“Blow-by” is a word that is used to describe a variety of engine types, including diesel, gasoline, and other fuels. Gas leaks from diesel engines’ piston ring into the crankcase when the cylinder bore’s pressure from compressed air and fuel is greater than the oil pan’s pressure. As soon as the pressure in the combustion chamber becomes too high (which normally occurs first during the engine’s power stroke and subsequently during the compression event), it must be released.

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It’s crucial to highlight that some blow-by is typical, as rings aren’t completely infallible—not even in a brand new engine—and should be expected. Recent generation variable-valve diesel, however, include individual pressure sensors, which feed data to an engine control module (ECM) that allows it to correct for fluctuations in-cylinder pressure. 

For the most part, however, blow-by in a diesel engine indicates that the rings are stuck to the cylinder walls, that they are significantly worn, or that they have been completely damaged; these circumstances are caused by engine usage (mileage) and/or a lack of engine maintenance. Additionally, damaged pistons or cylinders are a possibility that can only be detected after a thorough takedown and examination. Whether you believe it or not, engine brakes can play a role in blow-by because those systems force rings to move and vibrate, causing them to break their seals when they are triggered.

High-mileage diesel engines, especially those with high-performance components such as compound or triple turbochargers, have a larger likelihood of experiencing excessive blow-by, especially if the necessary service schedule is not followed.

First and foremost, harsh idling and misfiring can be indicators of a problem. Excessive blow-by, on the other hand, can be identified by the presence of white smoke pouring from the oil-fill tube or the aperture on the valve cover. This can be verified by placing the oil-fill cap upside down on the tube or aperture.