How to Use a Fuel Pressure Testerby Justin Cupler
The fuel pump on a vehicle may fail over time. There are a few ways a fuel pump can fail: Ignition pressure, idling pressure, acceleration pressure and residual pressure. Ignition pressure is how much pressure is built up once the ignition key is turned to the "On" position. Idling pressure is the amount of fuel pressure during idle. Acceleration pressure is the fuel pressure delivered when accelerating. Finally, residual pressure is the pressure that remains in the lines after the key is turned "Off."
Pressure Tester Connection
Open the vehicle's hood.
Locate the under-hood junction block, it is a black box typically located near the battery. Remove the junction block's lid.
Turn the lid over and notice the fuse and relay "map" on the underside. Locate the fuse labeled "fuel pump" on the map.
Remove the fuel pump fuse from the junction block.
Start the vehicle and allow it to run until it stalls, this relieves all fuel pressure from the system. Turn the key off, reinsert the fuse and close the fuse box lid.
Locate the fuel rail, the metal rail on top-center of the engine and runs the length of the engine. Uncap the small valve on the fuel rail, known as a Schrader valve.
Place the fitting on the end of the pressure tester's hose on the Schrader valve and press it on to the valve. Listen for the clicking sound indicating the pressure tester is connected.
Turn the ignition key to the "On" position, but do not start the vehicle.
Read the psi level on the pressure tester.
Compare that reading -- known as the ignition pressure -- to the recommended pressure listed in the repair guide. If the pressure is more than a few pounds low, there is a problem somewhere in the fuel system -- likely the fuel pump.
Turn the key and start the vehicle. Allow it run for roughly five minutes.
Check the psi reading on the fuel pressure tester -- this is known as the idling pressure -- and compare it to the recommended pressure in the repair manual. If it is even a few pounds low, there is a problem in the fuel system, such as: weak fuel pump, low fuel pump voltage, clogged fuel filter or bad pressure regulator.
Locate the return fuel line, the smaller of the fuel lines coming from the fuel rail. Pinch off the return fuel line, using the needle-nosed pliers. Read the psi on the pressure gauge, the pressure should be nearly double the recommended idling pressure. If not, there could be several problems with the fuel system.
Remove the needle-nosed pliers from the return fuel hose.
Turn the vehicle off and remove the ignition key. Allow the vehicle to sit for 5 to 10 minutes and check the psi reading on the fuel pressure gauge, this is known as residual pressure. Compared the pressure loss to the acceptable amount of pressure loss in the repair manual. Excessive pressure loss indicates an internal or external fuel leak.
Pressure Tester Removal
Open the under-hood junction block's lid and remove the fuel pump fuse.
Start the vehicle and allow it to run until it stalls, purging it of all fuel pressure.
Remove the pressure tester hose by pulling back on the locking ring on the connector and pulling the connector from the fuel rail.
Reinstall the fuel pump fuse and close the lid of the under-hood junction block.
Reinstall the cap on the Schrader valve and close the vehicle's hood.
- If any of the pressures are inaccurate, continue with the proper diagnostics.
Things You'll Need
- Repair manual (Chilton's or Haynes)
- Needle-nosed pliers
- Never smoke while performing this test.
Justin Cupler is a professional writer who has been published on several websites including CarsDirect and Autos.com. Cupler has worked in the professional automotive repair field as a technician and a manager since 2000. He has a certificate in broadcast journalism from the Connecticut School of Broadcasting. Cupler is currently studying mechanical engineering at Saint Petersburg College.