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Home >> GMC >> 1987 >> Forward Control 6.2 J, Automatic, 400/M40 >> Repair and Diagnosis >> Engine Performance >> System >> CCC TBI Tests W/Codes >> Diagnostic Charts >> Chart A6 - Fuel System Pressure Test >> Test Description

Test Description

NOTE: Test numbers refer to test numbers on diagnostic chart.
  1. Pressure is present, but is less than 9 psi (.63 kg/cm2 ), fall into 2 categories:

    Regulated Pressure, But Less Than 9 PSI (.63 kg/cm 2  )  Amount of fuel delivered to injector is adequate, but pressure is too low. System will run lean and may set Code

    Restricted Flow Causing Pressure Drop  Normally, a vehicle with fuel pressure of less than 9 psi (.63 kg/cm2 ) at idle will not be driveable. However, if the pressure drop occurs only while driving, engine will normally surge, then stop as pressure begins to drop rapidly.

  2. Restricting fuel return line allows fuel pump to develop maximum pressure (dead head pressure). When battery voltage is applied to pump test terminal, pressure should be 13-18 psi (.91-1.27 kg/cm2 ).
  3. This determines if high fuel pressure is due to restricted fuel return line or a faulty pressure regulator on throttle body unit.
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