How Much Blow-By Is Too Much On A Diesel Engine?

In today’s world of automobiles, internal combustion engines are available in a wide array of induction systems. We have diesel engines, gasoline engines, turbocharged engines, and more. One condition that can affect them all is blow-by. This is especially true for diesel engines. If the term is unfamiliar, engine blow-by happens when combustion gases make their way past the engine’s pistons and cylinder wall to land in the crankcase.

Thankfully, this condition is perfectly normal. All engines will experience some minor level of blow-by. The rings that create a seal between the piston and cylinder wall aren’t indestructible, and like all things, will wear down with time. Most engines may generate 1.5 CFM (cubic feet per minute) when the engine is operating normally to 3 CFM during colder engine temperatures. 

Although it’s a normal occurrence, too much engine blow-by can affect engine performance and even damage it beyond repair. In some cases, drivers could experience diesel engine runaway – which is just as scary as it sounds.

Worn piston rings are usually the first thing that causes you trouble. Situated near the top of an engine’s pistons, these rings can stick to cylinder walls from carbon buildup, and tend to wear down from outside contaminants introduced through the engine’s combustion chamber. This will cause the seal on the piston rings to break, which can let the compressed air of the combustion chamber escape. This, in turn, allows combustion gases to leak into the crankcase. The crankcase is an important part of the engine that keeps many components lubricated and prevents them from overheating.

Just as the piston rings can wear down, so too can the pistons and cylinder walls. When diesel engines are not properly maintained, their conditions usually go unnoticed. If you drive a diesel engine and have experienced decreased fuel efficiency or performance, a sudden loss of power, or engine misfiring, then your engine may be experiencing blow-by.

A surefire way to check is to remove the oil-filler cap and place it upside down on the opening of the tube. If the crankcase pressure is too much, the cap will get blown off. Auto repair shops also have a few quick tests they can run to check for blow-by. Then, since you’re already there, you may as well get your vehicle serviced and fix what you need to and cure your engine of too much blow-by.