This video covers the basics of fuel pump pressure, current, power and ground testing that can be applied to ANY electric fuel pump. We also go into some unique properties of the GM CPI “poppet” style injection system, and where it differs from typical fuel injection systems. Lower than normal fuel pressure is often misdiagnosed on the central port injection system. For example: Fuel psi at 49 psi during cranking will be a no start! Just about every other fuel injection system will start and run fine with this kind of pressure. But the GM CPI needs a minimum of 50 psi to open the mechanical injector poppet nozzles. This design was used on the 4.3, 5.0 and 5.7 engines throughout the 90s.

Engine Performance Diagnostics chapter 16 pages 2 - 6, 9 - 16

symptoms 50

  • intermittent no start condition
  • starts when a fuel source is used (carb. Clean, starting fluid etc) and may continue to run normally after it starts
  • possible hesitation/bog on acceleration once running depending on how weak the pump is
  • fuel system lean trouble codes (P0171, P0174)

fix 50

  • fuel pump replacement

test shown enh 50

  • how to check fuel pressure on a GM CPI system
  • how to use a GM fuel pump test connector and fused jumper wire to energize a fuel pump
  • using a low amp probe and scope to check an electric fuel pump
  • how to check an electric fuel pump power and ground
  • understanding the impact of a plugged fuel filter on fuel pump current

Tools used

  • fuel pressure gauge
  • low amp probe
  • fused jumper wire
  • lab scope

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(Chapters 14, 15,16) Fuel Delivery and Fuel Pressure Testing

Related videos:

GM Fuel Pump Relay Testing:


Low Fuel Pressure from a bad fuel pump ground (Ford Ranger):


How to identify and repair a leaking fuel pressure regulator (GM CPI system) part 1:


How to identify and repair a leaking fuel pressure regulator (GM CPI system) part 2:


Fuel Pump Electrical Circuits Description and Operation:


Symptoms and Causes of Low Fuel Pressure (Part 1):


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For more information on this topic, I have written a “field manual” called Engine Performance Diagnostics which is available as an eBook or paper book.

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