What Causes Blow-by on a Diesel Engine – How to Minimize Engine Blow-By?

Blow-by is the last thing you would want on your engine. Unfortunately, most drivers have had to come to terms with the fact that blow-by affects all engines, regardless of the budget, brand, or builder. Blow-by also goes by crankcase pressure.

Blow-by is a relatively common term and an occurrence that happens on all engine types—diesel, gas, etc. For diesel engines, this phenomenon occurs when the pressure from the compressed air and fuel in the cylinder bore exceeds that in the oil pan. This forces the gas to leak past the piston rings and into the crankcase, hence crankcase pressure. 

This article explores some of the reasons why engine blow by happen and what you can do to keep it away from your rig’s engine. 

What Causes Blow-by on a Diesel Engine?

what causes blow-by on a diesel engine

Blow-by in diesel engines is a phenomenon that occurs during the combustion process where the burned gases force their way past the piston rings and get into the crankcase. In other words, when the pressure in the combustion chamber gets exceedingly high, it finds a route to escape.

It is important to note that blow-by is a normal occurrence since the rings are never 100% infallible. Therefore, it is vital to measure, troubleshoot, and minimize blow-by in engines so as to increase horsepower. 

More often, this crankcase pressure will escape via the breathers and sometimes it may be too much which causes them to drip or blow oil. In worst cases, the pressure can exceed what the breathers can handle and cause the gaskets to blow out and start leaking. 

Even in ideal conditions, where the pistons and rings fit perfectly in the cylinders, there would be a minute amount of blow-by attributed to the necessary side, back, and end gap clearance. The fact that there has to be some clearance is the main reason why it is difficult to completely eliminate blow-by. 

Another cause for engine blow-by is the piston ring end gap. This gap is included in the design to facilitate the running of the engine in normal temperatures. Theoretically, the gap should be negligible, but still sufficient enough to ensure the ends do not butt together. 

Besides rings and pistons that are worn out or were improperly prepped during a prior rebuild as causes of blow-by, overheating, detonation and lack of lubrication can also be blamed for the same. 

How to Minimize Engine Blow-By?

Blow-by may be the result of the inevitable combustion process and is thus impossible to eliminate, but efforts should still be made to reduce it to a minimum. This is primarily because any gas leak past the rings always translates to lost horsepower. 

You can minimize the blow-by by using a set of premium, precision-machined pistons that can fit perfectly round and true cylinders that have been honed with a deck plate in place. These pistons should be held in place with rings that are perfectly sized and end-gapped for the specific application.

Moreover, these rings should be carefully checked to ensure that they all come with the proper width and radial wall thickness—verifying that they are the ideal ones for the selected pistons. 


If you overfill the crankcase by half a quart or thereabout, your engine is not at risk of experiencing this problem. In older engines, excessive blow-by is likely to be caused by piston rings that wear out and allow too much stuff to get by them. This becomes an even more serious and expensive problem. 

Injector blow by can occur when the injector does not seal against the injector seat in the cylinder head. You may hear a chuffing sound or notice some black tar around the injectors. Your engine could also experience more symptoms you can look out for. 

All engines tend to have minute levels of blow-by since the piston rings are never completely watertight, even in new engines. Basically, a 12-liter engine that is in good condition is likely to experience 1.5 cubic feet per minute of blow under normal operating temperatures. This number will increase to 3.5 cfm in colder temperatures. Either way, excessive blow-by could lead to larger issues that need immediate attention. 


Engine blow by is an unfortunate phenomenon that affects all diesel engines, including those in commercial generators. It is thus advisable to know the signs and symptoms so you can avoid the issue or minimize it in future. Outlined above are some of the causes of blow by and solutions you can apply to mitigate the same.