What is blowby on a diesel?

Blowby is a phenomenon that occurs in diesel engines where combustion gases leak past the piston rings and enter the crankcase. It is a common issue in older or poorly maintained diesel engines, but can also occur in newer engines under certain circumstances.

The blowby gases consist of unburned fuel, combustion by-products, and air. When blowby occurs, these gases enter the crankcase and mix with the engine oil. This can lead to several problems:

1. Reduced engine performance: Blowby reduces the efficiency of the engine by allowing combustion gases to escape from the combustion chamber rather than contributing to power generation. This can result in a decrease in engine power and acceleration.

2. Increased oil consumption: As the blowby gases mix with the engine oil, it can cause oil dilution. This leads to increased oil consumption and a decrease in lubrication properties, potentially causing engine damage.

3. Contaminated engine oil: Blowby gases introduce contaminants, such as soot and acids, into the engine oil. These contaminants can cause increased wear and corrosion of engine components.

4. Pressure build-up in the crankcase: Blowby can cause an increase in crankcase pressure, leading to seal leaks and oil leaks. It can also result in oil mist being expelled from the crankcase breather, causing a buildup of oily residue in the engine bay.

To address blowby in a diesel engine, it's important to identify and address the underlying causes. Some common reasons for blowby include worn piston rings, worn cylinder walls, excessive cylinder pressures, or clogged or malfunctioning crankcase ventilation systems.

Regular maintenance, such as performing oil changes at the recommended intervals and using high-quality engine oil, can help reduce blowby. Additionally, ensuring that all engine components are in good condition and not worn beyond specifications can minimize the occurrence of blowby.

If blowby is severe or persists even after maintenance efforts, it may be necessary to repair or replace worn or damaged engine components. Consulting with a qualified mechanic or diesel engine specialist is recommended to determine the best course of action.