How to Test an Automotive Fuel Pump

Explanation

Your car's fuel pump is used to supply gas to the engine's fuel injectors weather it be gasoline or diesel fuels. Depending on the fuel system design you can have one or more styles of pumps in various locations both mechanical and electric models. Direct injection fuel systems can be both diesel and regular gasoline which is comprised of a primary pump in the fuel tank and a secondary "high pressure" pump which mounts and is driven by the engine. Regular pressure fuel injection system have just one main pump in the fuel tank. A fuel pump test will show the performance condition of the pump.

What goes wrong?

As a car ages the internal parts of the fuel pump can wear causing a loss in pressure to the fuel injection rail and the injectors resulting in hard starting, hesitation and low engine power and when the internals of the pump fail completely the engine will not start.

Fuel Pump

  • One or two fuel pumps depending on design
  • Primary or main pump is located in the fuel tank
  • Can cause low power, hesitation and no start conditions
  • Is controlled by an electric relay or the revolution of the engine
  • Needs an external gauge to check pump performance
  • Is between two and three hour job to replace (most cases)
  • Secondary or "high pressure pump" is mounted to the engine

What's the cost?

Most people do not have a fuel pump test kit but fear not, auto parts stores will lend you one for free in most cases. If you suspect the fuel pump is getting weak in your car and you take your car to a shop to have the test done you can expect to pay between $90.00 and $150.00 US.

Let's Get Started

Begin with the vehicle on flat ground in park with the emergency brake set, engine off (no smoking or open flame). This test is for in-tank fuel pumps only.

  1. Pressure Port: Some cars are designed with a convenient pressure port on the engine fuel rail (remove cap) to attach the gauge to while other designs will need an adapter from the test kit which can be attached at the fuel filter or fuel pressure feed line headed to the engine's fuel rail.
  2. Remove the fuel pressure test gauge from the kit. Most gauges have a quick disconnect coupler that will allow several different adaptors for various manufacturers. The kit should have them labeled but if not look for the hose connector or fitting that will work for your car.
  3. When attaching the pressure gauge (engine off) a small amount of fuel may be present so don't be alarmed have a shop towel ready.
  4. Without starting the engine turn the ignition key to the on position the gauge should jump up to between 45 psi and 58 psi (DPI) direct port injection systems, throttle body injection (TBI) should be between 13 and 17 psi in most cases as the system primes. If no pressure is present recheck the hose or adapter connection to avoid any false negatives, re-cycle the ignition switch to recheck. Still no pressure? suspect a bad fuel pump, fuel pump relay or fuse.
  5. Now start the engine so we can test the performance of the pump. While the engine is running, the fuel pressure should drop about 5 psi, from the static prime pressure (DPI) then snap the throttle, the fuel system pressure should jump up about 5 psi. this means the fuel pump and pressure regulator is working correctly. If the system fuel pressure does not respond inspect the regulator vacuum feed line and if okay the fuel pressure regulator has failed (if equipped).
  6. The final check will be the load test. This test will determine how the pump performs under load of use. This can be achieved by one of two ways, first you can load the engine by having the transmission in reverse (automatic) and hold the brake while gently giving the engine a slight amount of throttle. This will load the engine while not allowing the car to move effectively loading the engine and fuel pump. The second way is to close the hood to the first safety catch and drive the car (not at highway speeds) while the gauge rests outside being visible to the driver. The hose will route from the side of the hood while using masking tape to secure the gauge. The pressure should hold continuously throughout the test. If the fuel pressure is low under load or while idling the fuel filter (if equipped) could be clogged, or the fuel pump has failed and needs replacement.
  7. Remove the fuel pressure gauge and reinstall the dust cap or remove the fuel line adaptor clean up any excess fuel.

Watch the video!

Fuel Pump Pressure Test

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