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The Blowby Gasses

Blow-by gasses form inside the crankcase and are a result of the engine's combustion. Water, fuel and other pollutants contaminate the engine oil and evaporate when the engine reaches operating temperature. These gases need to be vented, which happens through an opening on top of the valve cover or any other location on the crankcase. Cars that are younger than 50 years (>1970s or so) have a so-called "PCV" (Positive Crankcase Ventilation) system that removes blow-by vapours from the crankcase and creates a mild vacuum inside the engine.

Engine Blowby Gasses
Engine Blowby Gasses

Most blow-by occurs during combustion inside the compression chamber. Compressed air, soot, combustion gases and fuel vapours make their way to the crankcase across the piston rings and cylinder walls. The cylinder liners and piston rings wear out over time, and you will notice an increased blow-by effect. The piston rings and the engine oil never seal a combustion chamber perfectly and blow-by occurs naturally in any engine. But when rings and cylinder liners are worn, more blow-by gasses form. This is usually bad because they contaminate the engine oil and you will have to change it more regularly. Also, the car experiences low performance due to inefficient compression.

The PCV System

The PCV system (Positive Crankcase Ventilation) removes excess gasses and pressure buildup from the crankcase. The blow-by gasses also create a positive pressure inside the crankcase, which needs to be vented. The PCV eliminates these gasses from the crankcase by rerouting them into the intake and mixing them with fuel to be burned in the engine. The PCV valve only lets gasses flow in one direction and prevents a reverse pressure buildup from a turbocharger or backfire. With a healthy engine, you actually don't need a vacuum inside the crankcase. It is enough if the crankcase can breathe and release the pressure into the atmosphere. Venting blow-by gasses back into the combustion chamber is mainly done to reduce emissions and to stay compliant with regulations. If your pistons, piston rings and cylinder walls are worn, your engine creates a lot of blow-by gasses that need to be vented. In this case, a vacuum helps to get rid of exhaust gasses, air, vapour and fuel fumes.

PCV System
PCV System

Engine Oil Contamination

The blow-by gasses and the soot slowly contaminate the engine oil destroying the additives, its wear protection and making it more acidic. That's when engine oil starts to show discolouration and get dark. The oxidation resulting from the air and fuel, as well as the heat and the soot, turn the engine oil black. With Diesel engines, the oil turns black faster because they tend to run hotter and have more compression, leading to more blow-by. That's why it's so important to regularly change the engine oil and even increase the intervals with older and higher mileage cars.

Good vs Contaminated Oil
Good vs Contaminated Oil

The Problem with Crankcase Ventilation

The oil, soot and fuel vapours, which get sucked in from the crankcase ventilation "PCV" system into the intake manifold can build up a thick layer of gunk on the walls, the intake ports and the valves. (Exhaust Gas Circulation "EGR" is another reason for gunk). This reduces your engine's power output drastically and decreases the engine's lifetime. It is a huge mess to clean this. In order to avoid the gunk buildup, it makes sense to install an oil catch can or reroute the breather hose into the atmosphere. Depending on the country, venting the blow-by gasses could be illegal since toxic oil vapours and droplets get released into the environment. For the car, it’s absolutely no issue. All pre-60s cars used to do it like this.

Excessive Intake Soot
Excessive Intake Soot

Testing for Blowby

If you want to do a health check on an engine, the blowby test is a great and easy-to-do method. To check if your engine has worn piston rings and if it produces excessive blow-by follow these steps:

1. Twist open your oil cap while the engine is idling

2. Pinch or disconnect the PCV hoses to prevent the vacuum from forming inside the crankcase

3. Observe what the oil cap is doing. If it is intensely dancing around from the gasses which are trying to escape, you will have too much blow-by. If the lid is even flying off the cylinder head cover, your engine is badly damaged and needs to be rebuilt.

Here is a video of a blowby test I did on a high-mileage (490'000 km) Mercedes diesel engine.

1991 Mercedes 300TD Inline 6 Blowby Test

Excessive Blow-by

Excessive blow-by usually means worn piston rings and cylinder walls, but it can also be a symptom of a blown head gasket. If your engine suddenly started showing excessive blow-by, it's usually a blown head gasket that will need to be replaced. The best thing to get rid of excessive blowby is to rebuild the engine, but this is costly and time-intensive. You can try to use oil-additive products to reduce blowby and extend your engine's lifetime. I'm using a product called "Stop Smoke" from Liqui Moly which helps reduce blowby and restores compression. You can also try to first bump up the warm SAE oil rating by 10 points (For example 10W40 -> 20W50) to increase its viscosity. Please refer to the article: Classic Car Engine Oils. You will be surprised how well this can work. Every time I'm changing the oil on my high-mileage engine, I flush it (Flushing the Engine) and add the stop smoke additive. It won’t fix your engine, but it can prolong the life of your car without having to rebuild everything.

Liqui Moly Stop Smoke
Liqui Moly Stop Smoke


In order to keep a classic car alive as long as possible, more maintenance and repair tasks need to be done correctly. You can find detailed instructions and explanations in my book:

Nils Willner

Nils is a Swiss-German engineer who is obsessed with old cars and engines. He is the author of "The Ultimate Classic Car Guide - How to Buy, Maintain & Repair Classic Cars" and the founder of EVC. His passion has always been with old cars and everything with wheels and engines.