C7 and C9 Industrial Engines Troubleshooting – Oil Contains Fuel

Always verify fuel dilution. Submit an oil sample to a Caterpillar regional SOS laboratory for analysis.

Note: For additional information, refer to Special Instructions, REHS3007, “Determining the Cause of Fuel Dilution of Engine Oil”.

Probable Causes


  • Leaking seals 
  • Fuel injector tip 
  • Fuel transfer pump seal 
  • Cracked cylinder head 

Recommended Actions

Note: The procedures have been listed in order of probability. Complete the procedures in order.

Table 1
Troubleshooting Test Steps Values Results
1. Leaking Seals

A. Remove the injector from the affected cylinder and visually inspect the injector O-ring seals for damage.

If leakage is found and the O-rings are in good repair, there may be a problem with the injector. Replace the injector. Ensure that the new trim files are installed. Refer to Troubleshooting, “Injector Trim File – Install” for details. Use Caterpillar Electronic Technician (ET) to perform a “Fuel System Verification Test”. Refer to Troubleshooting, “Injector Solenoid – Test” for details.

If the leakage is not located, add the appropriate dye to the fuel tanks. Run the engine at high idle for several minutes. Shut down the engine and remove the valve covers. Use a black light to check for traces of dye around each injector.

O-ring seal Result: The injector O-ring seal is damaged.

Repair: Replace the damaged seals. Install the injector onto the engine and confirm that the repair has resolved the problem.

Result: The injector O-ring seal is not damaged.

Proceed to Test Step 2.

2. Fuel Injector Tip

A. Use Cat ET to perform the “Cylinder Cutout Test” in order to identify any cylinders that may be overfueled.

B. A fuel injector tip that is broken or fractured is a possible cause of a cylinder that is being overfueled. If a cylinder is identified by the test, Remove the injector and check the injector tip for damage. Check the fuel injector tip for cracks or breakage.

Injector tip Result: The fuel injector tip is damaged.

Repair: Replace the suspect injector.

Result: The fuel injector tip is not damaged.

Proceed to Test Step 3.

3. Fuel Transfer Pump Seal

A. Check for fuel leakage around the shaft seal for the fuel transfer pump. Ensure that the weep hole is not plugged.

Pump seal Result: There is fuel leakage around the shaft seal.

Repair: Replace the fuel transfer pump.

Result There is not fuel leakage around the shaft seal.

Proceed to Test Step 4.

4. Cracked Cylinder Head

A. Look for signs of damage to the internal passages for the fuel supply to the injectors in the cylinder head. Check for pin hole leaks. Refer to Disassembly and Assembly.

Cracked head Result The internal passages for the fuel supply to the injectors are damaged.

Repair: Repair or replace the cylinder head.


Posted in C7

Leave a Reply

Your email address will not be published. Required fields are marked *