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Irregularities in Indicator Diagram

Updated on July 20, 2013

Irregularities in the shape of indicator diagrams are indications of faulty engine operations. If irregularity seems for all units, problem may be common like faults in fuel oil system, intake air, turbocharger, air cooler, exhaust, etc. When the irregularity is confined to a single unit, problem could be of fuel injectors, fuel pump, exhaust valve, etc.

Early Ignition

Indications and Effects

  • Abnormally high peak pressure of the unit is recorded at the top of the piston stroke.
  • Knocking sound comes out of the engine due to heavy loads passed to bearings via running gear.
  • Early ignition causes increased thermal efficiency of the engine. Also exhaust temperature reduces since combustion starts long before it is supposed to. But the shock loads and vibrations results in damage of the engine.


  • Incorrectly adjusted or accidently changed fuel pump timing
  • Damaged or incorrectly set fuel valve or fuel injector
  • Undesired fuel quality
  • Parts inside the cylinder are overheated.

Early Ignition
Early Ignition

Late Ignition

Indications and Effects

  • Low peak pressure is indicated for the unit after top dead center.
  • Combustion continue during expansion stroke, give rise to incomplete combustion of fuel, loss of energy, elevated exhaust gas temperature for the unit, and black smoke at the engine exhaust.
  • Reduced power of the engine due to incomplete combustion of the fuel and energy lost in the exhaust.


  • Faulty fuel injector or injector spring tension tighted beyond setting.
  • Poor fuel quality
  • Wrongly timed or leaking fuel pump
  • Engine parts inside cylinder are under cooled
  • Incorrect atomization
  • Compression pressure low
  • Combustion air supply is low

Late Ignition
Late Ignition

After Burning

Indication and Effects

  • A rise in expansions line is recorded in the later part of piston stroke
  • Since burning of fuel continues, exhaust gas temperature and pressure increases, causing black smoke at engine exhaust.
  • Unburnt carbon deposits fouls exhaust system, cause damage to exhaust valve and seat, turbocharger surges and there are chances of uptake fires in exhaust gas economizer.
  • Elevated temperatures inside cylinders cause breaking up of lubrication and increased wear of liners, piston rings, burning of piston crown, etc.


  • Slow fuel combustion
  • Quality of fuel is less
  • Low temperature of fuel (Means high viscosity)

After Burning
After Burning

Leaky Fuel Injector

Indications and Effects

  • Reduced power in the affected unit, high exhaust temperature and presence of black smoke in exhaust.
  • Possible knocking sounds or pressure waves in fuel injection system.
  • Sudden up and downs in indicated diagram in the fuel injection and expansion side.
  • After burning due to incomplete combustion of fuel.


  • Leaking fuel injector.
  • Chocking of fuel injector spray holes, which leads to improper atomization and dripping of fuel.

Leaky Fuel Injector
Leaky Fuel Injector

Partly Choked Fuel Valve

Indications and Effects

  • Low exhaust gas temperature of the unit
  • Power card and draw card indications
  • Loss of engine power


  • Fuel oil contamination and improper purification
  • Carbon formation at injector tip
  • Carbon deposits on fuel valve due to over heating

Partly Choked Fuel Valve
Partly Choked Fuel Valve

Low Compression

Indications and Effects

  • Low pressure in the indicator card
  • Reduced power of engine


  • Improper combustion
  • Insufficient air for combustion
  • Leakage of air in between piston rings and liner while compression stroke due to worn out liner or piston rings.

Low Compression
Low Compression

Exhaust Valve Opening

Light spring diagram gives indications on faulty exhaust valve and intake port operations.

Early Opening

  • Elevated exhaust gas temperature and fouling of exhaust system
  • Loss of engine power

Late Opening

  • Reduced blow down effect and hence reduced scavenging efficiency
  • Low quality of exhaust gas delivered to the turbocharger inversely affect its operation

Exhaust Valve Opening
Exhaust Valve Opening

Choked Exhaust

Indications and Effects

  • Power loss in the unit
  • Increased exhaust temperature
  • Turbocharger surging
  • adversely affect scavenging efficiency


  • Improper combustion
  • Increased cylinder lube oil

Choked Exhaust
Choked Exhaust


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    Post Comment
    • profile image


      4 years ago

      I want to say Honda XR250, as the cam cover and rocker caps, cltcuh arm and cltcuh casing would match, but not sure about the shaped trim cover over the oil filter.

    • Haseena Firdousia profile image


      6 years ago from India

      Greta hub. Voted up.

    • profile image


      6 years ago

      first time reading about indicator diagrams. good


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