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Fuel System Diagnostics for Successful Fuel Pump Installation

Misdiagnosis is the leading cause of fuel pump problems and returns. If the engine runs but displays drivability symptoms that you suspect are fuel-related, attempt to eliminate other possible causes by:

  1. Checking the vehicle’s OBD system
  2. Checking the ignition system
  3. Checking for vacuum leaks
  4. Checking the EGR and PCV systems
  5. Running a power balance test

If ignition and compression are both OK, that leaves fuel as the obvious culprit. Now the question is what’s wrong with the fuel delivery system? The most likely causes are:

  1. A dead fuel pump (could be the pump, pump relay, or wiring circuit)
  2. A clogged fuel filter
  3. Low fuel pressure (weak pump or restricted line)
  4. No pulse signal to injectors (bad injector relay or PCM driver circuit)

If you’ve eliminated the other possible causes there are two tests to diagnosis a faulty fuel pump:

  • Dead Head Pressure: This checks the maximum output pressure of the fuel pump. With the return line blocked the pump should produce pressure that’s significantly higher than its normal operating pressure at idle. If the pressure rating does not go up with the return line blocked, the pump may not be able to deliver enough fuel at higher engine speeds. Possible causes include: a worn pump, low voltage at the pump, a clogged fuel filter, a clogged inlet sock in the tank, an obstructed fuel line, or an almost empty fuel tank.
  • Fuel Volume Test: A fuel pump that delivers normal pressure may still cause drivability problems if it can’t deliver enough fuel volume to meet the engine’s needs. A fuel volume test may therefore be the best way to evaluate the pump’s condition. This test measures the volume of fuel delivered over a specified interval and can be done by connecting a fuel flow gauge into the fuel supply line, or by disconnecting the fuel supply line and placing it into a measured container.

Caution: Make sure there are no open sparks or flames nearby while doing this test!

With the engine off, energize the pump and measure the volume of fuel delivered during the specified interval of time. As a rule, a good pump should deliver about one quart of fuel in 30 seconds.

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