A lumber yard reported that a Toyota 8FGU25 (4Y engine) forklift was burning a liter of engine oil every 8 hours of operation, emitting blue smoke at idle, and running roughly with a noticeable misfire. The engine oil dipstick level was consistently dropping, but there were no visible external oil leaks on the ground.
The technician first checked the spark plugs. Cylinder #1 plug was fouled with wet, black oil, indicating oil was entering the combustion chamber. He performed a leak-down test, which showed 15% leakage on Cylinder #1, suggesting worn valve guides or piston rings. However, before disassembling the engine, he removed the oil filler cap while the engine was idling. A strong puffing of air blew the cap off the valve cover, indicating excessive crankcase pressure.
He checked the "Positive Crankcase Ventilation (PCV) Valve." The PCV valve was completely seized shut by carbon buildup. With the PCV blocked, the exhaust blow-by gases could not be vented back into the intake manifold. This pressurized the crankcase, forcing oil vapor up the valve guides and into the combustion chamber (causing the fouled plug and blue smoke).
Furthermore, the 8FGU25 is turbocharged. The "Turbocharger Oil Drain Line" (a pipe returning oil from the turbo to the oil pan) was packed solid with thick, sludgy oil. The turbo relies on gravity to return its pressurized lubricating oil. Because the drain was blocked, the oil backed up inside the turbocharger bearing housing. The crankcase over-pressure combined with the blocked drain forced the oil past the turbine compressor seals directly into the intake manifold. The engine was literally inhaling oil from two sources.
The repair involved replacing the PCV valve and cleaning the breather hoses. The turbocharger drain line was removed, soaked in solvent, and rodded out to clear the sludge. The turbocharger compressor housing was cleaned of oil sludge. The fouled spark plug was replaced. After the repair, the crankcase pressure normalized, the blue smoke cleared, and the oil consumption stopped completely over the next 40 hours of operation.
