Diesel knowledge

Start with the symptom. Test before replacing.

A structured diesel diagnosis should identify the system causing the fault before parts are replaced. This guide is educational and does not replace vehicle-specific testing.

Hard start / no start

  • Fuel supply and restriction
  • Low rail / injection pressure
  • Air entering the fuel system
  • Injector return / leak-off
  • Compression or mechanical condition

Lack of power

  • Fuel delivery under load
  • Rail-pressure control
  • Restricted fuel supply
  • Injector balance / delivery
  • Air and boost system condition

Rough idle / diesel knock

  • Injector correction and balance
  • Spray quality and nozzle condition
  • Injection timing / control
  • Fuel quality or contamination
  • Cylinder mechanical condition

Smoke

  • Air-to-fuel imbalance
  • Injector over-fuelling or poor atomisation
  • Air restriction / boost fault
  • Fuel quality
  • Engine mechanical condition
Workshop principle

Fault codes are a starting point, not a parts list.

Electronic fault information can narrow the search, but pressure, delivery, return flow, electrical control and mechanical condition may still need to be measured before condemning an injector or pump.

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