Test a fuel pump: A Step-by-Step Guide

Learn how to test a fuel pump using this Step-by-Step Guide

Replacing a fuel pump without testing it is an expensive mistake. The fuel pump in a fuel-injected vehicle is mounted inside the gas tank. It’s a major job to replace it and these pumps are expensive. So before you replace one, test the fuel pump using a fuel pressure testing kit.

I’ve seen too many DIYers make the assumption that their engine running problems are due to a bad pump. Several hundred dollars later, they still have the same problem.

Here’s the proper way to test a fuel pump

• Rent or purchase a fuel pressure test kit. Not all vehicles have a Schraeder valve to connect a gauge to. So it’s best to rent or buy a complete kit that will have the fittings you need to connect to the fuel line.

• Perform the fuel pressure check first thing in the morning with the engine cool. Fuel pumps have a check valve that prevents the fuel from draining back into the tank when the engine is shut off. If the valve isn’t working properly, you’ll have a fuel drainback, which causes hard starting first thing in the morning.

• Shop manuals have a specific procedure for performing this “leakdown” test if the vehicle is brought in warm with high fuel pressure. Usually, the tech looks for pressure to stay within 5 lbs of the running pressure.

• Check fuel pump volume. If your fuel pressure and leakdown readings are within spec, you’ll need to check fuel delivery volume. Yup, that’s an important test especially if your performance problems occur at high speeds or heavy throttle. Some manufacturers list a volume of fuel for a specified period of time. If you can’t find a spec, use this rule of thumb—a fuel pump should deliver 1 quart of fuel in 30 seconds. If yours passes that test, move on to the electrical tests.

• Fuel pump current draw test— The electrical test can spot a fuel pump problem that evades all other diagnostics. Remember, the pump is always pumping against resistance (the pressure regulator and he fuel injectors). Just like any other electric motor that meets resistance, it uses more power. So like an EKG at your doctor’s office, measuring current draw at different throttle openings tells you how the pump is performing. A properly operating fuel pump should draw from 3 to 6.5 amps.

If you own a scope, you can get even better readings. A scope will tell you if you have a bad winding in the pump since it detects every lobe on the armature.

If the fuel pump fails the leak-down test, pressure test, or volume test, replace the pump.

© 2012 Rick Muscoplat