Smoke from Oil Cap – Causes, Reason & How To Fix!

A nightmare scenario for many car owners is starting their vehicle one day and seeing smoke fuming out of the oil cap. It’s usually an indicator that either the valve stem seals or the pistons have strained out.

On the other hand, it’s also entirely possible that it’s just the regular amount of smoke coming out. Car owners need to know why the smoke from oil cap is coming out. Learning about the indicators will be vital for identifying the right solution quickly.

Our article will help you diagnose the smoke from the oil cap and elaborate on the reason behind this phenomenon. Furthermore, we will help you explore the various solutions to help you find the right one for your situation.

How to Test for Smoke from Oil Cap?

How to Test for Smoke from Oil Cap

Although many believe that identifying the smoke in oil caps is harmful or not is a complex task. However, it’s an extremely simple process that can be completed by following a checklist.

It’s best to make this diagnosis early in the morning before you start driving your car. Start the car’s engine and let it stay idle for a while. After a few minutes, slowly loosen the oil cap until you open it completely.

Start observing the amount of fume coming from the inside. If you find the amount of smoke to be little, then it’s safe to assume that your car isn’t in danger. It’s perfectly normal for a bit of smoke to come out from the oil cap of a running car.

However, excess smoke coming out of the oil cap space is a bad omen. If the fume seems contaminated, then it’s crucial that you to the car to a mechanic immediately.

There are a few precautions you must take while making this diagnosis. For starters, do not make the diagnosis when your car’s engine is hot. When you start this process, open the oil cap within 1-3 minutes of starting the engine to avoid burning your hand.

Be sure to close the oil cap once you complete the diagnosis firmly. Not closing it properly or leaving it off can lead to fatal accidents. Lastly, be sure to clean up any oil residue that might accumulate when conducting this process.

Why is Smoke Emitting from Oil Cap?

Generally, smoke emitting from the oil cap by the engine happens for two reasons. The most probable reason is if the car’s valve steam seal or its rings have worn out. Another reason is if oil bypasses through them, causing fumes to come out.

Engine parts usually have a certain tolerance level which lets them operate smoothly. The frictionless performance of the car depends on oil flowing to all the components without obstruction. Deficiency in oil flow results in the car seizing up while driving.

Rigorous driving of the car results in the valve steam seal and rings wearing out and decaying after some time. Once they thin down, then smoke starts coming out of the oil cap.

Gaps in the car’s components cause the combustion chamber to start leaking oil. Oil seeps through the valves and piston and infiltrates the combustion chamber during this time, causing smoke to produce at an abnormal rate.

How to Stop Smoke from Coming Out of Oil Caps?

In most cases, your car won’t require repair just because smoke is coming out of the oil cap. It’s perfectly normal for this component to emit a little smoke when the vehicle has been operational for a while.

However, if the smoke emission is at an abnormal level, start by checking the stem valves and piston rings of the car.

Replace them immediately if you notice signs of damage or wear on any of them. It’s best to change the whole set instead of just one.

Replacing the damaged component only will cause an inconsistency in the car’s performance. The old part will not be able to keep up with the replacement, causing it to get damaged too. Therefore, it’s best to change it beforehand and avoid the hassle.

Changing the steam valve only will prevent the extra pressure from going through the piston rings due to the difference in strength.

On the other hand, only by changing the piston rings the system won’t contain enough pressure.

Risks Involved with Replacing Steam Valve & Piston Rings

Replacing either the steam valve or the piston rings are a very sensitive task and requires a high level of mechanical skill. Therefore, we highly recommend finishing this task with the help of a professional mechanic.

It’s a lengthy task that will take around 20 hours to replace the entire set. Therefore, be prepared to leave the car at the workshop overnight.

Changing the engine oil on time is a must to ensure that the car’s piston rings or steam valve doesn’t require frequent replacements.

Trying to replace these components at home is a risky endeavor and can cause more damage to the car. The piston rings are inside the engine block and are covered by the pistons. A lack of mechanical skill can cause harm to the surrounding parts too.

Furthermore, accomplishing this process will require the complete dismantling of the engine block. Therefore, a high level of skill is necessary for the reinstalling of these parts.

Frequently asked questions (FAQs)

1. Why does smoke come out when I take my oil cap off?

Gasoline vapor and exhaust gases come out of the oil cap when you take it off. It mixes with the air, causing smoke to come out.

2. Is it normal for air to come out of oil cap?

Air can come out of the oil cap due to the pressure in the crankcase. Therefore, it’s perfectly normal for a little bit of air to come out of it.

3. Are there any risks to losing the oil cap?

Not having an oil cap causes the car’s PCV system to pull in air, which leads to a vacuum leak.


The emission of smoke from oil caps is a normal phenomenon in a car that has been used for quite some time. The worst-case scenario is that it’s an indicator that the vehicle’s steam valve and piston ring require immediate change.

Making the replacement quickly eliminates any possible risks. Therefore, there’s no need to worry about dangers once you take action.

Conducting a diagnosis regularly will help you understand when a change is due. Soon enough, your car will be zooming through the streets effortlessly with no smoke coming through the oil cap. 

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