DIY diesel problem diagnosis

You can make a rough-and-ready diagnosis of many diesel problems by simply ‘reading’ the exhaust smoke (this is generalised advice for all types of diesel engines).

Black smoke means there's too much diesel and not enough oxygen, so the carbon in the fuel doesn’t have enough oxygen to combine with. This means that part of the diesel fuel remains unburnt and turns into black soot. This black soot is what makes the exhaust look black.

This problem may be caused by either:

  • A dirty air cleaner
  • Faulty injector pump
  • Faulty injectors
  • Faulty turbocharger / intercooler / hoses
  • Clogged and/or sticking cylinder head valves
  • Clogged or faulty EGR valve.

White smoke means that the diesel fuel is not burning correctly due to a lack of heat in the combustion chamber. This unburnt diesel contains minor toxins that may sting your eyes. This problem may be caused by:

  • A clogged fuel filter
  • Incorrect injector pump timing
  • Worn engine (low compression)
  • Water or petrol contamination of the diesel fuel.

Blue smoke is caused by excess lubricating oil within the engine cylinders during combustion. This excess oil then gets burnt and emitted as blue smoke. This problem is usually caused by either:

  • A worn-out engine. A worn-out engine is likely to be coated in muck. It's also likely to be dribbling oil and be very rattly. Sometimes sellers carefully clean the engine to disguise the signs of old age.
  • Worn valve stem seals. This is more of a problem with older, non-turbocharged engines. Regardless of engine type, worn valve stem seals tend to cause smoke at start-up and at idle. Constant blue smoke on a turbocharged diesel is unlikely to be caused by worn valve stem seals. Instead, the problem is more likely to be:
  • Leaking turbocharger seals, which are allowing oil into the intake and/or exhaust system
  • Excess engine oil in the sump, which is getting splashed up into the bores.
  • The wrong grade of oil (which is too thin for this type of engine). If the oil is too thin, some of it may sneak past the oil control rings and be burnt as part of the combustion. If the problem is oil that’s too thin, there will probably be blue smoke at all revs, but especially after a period of idling and also when the engine is at high revs or under heavy load.
  • Diesel fuel contaminating the oil. This may be caused by faulty injector pump and/or lift pump (which allows engine lubricating oil to mix with the diesel).

This problem may also be caused by faulty injectors (they're squirting in too much fuel, and the excess is dribbling down the bores into the sump).

It's also worth noting that many modern diesels suffer from a poorly designed Diesel Particulate Filter system, which deliberately over-fuels the engine for brief periods in order to create enough heat to burn carbon off the Diesel Particulate Filter. If this over-fuelling isn’t kept to an absolute minimum, it can wreak havoc with the engine.

Again, if the oil has been thinned by contamination with diesel, some of it may sneak past the oil control rings and be burnt as part of the combustion. In this case, there will probably be blue smoke at all revs, but especially after a period of idling and also when the engine is at high revs or under heavy load.

Excess lubricating oil entering the combustion chamber due to overheating. See our explanation of EGR problems here: http://www.dogandlemon.com/articles/diesel-egr-problems.

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