How Much Blow-by Is Normal?

The engine is the heart of a car, but different factors, including a blow-by, can damage it. As a car owner, you might be wondering what a blowby is and how much is normal or excessive for your engine. To answer your question, we have done proper research and consulted experts. Here is their response. 

Even new engines have moderate degrees of blow-by because piston rings aren't watertight. Under normal operating conditions, a 12-liter engine in good condition may experience 1.5 cubic feet per minute of blow-by.

Blow-by affects all types of engines that use diesel or fuel. It's essential to know the causes and symptoms of a blowby in your engines. Keep reading further to learn more about blowby and signs you should watch out for in your engine. 

Engine repair using socket wrench to tighten bolt, How Much Blow-by is Normal

What is Engine Blow-by? 

When you have weak piston rings, weak pistons, or a damaged cylinder wall, your engine is likely to be affected. This is because the pressurized air and fuel in the cylinder combustion chamber pass through the piston rings and into the crankcase ventilation. 

Internal combustion engines work by igniting the mixture of air and fuel. A blow-by occurs when the explosion travels through the piston rings to the crankcase and then out of the engine. 

untighten socket wrench to check and do maintenance of a engine

You may notice that the strength of the pistons will be drastically reduced if they are leaking, which could also be a reason for the blow-by in your engines. If you don't handle this problem immediately, it may cause further issues to your engine. This may even make your engine consume more engine oil than usual. 

How Much Blow-by is Normal? 

professional mechanic doing maintenance on an engine

It is important to note that there are two types of blow-by: power blowby and compression blowby.

Compression blow-by will have gasses pass through the cylinder walls, the piston, the piston ring, and down into the crankcase. For the piston to work smoothly inside the cylinder, it must be fixed properly. 

If the piston is fixed too tightly, it might get stuck inside the cylinder, and your engine won't work. If it's too loose, then you end up with excessive blowby.

The pressure inside the cylinder will always find a way to be released, but it should not be more than 10%.

All engines have blowby, whether it is a diesel or gas engine because there is no such thing as a perfect seal. Because of this, engines have vent holes to release air pressure that builds up inside the crankcase. 

Blow-by can be considered abnormal or excessive if the pressure caused by air is enough to bring out oil from your engines. When you notice oil pushing out from the oil cap of your engine after using your vehicle, it means the blowby has gone beyond normal. 

How Do You Check For Blowby? 

You can notice if your engine has excessive blow by observing some of these signs.

overheated engine causes to smoke


A sign you should look out for is white smoke. Check if white smoke is coming out of the oil tube or valve cover. 


If the noise coming from the engine is very loud and is also accompanied by fumes or a large amount of exhaust cloud, it is a sign that your engine is having a blowby. 

Oil Stains

If the oil tube of your engine is covered with stains of oils, it is a sign of a blowby that can affect your engine. The cause of this is because unburned fuel mixes into the oil, and this enters the crankcase. 

Increased Usage of Oil and Fuel

When your engine is leaking the oil or fuel into the crankcase, it will cause your engine to use more oil and fuel.  When too much oil or fuel gets into your crankcase, it causes a blowby. 

Buildup of Soot

Blow-by contributes to increased incomplete combustion, which produces soot held within the cylinder walls, resulting in soot buildup. 

Causes of Blowby

Blow-by occurs when impurities such as air, gasoline, and moisture are forced past the piston rings and into the crankcase by internal combustion in your engine's combustion chamber.

If your engine's piston rings aren't well-fitting or sealed, they won't be able to control the pressure created by combustion. Instead, they will be filled with leaking air-fuel mixture and impurities. 

Both pieces wear away over time as the engine's pistons move up and down against the cylinders during typical engine operations. The pistons become smaller, and the cylinders become wider. 

Additionally, the piston rings deteriorate over time, making them less capable of maintaining a tight seal. As the pieces grind against each other, these problems become more pronounced, and the number of blow-by increases. 

Furthermore, any soot and deposits from incomplete combustion will build on the rings, obstructing their seal and aggravating engine blow-by.

Blow-by can also be caused if the damage to fractured piston rings is not properly fitted. Another reason can be if the piston is damaged or defective and if the piston rings have gaps or spaces. 

How to Fix Blow-by

filling oil in an engine

If you notice your engine has a blow-by problem, you need to fix it. It may not be easy to fix a blow-by because of the worn-out piston rings and pistons, but there are other steps you can take first. 

Clean Your Crankcase Vent

First of all, make sure your crankcase vent is clean and clear of dust and impurities. Remove the hose and blow air through it to make it very clear. 

Use Some Oil

Occasionally, the piston rings can become trapped and refuse to seal against the cylinder walls. If you're lucky, pouring some diesel into the cylinders and letting them sit for a day or two will remedy the problem. 

Get Another Piston Rings

If the fuel or additive didn't work, the piston rings might need to be replaced. To replace the piston rings, you must first remove the pistons from the engine block, which is a significant repair. You can do a leak-down test to ensure that the piston rings or pistons caused the problem.

Replace Pistons

You should also inspect the pistons while replacing the piston rings. If the pistons are damaged, you may need to repair or replace them. When replacing pistons, it is common to resurface the engine block.

Replace Your Cylinder

You may need to replace your cylinder walls if they are scratched and worn out, or you may need to bore larger cylinders with larger pistons. Alternatively, the engine block can be replaced.

Can Blow-by Cause Overheating? 

Blow-by can reduce the efficiency of your engine over time by covering sections of the intake with oil and fuel.

In forced-induction and intercooler engines, blow-by covers the inside of the intercooler, restricting its ability to transmit heat and cool the intake charge.

These problems become more apparent as the car gets older and the problem is not given proper attention on time. 

How Do I Lower Crankcase Pressure? 

Crankcase pressure is a prevalent issue in various vehicles, including cars and tractors.

Jammed or worn piston rings cause crankcase pressure. Because the piston rings no longer adequately occupy the area, combustion gasses are pushed past the pistons and blown into the crankcase, causing pressure. 

The piston rings can be protected to prevent crankcase pressure. There is no oil coating on the working parts during a cold start. Metal-to-metal contact becomes more likely, causing the piston rings to wear out faster.

Many engine additives can help you avoid this. They adhere to all moving parts of your engine and form a thin film layer, effectively preventing metal-to-metal contact and wear.

Additionally, blocked or worn-out piston rings can produce crankcase pressure.

Combustion Gases

It should also be noted that the combustion gasses released into the atmosphere should be reduced due to environmental regulations.

s a result, the internal combustion engine features a crankcase ventilation system, which takes clean air into the intake manifold through the air filter and circulates it through the crankcase.

The fumes from the crankcase are subsequently discharged into the cylinder's combustion chambers, where they are consumed.

Black sludge can be produced when the crankcase ventilation system fails, posing a danger of total engine damage. 

The oil evaporates due to the hot combustion fumes, resulting in oil consumption. Furthermore, compression loss is potential, resulting in a power reduction.

Remember that crankcase pressure causes piston rings to be stuck, and this can only be loosened by applying a cleaning chemical or additive to it.  


Engine repair using socket wrench to tighten bolt

To ensure your engines are free from damage, always check for and clean out any dust or impurities.

If you notice your engine making loud noises or bringing out white smoke, you may need to replace some parts like the piston, piston rings, and cylinders. 

To read similar articles about vehicle engines, check out these engaging posts: 

How Long Can a Car Battery Last Without The Engine On? 

Transmission Hot Idle Engine: What Does It Mean? 

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