Why Oil Coming Out Of Crankcase Breather [Solved]

Oil coming out of crankcase breather is a problem every car owner or motorcycle owner encounters. It occurs due to excessive oil in the engine, worn-out oil seals, faulty compression rings, high crankcase pressure, and damaged PCV valves. 

You can run your engine smoothly without any oil blow-by if you address these problems. Fortunately, we provide solutions to those problems. 

Turns out, we will show you how to do a compression test at the end of this guide. It will help you find the source or the main culprits that cause oil to come out of the crankcase breather. So, continue to read. 

Why Is Oil Coming Out Of Crankcase Breather?

Here are the reasons that cause oil blow-by through the crankcase breather. 

1. Excess Oil In The Crankcase

Extra oil in your vehicle engine causes the oil to foam and blow out through the breather, feeding into the air filter. 

If it’s possible, disconnect the breather hose and push it into a tall bottle with a narrow throat like the coke bottle. Then, run your engine for 10 minutes. If oil leaks, check out the oil level. If the oil level is normal, we bet the oil comes out of the valves. 

Note: You can also read will high mileage oil cause leaks.

2. Worn Oil Seals

Oil blow-by through the induction system is a sign of worn oil seals on the inlet valves. Pressure leakage from worn-out or damaged seals causes the oil to force down into the inlet passage and back through to the inlet filter. 

Typically, excess oil will blow out through the crankcase breather too as the cylinder head or rocker cover gets pressurized, blowing back down to the crankcase. We recommend you do a compression test on cylinders. 

3. Faulty Compression Rings

Damaged compression rings are also the culprit behind oil blow-by through the crankcase breather. Generally, there are two ways for the oil to come out through the exhaust or the crankcase breather. Either way, it means your bike may get some dirt at some point. 

When compression leaks past the rings, it can pressurize the crankcase and push more oil through the breather. And when rings and valve guides get loose, oil gets into the combustion chamber. 

4. High Crankcase Pressure

Excessive oil coming out of the breather is an indication that you have high crankcase pressure. This can be due to the following things:

  • Cylinder head gasket leaking
  • Cylinder bore worn
  • Piston rings broken or worn
  • Valve stem guides worn

It would be worse when loaded would make all of these a possibility. We recommend you start with a compression test to see if the problem is isolated to one cylinder. 

5. PCV Valve Gets Clogged

A clogged PCV valve can also be the culprit behind the oil leak through the crankcase breather. You can easily inspect it by pulling or shaking. You should hear or feel the valve moving in most, but it should allow you to blow through it only one way.

This valve lets the intake system draw a small vacuum from your vehicle engine, removing the pressure. Cleaning or replacing the PCV valve is an easy fix for this problem. 

How To Fix Oil Leak From Crankcase Breather?

We don’t know what engine or car you are working on. So, it will be challenging for us to give you an exact solution to this problem. However, the following basic tips can help you solve the oil leak issue through the crankcase breather. 

  • Make sure you don’t overfill your engine with oil.
  • Inspect the oil seals on the inlet valves. If they get damaged or worn out, replace them. 
  • Ensure the compression rings are in good shape. 
  • Don’t forget to check out the cylinder head gasket. If needed, replace it. 
  • Replace the PCV valve if it gets damaged.

How To Perform A Compression Test On Your Engine?

Doing a compression test is the best way to explore the reasons behind the oil leak through the crankcase breather. And this chapter will help you how to do a compression test following the below steps. 

Disconnect Fuel Flow

First off, disable the flow of oil to your engine. Just pull the fuse that goes to the fuel pump. The fuel injection system may have two different fuses. That’s why it’s not pumping gas that you don’t want to burn anyway. 

Disable Ignition Systems

Disconnect the spark or ignition systems. Again, it’s a fuse on a modern car and you can easily disable it. On the Cougar, you can find a wire that comes off the coil and feeds the high-tension electricity out to the distributor. Just pull the cable off and keep it somewhere. 

Pull One Or More Spark Plugs

Pull your spark plug or you can pull them all at once. But ensure you keep track of where all the plug wires are. Just pull them all or do it one by one. 

Attach Compression Tester

Now, attach your compression tester to the spark plug hole with the right thread and size a gauge on the other end in PSI and a rubber hose between them. You just put the compression tester in where the plug you just removed. 

Crank the Engine

Finally, crank your car engine and keep doing it until the needle has peaked. Now, take the reading for each cylinder. If you get better PSI for every cylinder, you have got a healthy engine. 

If one of them is very low, you can try pouring some motor oil into that particular cylinder. Then, reattach your compression tester to run the test again. 

If it gets a better pressure, you may have a worn-out interface of your piston rings to the cylinder. You may have an issue with your valve and the valve seat if the pressure is low. 


How do you fix excessive crankcase pressure?

You can install a vacuum pump that continually eliminates the pressure out of the crankcase. 

What causes too much pressure in the crankcase?

A blown head gasket or cracked engine block causes too much pressure in the crankcase. 

What happens if the crankcase breather is clogged?

If the crankcase breather gets clogged, it will cause vacuum leaks. And because of the vacuum leaks, it will lead you to an incorrect air-fuel ratio. 


It’s typical to give an exact solution to what causes the oil to come out of the crankcase breather because the model & make of each car are different. However, we mentioned the common culprits that caused the oil to leak from the crankcase breather. 

If you can’t discover the actual reasons behind oil blow-by, do a compression test following the steps we mentioned. Otherwise, it would be best to hire a certified mechanic if you are not mechanically inclined. 

About John M

John contributed as a technical head at an automobile company just 2 years after his post-graduation in Automobile Engineering. He loves to lead a free life, so he left his job & started blogging. Now, he does research on every automotive problem, part & product and seeks a better solution & best products & shares his findings with his readers to help them as well as to minimize their struggle.

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