Why does my car emit blue smoke?

Thursday September 19 2019


By Paul Kaganzi

Hello Paul, my car briefly emits blue smoke during cold starts. Otherwise it runs well and doesn’t smoke as I drive. What could be the problem? Mukasa

Hello Mukasa, blue smoke from your exhaust, however brief, is associated with engine oil burning in the combustion chamber where there should, ordinarily, only be the fuel air mixture.

This means there is a small amount of oil leaking into the combustion chamber, where it shouldn’t be.

The only possible intrusion points are two: worn out spark plug seals or worn out valve stem seals. Spark plug seals can be affected by age, usually around 100,000 kms.

Look out for mileage tampering which may give you a wrong impression of your car age. When they spark plug seals wear out they leak oil to the spark plugs which burn the oil during ignition of fuel air mixture.

An inspection of the spark plugs will confirm this problem if they have engine oil on them. Spark plug seals are usually replaced together with the valve cover or top cover seal.
Use of low grade oils blended without additives that protect engine oil seals can accelerate seal failure.


The engine valve seals are often damaged by excess sludge or deposit build up in your engine from accumulation of after burn combustion gases and dirt. This puts pressure on the valve seals which work hard to prevent oil intrusion during the opening and closing cycles of the intake valves.

Again use of engine oils blended with active cleaning detergent molecules can help prevent this sort of damage.

Before you replace the valve seals try using a shorter mileage mono grade engine oil designed to reduce sludge build up and engine oil leakage in older engines such as Shell Helix HX3. Also monitor how much oil you need to top up over the oil drain period. If its less than 0.5 of a litre over 3,000kms you may live with it until you decide to repair or part ways.