Crankcase Vent Filter Replacement

What is the Crankcase Vent Filter all about?

The most common application for the crankcase vent filter is on high performance engines such as race cars, though it is widely used on older models of vehicles as well. Inside the engine, small amounts of combustion gases push by the piston in the cylinder and enter the crankcase, or the bottom end of the engine. These gases create a positive pressure inside the crankcase, which needs ventilation. Positive crankcase pressure robs the engine of power as the pistons have to fight against that pressure when performing each down stroke.

A breather tube is mounted at the top of the crankcase in the engine block that releases the positive pressure into the environment or into the intake to cleanly burn them before they enter the atmosphere. The breather tube contains a port or elbow at some point along its route where the crankcase vent filter is installed. The crankcase vent filter allows excess pressure to push out into the environment without carrying oil particles or other messy contaminants with the gases. It also prevents any debris or dirt from entering the crankcase if there is a situation where negative pressure occurs, such as engine cooldown. If the crankcase vent filter is plugged, the excess positive pressure inside the crankcase can blow out engine oil seals. If the crankcase vent filter isn’t in place, dirt can enter the crankcase and plug oil galleries or contaminate the oil, which can cause lasting engine damage.

Keep in mind:

  • The crankcase vent filter works contrary to how other filters operate. Its main purpose is to prevent contaminants in the engine from entering the atmosphere, while most other filters prevent contaminants from entering the engine.

How it's done:

  • The crankcase vent filter is verified that it needs to be replaced. The air filter housing is then opened.
  • The defective crankcase vent filter is removed by disconnecting the vent hose and removing the clip.
  • The new crankcase vent filter is installed and secured with a clip. The vent hose is then connected.
  • The air cleaner housing is closed. The crankcase vent filter is then tested for operation.
  • The vehicle is road tested to ensure proper operation of the crankcase vent filter.

Our recommendation:

The crankcase vent filter will eventually become plugged with oil that is carried by the exiting vapor. It should be checked whenever the engine air filter is replaced, and should be replaced at least as often as the spark plugs. If you experience symptoms that may be related to the crankcase vent filter, have one of our expert technicians diagnose and replace the vent filter.

What common symptoms indicate you may need to replace the Crankcase Vent Filter?

  • Oil on the engine
  • Plugged filter causing engine seals to leak
  • Decrease in engine performance

How important is this service?

When the crankcase vent filter is plugged, engine oil will leak when seals are pushed out. If the vent filter is missing or broken, contaminants can enter the crankcase. If the vent filter is not in place, an oily mess can occur under your hood. It is prudent to replace the crankcase vent filter as soon as needed.



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Tell us what the problem is (e.g the car is not starting or I need new shock absorbers). What kind of car you drive and your contact information.

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