Intake
The piston moves down and the cylinder fills with air. Restricted airflow, incorrect valve timing or poor cylinder sealing reduces the oxygen available for combustion.
Troubleshooting libraryDiesel engine
Fundamentals that make scan data useful
Diesel engine
theory & operation
You diagnose faster when you understand what the controller is trying to make the engine do. Start with air, compression, heat, fuel and timing.
Compression ignition
The four-stroke cycle
Compression
The piston compresses the air until temperature rises enough to ignite injected fuel. Low cranking speed or low compression hurts cold starting first.
Power
Fuel is injected near the top of the compression stroke. Atomization, timing, pressure, air motion and cylinder temperature decide how completely it burns.
Exhaust
The piston pushes spent gas out. Exhaust restriction, valve problems and turbocharger control faults can affect the next intake cycle.
System relationships
What the engine needs
One failed input can change fuel, airflow, turbo control and aftertreatment behavior at the same time.
Common rail
A high-pressure pump fills a shared rail. The ECM controls pressure and injector timing separately. Compare desired versus actual pressure, then determine whether a deficit comes from supply, control, leakage or pump output.
HEUI
High-pressure engine oil supplies the force that actuates the injectors. Oil level, viscosity, base-oil supply, injection-control pressure and electrical injector control all belong in the same no-start diagnosis.
Turbo & air path
The turbo recovers exhaust energy to move more air. Intake restriction, charge-air leaks, exhaust leaks before the turbine, vane/wastegate control and biased pressure sensors can all create low-power complaints.
Combustion clues
Black smoke usually points toward too much fuel for the available air. White can be unburned fuel, coolant or vapor. Blue often suggests oil—but temperature, aftertreatment and operating condition matter.