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Most diesel engine problems are related to the injection system. As such diagnosing diesel engine problems requires knowledge of engine and injection system operation.

This brochure provides you with the guideline to troubleshoot various diesel engine and injection system problems.


Diesel engine problems that are most frequently encountered may be identified as follows

1. Excessive Exhaust Smoke

2. Engine Knocking

3. Engine Missing

4. Hard Starting

5. Lack of Power

6. Poor Fuel Efficiency

7. Fuel Leaks

8. Clogged Fuel Filters

These potential problems are briefly discussed below.


Excessive diesel smoke is due to incomplete combustion, normally caused by faulty injection system or other engine troubles. A small amount of exhaust smoke is normal during initial start-up or rapid acceleration.

Type of Smoke

Abnormal Exhaust smoke may be black, white or blue. Each type of smoke indicates engine problems and these are discussed below:

Black Smoke

Excessive black smoke is caused by a rich air-fuel mixture. This may result form problems with the injection pump or infection timing, which may in turn be clue to a choked air cleaner, worn fuel injectors, adulterated diesel fuel or the engine itself.

White Smoke

White smoke occurs mainly during cold starts, when the fuel tends to condense into liquid and does not burn due to cold engine parts. The most common reason for white smoke are in-operative glow plugs low engine compression, a bad injector spray pattern, late injection timing or injection pump problems.

Blue Smoke

Excessive blue smoke indicates problems from low engine compression and/or worn piston rings, scored cylinder walls or leaking valve stem seals The blue smoke is caused by crankcase oil entering the combustion chamber and being emitted after partial combustion through the exhaust


All diesel engines produce a "knocking" sound when running. In a diesel engine fuel ignites when infected into the combustion chamber. This rapid combustion produces very high pressures generating a rumble or dull clattering sound Abnormally loud "knocking" may be due to diesel engine miss.


A diesel engine miss results from one or more cylinders not burning fuel properly. This is caused by injection system problems which include:

  •  Faulty injectors
  •  Clogged fuel filters 
  •  Incorrect injection timing 
  •  Low engine compression 
  •  Injection system leaks •
  •  Air leaks 
  •  Faulty injection pump

4.         HARD STARTING

If diesel engine does not start it may be due to:

  •  In-operative glow plugs 
  •  Restricted air or fuel flow 
  •  Bad fuel flow solenoid 
  •  Contaminated fuel 
  •  Injection pump problem 
  •  Low battery power

A slow cranking speed is a common cause for starting problem. Being a compression ignition engine, a diesel engine must crank fast enough to produce sufficient heat for combustion

5.         LACK OF POWER

·Lack of engine power may be caused by 

  • Slack throttle cables

  • Incorrect governor settings 

  •  Clogged fuel filters 

  •  Dirty air fillers 

  •  Low engine compression 

  •  Other factors affecting combustion


Poor fuel economy may be due to:

  •  Fuel leak ·

  •  Dirty air filter

  •  Corrected injection timing 

  •  Leaking injectors

7.         FUEL LEAKS

Leaking fuel lines or loose connections can adversely affect the performance of a diesel engine Pinpointing exact locations of fuel leak become much more easier when the engine is on Since fuel is injected at high pressure extra care must be taken as the leaking fuel can cause serious injury

If signs of fuel leakage are detected use a piece of cardboard to find the leak, move the cardboard around each fitting If there is a serious leak, it will strike the cardboard and not your hand, thereby avoiding serious injury to your hand


Other than the main filter installed in the fuel line for draining water, diesels have sock filters fitted in the fuel tank and some times in the injector assembly as shown below For optimum performance these filters must be kept clean.


Diesel Compression Test

Diesel engine compression test is similar to compression test for petrol engine. In diesels compression pressures are in the range of 3,000/ 4,500 psi Readings in each cylinder should be in the range of 50 to 75 psi of each other

Cylinder-Not-Firing Test

The resistance of each glow plug increases as the cylinders fire A no change in the resistance of any particular cylinder's flow plug will show that the cylinder in NOT firing. Pyrometer, a temperature sensing device can be used to detect temperatures at exhaust of each cylinder to confirm the NOT firing cylinder.

Injection Pressure Test

An injection pressure test uses special valves and high pressure gauge to test the following

  • Injector opening pressure

  • Injector nozzle leakage

  • Injection line pressure balance 

  • Injection pump condition

Due to the versatility of this tester it helps in quickly locating bad nozzle, clogged injector filter or faulting pump.


Most diesel engine problems are associated with the injection system Major injector system components have been discussed in driver energy tips No. 4 titled "Understanding and Maintaining Diesel Vehicles" In the following section information is provided on the proper functional characteristics of the diesel injector

Injector Opening Pressure

Typical diesel injector opening pressure is approximately 1,700 to 2,000 psi (pound per square inch) of opening pressure is not within service manual specifications, rebuild or replace the injector.

Injector Spray Pattern

Some diesel injectors make chattering sound during operation while others do not. However, all nozzles should make a swishing or pinging sound when spraying fuel.

As shown above there should be a narrow, cone shaped mist of fluid. A solid stream of fuel, uneven spray, excessively wide spray or spray filled with solid droplets indicates that the injector needs service and or replacement


  1. 1. Wear safety glasses when working on a diesel injection system. 

  2.  When in doubt, refer to a service manual for the make of vehicle being serviced. The slightest mistake could upset engine performance or cause engine damage. 

  3.  Always cap lines or plug fuel end fittings to prevent entry of foreign matter. 

  4.  Never drop a diesel injector or injection pump. They can be damaged. 

  5.  Remember that high pressure inside a diesel injection system can cause serious injury. 

  6.  Some diesel injection systems must be bled (air removed) after repairs. 

  7.  Clean around fittings before they are disconnected. 

  8.  Adhere to all torque specifications. This is extremely all the more important on a diesel engine. 

  9. . Never use a bent, frayed, or kinked injection 

  10.  line. Place a screen mesh over the air inlet when the engine is to be operated without the air cleaner. Rags and other objects can be sucked into the engine. Also, do not cover the air inlet with your hand with the engine running or injury may result. 

  11. Check fuel filters and water separators periodically. Water can cause extensive corrosion of injection system parts.

This is one in a series of ENERCON Driver Energy Efficiency Tips for car owners and auto mechanics. This series presents basic information on techniques for increasing fuel efficiency in vehicles. For further information contact, Fuel Efficiency In Road Transport Sector Project, ENERCON Building, G-5/2, Islamabad.

ENERCON Building, G-5/2, Islamabad, Pakistan 
Tel: + (92-51) 9206005, 9207572 
Fax: + (92-51) 9206003
 E- Mail:

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