What is blowby on a diesel?

Blowby is the leakage of combustion gases past the piston rings and into the crankcase of an internal combustion engine, most notably diesels. It is a natural phenomenon that occurs to some extent in all engines, but excessive blowby is a sign of engine wear or damage.

Here's a breakdown:

• Causes: Blowby occurs because the piston rings, while designed to seal the combustion chamber, cannot achieve a perfect seal. High pressures during combustion force gases past these rings. The primary causes of increased blowby are:

  • Worn or damaged piston%20rings: As rings wear, the gap between the ring and the cylinder wall increases, allowing more gas to escape.
  • Worn or damaged cylinder%20walls: Similar to worn rings, damage to the cylinder walls creates paths for gas to leak.
  • Piston damage: Cracks or other damage in the piston itself can cause blowby.
  • Excessive cylinder wear or cylinder%20glazing: Over time, the microscopic crosshatch pattern on cylinder walls can wear smooth, reducing the ability of piston rings to seat properly.

• Consequences: Excessive blowby has several negative consequences:

  • Reduced engine%20power: Loss of combustion gases translates to a loss of pressure pushing on the piston, reducing power output and fuel%20efficiency.
  • Crankcase%20pressure: Increased pressure in the crankcase can lead to oil leaks from seals and gaskets.
  • Oil%20contamination: Combustion gases contain unburnt fuel, water, and other contaminants that can dilute and degrade engine oil, reducing its lubricating properties. This accelerates engine wear.
  • Increased emissions: Blowby gases are typically routed back into the intake manifold to be re-burned (via the positive%20crankcase%20ventilation (PCV) system), but excessive amounts can overwhelm the system, leading to increased emissions and carbon%20deposits within the engine.

• Symptoms: The symptoms of excessive blowby include:

  • Smoke or fumes emanating from the oil%20fill%20cap or dipstick tube when the engine is running.
  • Oil leaks from engine seals and gaskets.
  • Excessive oil consumption.
  • Reduced engine power.
  • Rough running or misfiring.

• Diagnosis: Blowby can be diagnosed using a blowby meter, which measures the amount of gas escaping the crankcase. A compression test or cylinder leak-down test can also help pinpoint the source of the blowby (e.g., worn rings, leaking valves).

• Remediation: The fix for excessive blowby typically involves engine repairs, such as replacing piston rings, re-honing or re-boring the cylinders, or replacing the engine. The specific repair depends on the underlying cause of the blowby.