A little bit of oil in air filter check hose

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  • Hello.

    During the regular services I have found a little bit of oil in my air filter check hose.

    Can this be the result of too much oil in the engine during last season or revving bike high and hard,

    or it has always to be something serious like worn piston rings, blocked oil passages and similar?

    : puzzled

  • No, it's quite normal. That's what that hose is for.

    The reason for this oil is the crankcase ventilation system. Piston rings never seal perfectly, so you always get some gases that push past the piston into the crankcase during combustion. This would normally increase the pressure in the crankcase which would make the engine less efficient and cause sealing problems at the crankshaft radial seals. That's why you want to put in a vent, so the crankcase pressure is only slightly above ambient pressure. A long time ago, this was simply vented to the outside. For emissions / environmental protection reasons, this was forbidden quite some time ago (first CARB legislation I think) and it was mandatory to reintroduce these gases to the intake system. The gases always carry a little bit of oil mist with them. There is always some sort of oil separation system, but in motorcycles it is not very sophisticated yet, because there is no real need for it. For state of the art turbocharged direct injected car engines with the newest emissions legislation, these systems are a lot more complex. The reasons are that the small oil droplets can cause pre-ignition and also they are bad for raw emissions of the engine.

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