Smoke Coming Out of Engine Oil Cap – Reasons and Solutions

You’re reading this as you are an eye-opener of smoke coming out of engine oil cap and wondering what it might mean and why it happens. Could this be a problem you need to fix immediately? Well, it depends upon the engine fume amount. Read on to see the causes of smoke coming out of oil filler cap and simple ways to fix it.

smoke coming out of engine oil cap

Causes of Smoke Coming Out of Engine Oil Cap

There are two major reasons of smoke coming out of oil cap. While one shouldn’t bother you, the other should be of great concern. So let’s take a look at them.

1. The Usual Smoke

A small amount of smoke popping out of your engine cap is nothing to worry about. They are just engine fumes escaping from the oil filler cap as motor oil expands when heated. A great way to know that it’s nothing serious is if you see this fume when the engine is cold. So it’s probably a water vapor situation. But be sure that the engine cap smoke you see is small.

2. Failing Valve Stem Seals and Piston Rings

Valves and Piston rings prevent oil leakage and ensure engine oil doesn’t contaminate the air-fuel mixture in the engine combustion chamber. But another important function of the valve stem seals and piston rings is to seal the combustion chamber so exhaust gases don’t get into the oil cylinder.

However, these components can get worn out over time, letting exhaust gases pass through them. In addition, failing valve stem seals will let the piston rings push out exhaust gases from places it’s supposed to stay. These exhaust gases escaping from the valves and rings might find their way into the engine and build up pressure inside the engine. So when you open the engine oil filler cap, it comes out white smoke.

If the issue stems from a failing valve or ring, you must act immediately to avoid further damage. You could experience decreased horsepower and contamination of the engine oil tank.

To confirm your valves and rings are bad, look out for other inconsistencies in your car. For example, aside from emission of oil cap smoke, a bad piston ring can also cause the emission of blue smoke from your exhaust, more oil consumption, and oil leaks.

How to Test Smoke Coming Out of Oil Filler Cap

Having explained the causes of smoke leaving your engine oil cap, you now know what to look for.  However, we stated that the smoke could be normal or something serious. So how do you know if it’s something serious or not? It’s simple; if you are not sure of the real cause of the smoke, carry out this simple test.

  • Turn on your car’s engine and leave it idle for about 1 to 3 minutes (Don’t allow your engine to become too hot). Don’t go to the next step if your engine is too hot to avoid burns.
  • Lift the hood of your car, slowly loosen the oil filler cap and remove it.
  • Examine the amount of smoke leaving the oil cap as you remove it.
  • Put off the engine and return the filler cap on.
  • Clean any residue you may have spilled on your engine when taking out and replacing the cap.

What did you notice? If the smoke looks minimal, you have nothing to worry about; it’s normal smoke. If, you notice a substantial amount of smoke popping out, you may have worn or seized piston rings and valve stem seals. So typically, the quantity of smoke that pops out will determine whether or not you should be worried.

How to Fix the Smoke Coming Out From Engine Oil Cap

The fastest way to solve a problem is first to identify its cause. Here, we have explained why your engine oil cap brings out smoke. To fix engine oil cap smoke, do the following.

Diagnosing the Issue

When you notice smoke coming out of the engine oil cap, it’s critical to investigate promptly to avoid potential damage. Start with checking the consistency and color of the smoke, as it can give you a hint regarding the underlying issue. Moreover, evaluate the performance of your vehicle – is it consuming more oil or showing reduced performance? These signs can point towards a serious issue.


If the fumes seem small, let it be, they will go away by themselves. However, you may have worn rings or valve stem seals if the smoke is much. But first, carry out a proper diagnosis to ascertain the condition of your valves and pistons. Once you’ve confirmed the culprit, change it. Generally, you should change only the failing components.

For example, you can change only the piston rings with premium piston rings if they are bad. However, for optimal performance, you must replace both worn stem valve seals and piston rings since both work together. Why? Replacing only one will make the new one more powerful than the old part.

Technically, because the old part may not withstand the pressure from the new part, it will fail in no time. Thus, taking you back to the same job of replacing the other. For optimal performance of both parts and to save time and money, change both even if only one is faulty. Otherwise engine power will be reduced.

Note that replacing piston rings and valves requires expertise, so it’s best left to the hands of professionals. Generally, it can take up to 16 hours or more for a mechanic to replace all piston rings and about 3 hours to replace stem valve seals. You should change engine oil regularly for getting best performance and cool the engine before adding oil.

piston rings replacement

Cost to Solve Smoke from engine oil cap

The piston rings, for example, are located in the engine block inside the cylinder wrapped around the pistons. So it’s impossible to access them easily. So a mechanic would need to dismantle the engine to get to this part. Then after replacement, reassemble the engine and install it back into your vehicle.

And because it takes time, the replacement cost is also high. For example, piston ring replacement costs around $1000 – $5000, depending on the car model and labor cost. But labor per hour can range between $100 – $200, while the piston rings cost around $40 – $200 depending on the number of cylinders in your car. On the other hand, stem valve seals replacement costs between $900 – $1800, depending on your vehicle model. In addition, replacing an oil cap typically costs between $10 to $50, depending on whether you opt for an aftermarket or OEM part.


Smoke coming out of engine oil cap may mean something serious or nothing to worry about. First, however, you must understand why it’s emitting smoke and fix it as needed. If only a little smoke comes out, you may have nothing to worry about. You may be dealing with a faulty ring or valve if the smoke is much. You can use the simple test above to determine the quantity of smoke you see.

If there is much smoke, check the condition of your rings and valves and change them if needed. Remember, changing both is always recommended, even if only one is faulty, since both are connected and work together. This way, you can keep the performance of both components on the same radar and save time and effort. Repairing parts one after the other is usually more expensive and time-consuming.

Solomon Osuagwu

Osuagwu Solomon is a vetted auto mechanic with over ten years of experience in the garage and five years of experience as a service writer. He prides himself in writing accurate information on professional repair guides, DIY repair guides, buyer’s guides, comparisons, and car reviews. If he’s not in his repair garage, he’s writing automotive blogs to help car owners and fellow mechanics to troubleshoot and proffer solutions to several car problems.

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