Every engine has some level of blowby, but when it comes to large diesels, the concern is amplified. When you combine a large cylinder bore, high cylinder pressure through turbocharging, many hours of use and marginal maintenance, excessive blowby is the result.

The leakage of any combustion gases, air, or pressure into the engine’s crankcase is considered blowby. On a large diesel, about 60% of the blowby enters the crankcase by going past the piston rings. This occurs when the pressure differential in the cylinder bore is the greatest compared with the pressure in the oil pan. Thus, blowby is highest during the engine’s expansion (power) stroke and, secondly, during the compression stroke.

In addition, blowby is intrinsically linked to engine temperature and load. When measured in cubic feet per minute (cfm), a 12-liter engine in good mechanical condition can experience at idle 1.5 cfm of blowby at normal operating temperature but 3.5 cfm when cold. Under full load, the blowby may be 2.7 cfm.

The remaining 40% of blowby comes from sources that most do not consider, such as the turbocharger or the compressor for the air brakes on a truck. When diagnosing an excessive blowby condition, you need to look at any components on the engine that are linked to engine oil and, thus, the crankcase. 

The source of the blowby will determine how it presents and the potential long-term implications. Blowby that goes past the piston rings not only pressurizes the oil pan but also introduces combustion gases that contain unburned fuel, particulate matter, and oxides of nitrogen emissions. They also create condensation due to the temperature differential of combustion gases and the crankcase.

When mixed with engine oil, blowby produces sludge and acids that attack all engine parts. The unburned fuel dilutes the lubricity and viscosity of the engine oil, attacking engine bearings, the valve train, and cylinder walls.

When equipped with an engine brake, higher-than-normal blowby will be induced when the system is engaged. When evoked, the piston moves around and the rings flutter, allowing them to lose their seal. The engine brake is meant to help stop the vehicle and decrease friction wear, but it shouldn’t be used excessively.

Blowby tears the oil from the piston and rings. At first it vaporizes and then it becomes an aerosol, which you see as a film or fumes around the crankcase vent tube. 

The key to minimizing blowby is rooted in one word: sealing. An excellent seal needs to be created and maintained between the piston rings and the cylinder wall along with the other areas such as the turbocharger and, if applicable, a compressor. Keep the combustion gases and pressure where they are supposed to be, and blowby won’t be a concern.

Since every engine has some level of blowby, the crankcase needs a way to breathe. This is challenging with a turbocharged diesel vs. a normally aspirated gas engine since a PCV valve can’t be employed. A heavy-duty diesel, depending on its use and age, may have an open vent pipe. This is nothing more than its name implies. Its task is to relieve the pressure in the crankcase; it does little to remove any combustion gases or moisture. Newer engines may employ some style of separator, and that is considered a closed system. With this design, the engine oil is separated out and the combustion gases are fed back into the induction system. The oil is removed so it doesn’t damage the vanes on the turbocharger compressor wheel and foul the heat exchange capability of the intercooler. Some engines may have an oil separator and an open vent pipe. 

Familiarize yourself with the crankcase ventilation system function on every one of your diesels. If it fails, the engine will, over time, experience excessive wear and be prone to oil leaks from the pressure in the oil pan.

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