Testing A Fuel Pump

Fuel pumps in fuel injected vehicles are mounted inside the gas tank. It's a major job to replace it and these pumps are expensive. So before you replace one, you better do your homework. I've seen too many DIY mechanics make the assumption that their engine running problems are due to a bad pump. Several hundred dollars later, they still have the same problem.

If your diagnostics lead you to believe you have a fuel pump problem, here's the proper way to check out the pump. First, perform a fuel pressure test. You'll need a fuel pressure gauge and the adapters to fit your vehicle. Some auto parts stores rent this test gear. You'll also need the specs for your vehicle. Check out a printed or online shop manual for those.

Perform the fuel pressure check first thing in the morning, with the engine cool. A cool engine provides two things: better working conditions with less chance of a fuel-spill related fire, and a more accurate reading of residual fuel pressure after sitting for a long time. 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 fuel drain back and that 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.

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.

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.