This procedure addresses a common misunderstanding: the 6.7L Cummins diesel engine does not use ignition coils or spark plugs. Diesel engines use compression ignition, not spark ignition. If experiencing misfires or rough running, diagnose fuel injectors, glow plugs, or compression issues instead.

Warnings

Tools required

Preparation

1. Verify the concern with the customer or through diagnostic trouble codes
2. Confirm the vehicle has the 6.7L Cummins diesel engine (not a gasoline engine)
3. Allow engine to cool to safe working temperature
4. Park vehicle on level surface and engage parking brake
5. Disconnect negative battery cable if performing electrical diagnostics

Procedure

1. 1

   Verify Engine Type
   Confirm that the vehicle is equipped with the 6.7L Cummins turbo diesel engine by checking the engine code on the emissions label under the hood or verifying VIN specifications. This engine has six glow plugs (one per cylinder) but no spark plugs or ignition coils.
2. 2

   Scan for Diagnostic Trouble Codes
   Connect an OBD-II scanner and retrieve all stored and pending diagnostic trouble codes. Common codes for diesel performance issues include P0201-P0206 (injector circuit faults), P0380-P0396 (glow plug system faults), P0087 (fuel rail pressure too low), or P0088 (fuel rail pressure too high).
3. 3

   Inspect Glow Plug System
   If experiencing cold-start issues, locate the six glow plugs on the cylinder head. The glow plugs are threaded into the cylinder head and have electrical connectors. Check electrical continuity and resistance of each glow plug using a multimeter. Typical resistance should be 0.5-2.0 ohms per glow plug. Replace any glow plugs that are out of specification.
   Torque spec

   Sensor Bolts15 Nm (11 lb-ft)
4. 4

   Test Fuel Injector Operation
   If experiencing rough running, power loss, or black smoke, diagnose fuel injectors. Use a scanner with bidirectional controls to perform an injector contribution test. Each injector should cause a measurable RPM change when disabled. Listen for unusual ticking or knocking from specific cylinders that may indicate faulty injectors.
5. 5

   Check Fuel System Pressure
   Connect a high-pressure fuel gauge to the fuel rail test port. Key on engine off (KOEO) pressure should be approximately 5-10 psi from the lift pump. At idle, fuel rail pressure should be 5,000-6,000 psi, and under load can reach 23,000-26,000 psi. Low pressure indicates lift pump or high-pressure pump failure.
6. 6

   Inspect Air Intake and Turbocharger System
   Check for air leaks in the intake system between the air filter and turbocharger inlet. Inspect all intercooler boots and clamps for cracks or looseness. Verify turbocharger operation by checking for excessive shaft play or oil leakage. A boost leak or turbo failure can cause symptoms similar to ignition problems in gasoline engines.
7. 7

   Test Engine Compression
   If multiple cylinder misfires are present or engine lacks power, perform a compression test. Remove all glow plugs and install a diesel compression tester in each cylinder. Crank engine and record compression for each cylinder. Compression should be 350-450 psi and within 10% between cylinders. Low compression indicates valve, piston, or head gasket issues.

Reassembly

1. Reconnect negative battery cable if disconnected
2. Clear diagnostic trouble codes after repairs are completed
3. If glow plugs were removed, ensure proper torque is applied to prevent cylinder head damage
4. Verify all electrical connectors are properly seated

Verification

• Start engine and verify normal cold-start operation (glow plug wait light functions properly)
• Allow engine to reach operating temperature and verify smooth idle with no unusual smoke
• Perform a test drive to confirm normal acceleration, power delivery, and absence of misfires
• Re-scan for diagnostic trouble codes to ensure no new faults are present
• Monitor fuel rail pressure with scanner during test drive to verify normal operation

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