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The exhaust in a vehicle is often a good determinator of how healthy your vehicle is.  It can show several symptoms of poor health in terms of colour and odour. So what does the colour of smoke from the exhaust mean?

Blue smoke:

The engine is burning oil as the valve guide seals or pistol rings are worn out and oil is leaking away from where it should lubricate the moving parts. Instead the oil is leaking into the combustion chamber where it ends up being burned up with the fuel. This causes rough starts and can destroy the spark plugs.  Look out for consumption issues and check the oil regularly.  If the vehicle is old and the leak is quite small it’s worth just keeping an eye on but if the vehicle is newer and/or the leak is quite major it’s worth talking to your mechanic.

If the car is turbocharged blue smoke is a sign that the blower is in need of replacing or rebuilding.

Exhaust smoke

Black smoke:

Black smoke means that the engine is burning too much fuel and it’s a result of a rich fuel condition. The engine may misfire and not perform well. There can be several reasons as to why this is happening:

  • Carburetor choke may be stuck closed and the choke may need repairing or replacing
  • Clogged fuel return line
  • Leaking or dripping fuel injectors and may need replacing
  • Stuck closed fuel pressure regulator
  • The air filter may be dirty and need replacing
  • There could be other ignition problems so the distributor cap and rotor ignition module should be checked

All the above elements should be checked.  It’s usually easy to diagnose and fix these issues, the main issue is that burning fuel unnecessarily burns a hole in your pocket too.

White smoke:

Quite often thin vapour like white smoke is nothing to be concerned about, it’s often just condensation due to a build up inside the exhaust system.  If it is a cold day and the exhaust emits white smoke but doesn’t disappear when the car is warmed up and/or is thicker smoke then there could be a problem. It often means that coolant has got into the combustion chamber and is burning up.

Possible causes include:

  • Transmission fluid may be getting into the intake manifold via the vacuum modulator. Fix this: replace the vacuum modulator.
  • The cylinder head gasket(s) may be broken and allows the coolant to leak.  Fix this: replace head gasket(s).
  • The cylinder heads could be warped or cracked allowing coolant to enter the combustion chamber.  Fix this: the heads may need to be resurfaced or replaced – this is a professional job.
  • The engine block could be cracked, again letting coolant into the combustion chamber.  Fix this: replace the engine block.

All of these are quite costly repairs and should be looked at by a professional mechanic urgently.

Grey smoke:

This is a bit harder to diagnose. There could be several reasons for grey smoke to appear:

  • It could mean that the car is burning oil or has a bad turbocharger.  Treat it like you would blue smoke and check if there is excessive oil consumption.
  • Transmission fluid could be getting sucked into the engine and getting burned up because of a faulty transmission vacuum modulator.

A PCV valve could be stuck; building pressure and leading to oil leaks. The Positive Crankcase Ventilation (PCV) system reduces harmful emissions by sending them back into the combustion chamber. Stuck valves would stop this process happening correctly.

Blue/grey smoke:

Sometimes smoke can be a combination of blue/grey and indicates that oil has leaked past the valve seals and into the combustion chamber where it is burning.  The seals enable the intake and exhaust valves to move up and down as they provide oil for lubrication.  There should be just enough oil to lubricate the valve stems without any seeping into the combustion chamber.  However, the valve seats can get damaged over time and sludge can build on  top of the cylinder heads. The oil gets burned off in the engine and this causes the blue/grey exhaust smoke when starting up the vehicle.

The causes for blue/grey smoke are:

  • PCV valve seals are worn and need placing.
  • Too much clearance between the valve stem and the valve guide enables oil to leak past the gap into the cylinder.
  • Worn or damaged piston rings can cause blow-by which creates blue/grey smoke. You will need to replace the piston rings.
  • Worn cylinder walls can also cause blow-by.
  • The engine may have mechanical issues and so compression should be checked.

Other indicators of problems can be a drop in gas mileage or astrong smell coming from the exhaust, a bit like rotten eggs.

There could be several reasons for this:

  • The carburettor choke is stuck closed.
  • The engine could have mechanical problems and the compression may need checking.
  • The ignition timing may be set wrong and require adjusting or there could be other ignition problems so you should check and replace (if necessary) the distributor cap, rotor, ignition wires and spark plugs.
  • The engine could be running too hot as the cooling system may need repairing.
  • The fuel injectors are partially stuck open and need replacing.
  • An emission control device may not be correctly functioning.
  • There may be a fault in the computerised engine control system which will require scanning and testing by a professional.
  • The fuel pressure regular may be operating at pressure level that is too high and the fuel pressure regulator may need replacing.  Check it first with a fuel pressure gauge.

If you really aren't sure about what is going on then ask your mechanic.