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Cummins/Onan — Engine Compression Pressure Check

For these Cummins/Onan models: 3500MKII, SOOMKII 1.5.

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Before you start

  • Do not check compression by cranking engine with starter
  • Ensure compression tester gauge has been properly tested and calibrated before using

Procedure

  1. Start the engine and warm up to minimum temperature of 160°F (344 K)
  2. Stop the engine and remove drip manifold from the nozzle holder assemblies
  3. Connect a hose to the fuel return line from the pump and place the open end in a container to receive overflow fuel
  4. Start with the number 1 cylinder when checking the compression
  5. Remove the fuel injection nozzle and install the compression tester adaptor in the same manner as the fuel injection nozzle was installed
  6. Install the compression tester hose and gauge assembly
  7. Start the engine and run at approximately 600 rpm, taking several readings on gauge
  8. Remove the tester assembly and install nozzle holder
  9. Connect fuel injection line and fuel return line
  10. Repeat the operation on each remaining cylinder
  11. Install the drip manifold

Tools required

  • Compression tester gauge
  • Compression tester adaptor
  • Fuel injection nozzle holder

Specs & torque values

  • Minimum engine temperature: 160°F (344 K)
  • Engine speed: approximately 600 rpm

Source: manual:ALLIS-CHALMERS 3500 MK2 Engine Operating & Maintenance manual (05-1975).pdf. Figures and steps are as documented; always cross-check against the official manual and any later revisions for your exact serial number before servicing.