Smoke Coming From the Oil Cap! Here’s What’s Normal & What’s Not!

Smoke from the oil cap can be worrying, but depending on a few factors, it may not even be a problem. Is the smoke excessive? Is it condensation, or is it just a tiny amount of thin smoke you can put down to be as expected?

You have to remember the engine’s internals get very hot, and in an older, higher mileage engine, you have to expect an element of wear to the components inside. A small amount of wear will result in a small amount of fumes and fluids leaking, such as fuel and oil. Now, if a small amount of fuel or oil smolders in a part of the engine that isn’t hot enough to ignite or burn it, you can expect a small amount of smoke that only has one place to go, out of the oil cap. But that is not the case every time, and there are a few instances where there are problems that you should be concerned about, which are all included in this article. 

What Causes Smoke to Come out of the Oil Cap?

A few areas of concern can lead to smoke coming out of the oil cap, and most result in the engine burning oil in some form. But not all of them are such a big deal; they can also be easy to sort, as you will read.

Seals Or Gaskets 

Most issues involving smoke from the oil cap start with a leak from worn piston rings, engine gaskets, or oil seals. This means hot oil, fuel, and exhaust gases are forced into the head of the engine, where the oil filler cap is located. Although these areas of the engine are hot with the engine running, they aren’t hot enough to ignite the fuel; instead, they cause it to smolder away for ages, releasing smoke that will escape out of the oil cap. 

These issues can be minor problems; you can generally put this down to the age and mileage of the engine. Especially if there is only a tiny amount of thin smoke and the engine is still running as expected.

You will likely need an urgent repair if there are large plumes of smoke, which will involve replacing the offending items. Replacing gaskets or seals will require the engine to be removed and stripped down. I do not recommend anyone taking this on at home on the drive without the proper knowledge and training. 

Exhaust gases leaking

Exhaust gas leaks are slightly different; even with worn seals causing a problem, it may not be the problem. A small number of exhaust gases will always leak through the oil cap with it removed. This is called ‘blow by,’ which is the gasses bypassing the rings and entering the crankcase. They are usually removed from the engine when the crank rotates through crankcase ventilation. These gases can find their way into the cylinder head, which is why a small amount of exhaust smoke will come out of the oil cap.

Another issue you can have is a blocked PCV (Positive Crankcase Ventilation), which means the ‘blow by’ gases get removed through the oil filler cap and not through the PCV one-way valve. What happens is the exhaust gases get forced out past the valve stem oil seals due to a pressure build-up, which gives the gases an exit through the oil cap. It’s rarely an excessive amount of gas, as with a blocked PCV, you will experience other issues, such as the engine burning oil, which means you’ll see blue smoke and oil leaking from the exhaust.

However, suppose this is a substantial amount of smoke; you may find a blockage in the exhaust, causing a rise in back pressure. In that case, the typical culprit is a catalytic converter problem

is smoke coming from the oil cap normal

Low Oil Level 

When an engine is low on oil, the friction between moving components generates heat. The residue and contaminants burn to produce a small amount of smoke released from the oil cap. To rectify this issue is exactly as you would think: refill back to the correct level or completely change the oil and filter simultaneously.

Overfilled engine oil

When overfilled with oil, an engine will burn away the excess oil that seeps past the piston rings into the cylinder. You can expect to see an increased volume of smoke with a slightly different color. You may also see a small amount of the burning oil fumes escaping through the oil cap.

Fixing an overfilled engine can be simple, depending on your chosen method. Although messy, you can either fully drain and refill the oil or partially drain it through the drain plug. Another option is to suck the oil out of the engine with a fluid extraction or siphon pump through the dipstick tube; it requires you to keep removing the oil and check the level until it’s right again.

Water vapor

Suppose you find very thin smoke from the oil cap, which only happens when starting a cold engine. It may be water vapor from the condensation burning away. This isn’t something to be concerned about, especially if the smoke goes away after a minute or so when the engine warms up. 

How to Check if the Smoke From the Oil Cap Is Normal?

Checking if the fumes are normal is easy; you only need to be armed with common sense. Water vapor will be gone in about a minute from starting the engine from cold; it’s normal, so you won’t need to worry if the problem disappears immediately. If you find your car excessively overfilled or underfilled with oil, fix that issue and recheck the levels of smoke coming out of the oil cap, which should be reduced.

The next thing to do is check the color, the amount, and for how long the exhaust gasses are coming from the oil cap. Dark grey smoke that doesn’t go away will confirm exhaust gas is escaping, and an issue may need resolving by a mechanic. White smoke with a blueish tint means oil’s burning in the engine, forcing the fumes through the oil cap. Depending on the amount of gas released, this may require urgent attention.

How Do You Stop Smoke From Coming Out of the Oil Cap?

You first need to loosen the oil cap to stop smoke coming from the oil cap. Leave the engine idling so whatever is burning can burn. Be careful because the oil can spit when removing the cap entirely

You may be lucky and never see smoke coming from the oil cap again. But if it returns, you will, at least, be able to confirm if there is a problem. If you continue to see smoke, there will likely be a blockage that needs addressing in the PCV, or a seal/gasket needs replacing.

Frequently Asked Questions

Is There Supposed To Be Air Coming Out Of My Oil Cap?

Air coming out the oil cap is the positive crankcase ventilation (PCV) system operating. The PCV is a breather system that regulates the pressure and ventilation of fumes from the crankcase into the intake manifold, so you may see some air coming out the oil cap, this is completely normal providing it is not excessive.

How Much Blowby Is Normal?

In order to measure the volume of air you will need a blow by flow meter which measures the leaking gases that pass around the pistons, piston rings and valve guides. Normal levels vary by engine and temperature but a normal range should be between 1.5 and 3.5 CFM (Cubic feet per minute).

Final Words

To answer, is smoke coming from the oil cap normal? It can be if it is only in small quantities and disappears within a few minutes of starting a cold engine. If the leaking gasses are excessive and do not disappear, I strongly recommend that you have the engine inspected by a mechanic as soon as possible.

My name is Tom although my friends call me Tommy. Messing around with cars and bikes has always been a hobby of mine even from a young age. So I made it my day job 17 years ago. I am a fully qualified mechanic as you would expect. I've worked in all different areas of the motor trade, valeting, panel beating, engine repairs, I'm sure you get the idea. I enjoy sharing my wealth of knowledge and experience with others, which is the reason I spend a lot of time here writing for this website.

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