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Why Does My Car Burn Oil?

If you haven’t owned an old car (or you haven’t checked your oil lately), it might surprise you to find out that most cars burn oil. On top of this, checking the oil level and topping it up is one of the few things that you as the owner really need to do in order to make sure you can keep your car in good shape (and to keep your warranty).

But why do cars burn oil?

Well usually, motors burn oil by small amounts of engine oil getting past the pistons or valve stem seals and into the combustion chamber, though more often it’s engine oil passing the piston.

"Doesn’t the piston need to seal perfectly in the cylinder?" Someone who mildly understands engines asks.

Well yes. In an ideal world the pistons would seal perfectly in the cylinder creating a super powerful and efficient combustion process, and there would be no way for oil to ever enter the combustion chamber. However, as engines are made out of metal (which expands and contracts with heat), the pistons actually have to be slightly loose in the cylinder.

Now, the scientific amongst you might think, “If they’re loose in the cylinder, they can’t seal at all and all the combustion gasses can pass the piston”.

The geniuses that invented and perfected these motors over time also invented a compromise just to deal with this issue. By using small metal rings with a gap cut into them (allowing for the expansion), the piston can effectively create a seal for combustion.

As oil is used to lubricate and help seal the rings and piston in the cylinder (aka. compromise), there is usually the smallest amount of oil that passes the piston with every rotation, which can account for the oil burning phenomenon.

These rings and the cylinders themselves can also wear down with age, allowing more and more oil to bypass the rings entirely which increases the burning of oil and can even cause blue smoke from the exhaust.

No matter what though, this can usually be mitigated by just checking your oil level more regularly.


If you have any questions about this blog post, would like a second opinion from a mechanic or would like to find an honest mechanic in Brisbane, check out Kashy here.

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