jagged and uneven.
- Use a professional-grade scan tool: Some high-end scan tools allow remote fuel pump testing. By pushing a couple of buttons on the tool, you can activate the pump to see if it runs.
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