What is the normal crankshaft case pressure?
3 Answers
The normal negative pressure value for the crankshaft case is -350mbar. The engine's crankshaft ventilation system is a forced ventilation system, where the crankshaft case is connected to the intake pipe. The vacuum from the intake system draws the gases from the crankshaft case into the cylinders. As a result, the crankshaft case maintains a negative pressure state under most operating conditions, with the magnitude of the negative pressure related to the intake system pressure. To prevent excessive crankshaft case pressure, extend the service life of the engine oil, reduce component wear and corrosion, and prevent engine oil leaks, crankshaft case ventilation must be implemented. The purposes of crankshaft case ventilation are as follows: Prevent lubricating oil deterioration and fuel dilution of the oil, reducing wear and corrosion of engine components. Pressure reduction, temperature reduction, and leak prevention: Gases leaking into the crankshaft case increase the pressure and temperature inside, causing oil to leak through the crankshaft seals and gaskets. Recover combustible gases and reduce pollution. The gases that enter the crankshaft case are drawn back into the cylinders for combustion, recycling CH compounds and reducing atmospheric pollution.
That's a great question! The normal crankcase pressure is generally in a vacuum state, typically maintained between -15 to -25 inches of water column. Based on my experience working with various engines, if it exceeds atmospheric pressure, it becomes a serious issue—engine oil will start leaking everywhere. Last time I worked on an old Ford, a vacuum reading of -5 was a classic sign of a clogged crankcase ventilation valve. You can measure it using a U-shaped water manometer connected to the ventilation hose—simple and accurate. High pressure usually indicates a failed PCV valve or leaking piston rings, allowing combustion gases to escape into the crankcase; low pressure might point to an intake manifold leak or a blocked ventilation hose. Checking the ventilation system every 20,000 kilometers can prevent problems before they arise. By the way, for turbocharged cars, due to higher combustion chamber pressure, readings around -10 can still be considered normal, but it ultimately depends on the manufacturer's specifications.
Crankcase pressure is a critical factor. The normal range is negative pressure below atmospheric pressure, approximately around -20 inches of water column. I recall a certain German brand's technical manual explicitly stating it must not exceed -30. Uncontrolled pressure can lead to severe consequences, such as directly rupturing the oil pan gasket or causing the crankshaft oil seal to spray oil. As for measurement methods, a common manual pressure gauge can handle it, but beginners often overlook the temperature effect—pressure differences of three to four inches between cold and hot engines are quite common. Maintenance advice includes regularly replacing the PCV ventilation valve and cleaning the breather pipes, especially in northern areas with high dust and sand. By the way, modified car owners should pay attention: adding a turbo without matching the ventilation system often causes pressure to spike uncontrollably.
