Replacement of a Jerk Type Fuel Pump

1. Jerk Type Fuel Pump – Description

The jerk type helix-port controlled fuel pump is widely used on slow-speed and medium-speed marine diesel engines.

The quantity of fuel injected and the end of injection are controlled by the helical groove on the plunger, which uncovers the spill port in the barrel.

The beginning of injection is determined solely by the plunger lift, which depends on the cam profile.

2. Safety Precautions Before Replacement

Before replacing the fuel pump, the following safety measures are taken:

• Engine stopped and turning gear engaged
• Starting air supply shut off and blocked
• Fuel oil and lubricating oil supplies isolated
• Indicator cocks opened
• Fuel pressure released and fuel oil drained before dismantling

3. Initial Setting and Governor / VIT Connections

• Existing fuel rack position is marked before dismantling
• Initial fuel rack index is checked and marked using the manufacturer’s template
• The cross-bore of the plunger is aligned with the lower cut-off holes in the barrel
• Alignment is confirmed by visual inspection or using a torch light
• Governor fuel rack linkage is disconnected only after marking its position
• If Variable Injection Timing (VIT) is fitted:
  • VIT index arm is pulled to zero position
  • Linkage position is clearly marked before disconnection

  4. Replacement Procedure of Jerk Type Fuel Pump

  • Fuel pump top cover is removed
  • Barrel and plunger assembly is dismantled
  • Fuel inlet pipe is disconnected
  • Nuts at the pump base are removed
  • Pump housing is lifted using approved lifting tools
  • Spare fuel pump assembly is fitted in position
  • Free movement of fuel rack is checked
  • Pump is aligned and tightened to the base
  • Fuel rack zero setting and calibration are carried out
  • Governor and VIT connections are reconnected and matched with initial markings

  5. Adjustment of Actual and Effective Stroke

  The timing of a jerk type fuel pump is measured by fuel pump lead.

  (a) Actual Stroke Adjustment

  • Achieved by turning the fuel cam disc
  • A change of 1 mm in fuel pump lead alters the peak cylinder pressure by approximately 3.5 bar

  (b) Effective Stroke Adjustment

  • Achieved by adding or removing shims between the pump housing and top cover
  • Each shim changes peak pressure by approximately 1.75 bar

  6. Common Faults and Their Effect on Engine Performance

  Fault	Effect on Engine Performance
  Worn plunger and barrel	Increased leakage, higher fuel index required for same load, reduced engine power
  Incorrect timing	Uneven peak pressures, inefficient combustion, increased thermal loading
  Defective suction / puncture valve	Low injection pressure, unit misfires, low exhaust gas temperature
  Cavitation damage	Erosion near helix edge, unstable and irregular fuel delivery
  Fuel leakage into camshaft space	Deterioration of lubricating oil quality and pressure
  Plunger seizure	Sudden loss of fuel supply, risk of severe engine damage

  Prompt attention is essential to prevent:

  • Power imbalance between cylinders
  • Excessive thermal and mechanical stress
  • Progressive component damage
  • Possible serious engine failure