Cylinder Washout
A condition where excess fuel washes away the lubricating oil film on cylinder walls, causing low compression and piston wear.
Cylinder washout (or cylinder wash) is a detrimental engine condition where excessive liquid fuel enters a combustion chamber and washes away the thin film of engine oil lubricating the cylinder walls. This oil film is critical for preventing direct contact between the piston rings and the cylinder bore during operation.
Without the lubricating oil barrier, friction increases dramatically, causing metal-on-metal wear and scuffing of the cylinder walls. Furthermore, because piston rings rely on the oil film to help establish a tight compression seal, cylinder washout leads to a sudden loss of engine compression. The excess fuel also leaks past the rings into the oil pan, diluting the engine oil and degrading overall engine lubrication.
Cylinder washout is commonly caused by stuck-open fuel injectors, excessive engine cranking during a no-start condition, or extremely rich fuel mixtures. Symptoms include a fast, smooth cranking sound (indicative of no compression), raw fuel smell from the exhaust, and fuel-diluted oil. Diagnosing it requires checking fuel pressure bleed-down and performing a compression test before changing the contaminated oil.