Why is there oil in my air filter?

Published: 02 August 2010

Fellhopper: I decided to check the condition of the air filter on my GSX-R750 K2, it is a bit oily so I've ordered a new K&N filter to replace a standard one fitted last year (done about 11k).

I noticed some oil deposits in crankcase breather filter and a little has collected at the bottom of the crankcase breather housing. Is this normal?

The engine uses a few hundred ml of oil every 2-3000 miles and has done 22,000 in total.

It performs well and does not smoke or make any horrible noises, so am I worrying about nothing?

littleredrooster: Provided it's not absolutely soaked in oil, I wouldn't worry about it. Slight blow into the airbox is acceptable ­ sometimes caused by revving the engine when it's still cold.

surveyor57: The K&N will be oily when you get it. They come pre-oiled and should be maintained I'd say annually, by cleaning and re-oiling.

The oil traps fine particles in the induction air that would otherwise pass through the filter and into the engine.

MikeRotherham: Oil blowback through the induction system is a symptom of worn oil seals on the inlet valves.

Pressure leakage from worn seals causes the oil to be forced down into the inlet passage and back through to the inlet filter.

Typically you will get an excess of oil being blown out through the crankcase breather too as the cylinder head/rocker cover is pressurised blowing back down to the crankcase.

Best way of checking is to do a compression test on the cylinders and see what it is compared with the manual spec.

littleredrooster: Sorry mate, but I think you're a bit wide of the mark there. Worn inlet valve stem seals allow oil into the inlet tract which is then burnt in the combustion chamber.

It cannot pass to the breather. The inlet tract and valve always operates under vacuum, so cannot blow past the stem seals into the rocker box.

The only route for oil to get into the air filter is via the engine breather and usually indicates blow-by from the pistons/rings causing crankcase pressurisation.

fuzzrr: Agreed. Oil blow back into the airbox is normal. One thing you've got to think of though: if you are putting a K&N in it, and have a pipe also, are you going to get the ECU remapped? It will probably not be able to compensate for a pipe and air filter...