Smoke Coming Out of Oil Cap (Causes & Fixes)

Hasan Amjad
Smoke Coming Out of Oil Cap

The sight of smoke from your vehicle’s oil cap can be quite alarming. Mostly, it is a sign of some underlying issue

In some cases, the white smoke is normal as it is moisture evaporating. However, persistent dense smoke from the oil cap and other engine symptoms suggest that something is wrong with engine components; either the piston rings are worn out, or fuel/oil is bypassing through the bad valve stem seals.

Further, this article will delve into the primary reasons why the smoke comes from the oil and what you can do to fix the issue.

Before we move on to the reasons for the problem, it is necessary to distinguish between the normal and abnormal smoke coming from the oil cap.

Cracked Expansion Tank

Everyday Smoke vs. Abnormal Smoke

Sometimes, a small amount of vapor or “smoke” coming from the oil cap may not be a cause for concern. This normal “smoke” is usually a mixture of water vapor and trace amounts of oil mist, a byproduct of the combustion process.

When an engine is cold, water vapor can condense within the crankcase and mix with the oil. As the engine warms up, the moisture evaporates, and you may notice a small amount of vapor coming out of the oil cap.

This is considered normal, especially in colder climates or during winter months. These white vapor fumes should disappear as the engine reaches its average operating temperature.

To determine if the smoke coming from the oil cap is normal or indicative of a problem, consider the following factors:

1. Color and consistency

Water vapors are generally white or light gray and have a thin consistency. Abnormal smoke is typically bluish, white, or gray and may have a thicker, oily consistency.

2. Engine temperature

If the smoke is present only when the engine is cold and disappears as it warms up, it may be normal condensation. However, if the smoke persists even after the engine reaches its normal operating temperature, it indicates an underlying issue.

3. Other symptoms

Suppose the smoke is accompanied by other symptoms such as poor engine performance, excessive oil consumption, loss of power, or engine overheating. In that case, it is more likely a serious issue that needs immediate attention.

Why Smoke Rises from Oil Cap? [3 Main Reasons and Fixes]

Here are the three major reasons why the oil cap is leaking out smoke;

1. Worn Piston Rings

All engines have piston rings that seal the inner combustion chamber and separate it from the crankcase. They prevent the lubrication oil from leaking into the chamber. Furthermore, they help in the smooth movement of the piston inside the cylinder during a power stroke.

When the piston rings wear out, they fail to seal adequately, allowing combustion gases and oil to mix. When this oil burns along with fuel gases, the smoke escapes through the engine oil cap.

Fix: Replace Worn Piston Rings

Piston rings reside inside the engine cylinder, and it’s not feasible for you to remove or replace them yourself. So, it’s best to get your car to a garage. They will inspect, disassemble the engine and replace the piston rings.

car piston rings

2.  Fuel Bypassing Worn-Out Valve Stem Seals

Valve stem seals also play a crucial role in preventing oil from entering the combustion chamber while allowing the intake and exhaust valves to operate smoothly.

However, oil/fuel can bypass the seals and mix with the combustion gases when they wear out over time. This can cause excessive pressure in the crankcase and cause smoke to erupt from the oil cap.

Fix Replace Worn-Out Valve Stem Seals

This fix also requires professional attention. However, you may follow the procedure and a relevant video tutorial if you want to do it yourself.

  1. Consult your vehicle’s service manual for specific instructions on replacing valve stem seals.
  2. Remove the necessary components to access the valve stem seals, such as the valve cover, rocker arms, and camshaft(s).
  3. Using a valve spring compressor, compress the valve springs to expose the valve keepers (valve locks or collets). Carefully remove the valve keepers and release the valve spring compressor to remove the valve springs.
  4. Use needle-nose pliers or a valve seal removal tool to remove the old valve stem seals.
  5. Clean the valve stems and the area around the valve guides.
  6. Install the new valve stem seals, ensuring they are properly seated on the valve guides.
  7. Reassemble all the components you previously removed.

3. Clogged PCV System

Some cars have a Positive Crankcase Ventilation (PCV) system, which releases excess pressure in the crankcase by allowing combustion gases to be recirculated back into the intake manifold.

A clogged or malfunctioning PCV system can cause excessive pressure in the crankcase, leading to smoke from the oil cap.

Fix: Clean or Replace PCV System Components

  1. Locate the PCV valve. It is usually found on the valve cover or the intake manifold.
  2. Remove the PCV valve by disconnecting any hoses and unscrewing it from its position.
  3. Inspect the valve for signs of clogging or damage. If it appears dirty, clean it using a carburetor cleaner and a brush. If it seems to be damaged, replace it with a new one.
  4. Inspect the PCV hoses for cracks, leaks, or blockages. Replace any damaged hoses.
  5. Reinstall the PCV valve and reconnect any hoses.

Start the vehicle and observe for any signs of smoke from the oil cap. If the problem persists, get the engine checked by a professional.

Frequently Asked Questions

Why is smoke coming out of my engine oil?

The oil level has reduced if your car’s oil change is due. Then, this used oil starts to burn, producing smoke. It may also burn if leaking out through worn-out piston rings, valves, or seals.

How much does it cost to replace piston rings?

The replacement depends on your vehicle’s make and model and labor costs in your area. Generally, replacing piston rings can range from $1,000 to $3,000 or more, including parts and labor.

Why is smoke arising from the top of my engine?

Smoke arising from the top of your engine could be due to a leaking valve cover gasket, allowing oil to seep onto hot engine components and causing the oil to burn and produce smoke. Other potential causes include an oil cap leak, a faulty PCV system, or a more severe internal engine issue.

Final Words

The emergence of smoke from an engine is a significant concern that requires immediate attention to prevent extensive damage and potential engine failure.

This article aimed to provide insight into the possible causes of this issue. Though we have outlined the relevant problems and solutions, it is highly recommended to seek professional assistance, as these repairs tend to be major. We recommend you use our ask a Mechanic feature to chat with a master mechanic today.

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