Have you ever witnessed some white fumes when you open the oil cap? It is normal if you notice a small amount of smoke from the oil cap. Water vapors trying to escape from the engine are the only thing causing these smokes. Sometimes, you might just feel a rush of air coming from the cap when you open it.

However, excessive smoke could signify exhaust leakage caused by worn-out piston rings and valve stems.

Therefore, it is crucial to ensure that the smoke or fumes from the oil cap are caused by any other means or are water vapors. The following test should be taken in that regard.

How to Test Smoke Coming From an Oil Cap?

To determine the cause of fumes coming from the oil cap, follow these steps: 

  1. Start the engine and let it warm up at idle for a few minutes.
  2. Carefully remove the oil cap. Don’t open the cap when engine is hot.

After you open the oil cap, you should now observe a few things:

  1. When you open it, do you feel any pressure, or do the fumes release some pressure?
  2. Do the fumes occur in massive quantities?
  3. Do the fumes smell like burning engine oil/burning fuel? 

If any of these conditions are true, likely, the fumes being released aren’t just water vapors. Let’s determine what is causing the smoke coming from the oil cap.

Smoke Coming From Oil Cap Causes

There could be several causes for the smoke coming from the oil cap. Let’s start by talking about the issue that is probably to blame for the smoke coming from the oil cap. 

Smoke from the oil cap is most likely caused by exhaust gas leaking into the oil cylinder or the oil entering the combustion chambers. This problem arises from worn-out piston rings or valve springs. Let us discuss in detail:

Oil Entering the Combustion Chamber

The piston ring transmits heat from the piston to the cylinder block. A worn-out piston ring or valve stem might allow engine oil to pass through and into the hot combustion chamber.

This oil will burn in the combustion chambers with the gasoline-air mixture, emitting white fumes into the oil tank.

Exhaust Gasses Leaking in the Oil Cylinder

In the combustion chamber, the gasoline and air mixture is burned. The worn-out valve stem or piston ring can cause the exhaust to bypass the sealed area and enter the crankcase. 

Smoke comes from the oil cap as a direct consequence of the piston pushing the exhaust gasses into the oil tank.

Other potential causes for engine oil to bypass the valve stems or piston rings include:

  1. When the crankcase is overfilled, there is too much pressure on it, which causes piston rings or valve stem to push engine oil into the combustion chamber.
  2. When the engine is heated, low-viscosity oil becomes even thinner. Oil may enter the combustion chamber. As a result, resulting in smoke.

How Do I Stop My Oil Cap From Smoking?

Here are a few things you can do to stop the oil cap from emitting white smoke:

1. Replace the worn-out valve stems and piston rings

If your piston ring and valve stem are worn out, the only fix is to replace them. The cost of the parts will be between $75 and $100, whereas the cost of the labor will be between $1,800 and $3,500.

The replacement of the piston rings will take longer. While replacing a valve stem only takes a few hours, replacing a piston ring will take about 16 hours.

2. Use engine oil suitable for your car

Use oil with a high viscosity grade if you live in a hot area. This is because low-viscosity oil can enter the combustion chamber when heated and becomes too thin. 

Use high-quality oils because they have anti-burn additives that prevent them from burning at high temperatures.

FAQ’s on Smoke From Oil Cap

Like gasoline-powered vehicles, diesel engines produce smoke from the oil cap for the same reasons. Compared to petrol engines, the diesel engine's combustion process is more powerful, and the air is compressed first, then fuel is injected.

The fumes of the burned fuel-air mixture can enter the oil tank if the piston rings or valve stems are worn out, resulting in smoke from the oil cap.


In conclusion, some smoke emanating from the oil cap is completely normal. However, if a lot of smoke comes from the engine and smells like burning oil, there may be an oil leak in the combustion chamber. 

Worn-out piston rings or valve stems could cause the combustion gasses that result in smoke in the oil tank. Make sure to replace the valve stem or the piston rings in that case. Make an appointment with the mechanic to have the engine thoroughly checked for the source of smoke from the oil cap.

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