What Causes Engine Oil to Blow-By Into the Air Filter Box?

by Robert Moore

An oil saturated air filter or a puddle of oil in your air filter box seems like a strange occurrence by all means considering that its the last thing you would expect to find. Oil in the air filter or housing is known as blow-by and it can be caused by several different reasons that should be corrected immediately as it could be the result of or lead to serious engine damage. Determining the cause of the misplaced oil can be a bit tedious and will require at least one special tool that is likely available for rent at your local parts store.

Positive Crankcase Ventilation Valve

The positive crankcase ventilation or PCV valve is connected to the air intake tube via a rubber vacuum hose that uses the engines natural vacuum to prevent excessive pressure build up in the engine crankcase. In most casts the PCV valve can be removed by pulling it out of the valve cover grommet and then removing the rubber vacuum hose. Generally the PCV valve should be changed at every tune-up, but is often overlooked. Remove the PCV valve, and into the side that rests inside the valve cover to see the check valve. Shake the PCV valve, if the check valve doesn't move and a rattle is not heard, the PCV could likely be the culprit, allowing oil to pass through the valve and into the intake tube. Replace the PCV valve and check the filter and housing after 500-1000 miles, continue on if the PCV valve is properly functioning or replacing it doesn't solve the problem.

Clogged Oil Passages

Consider the vehicles routine maintenance schedule. Engine oil that becomes excessively old and worn will leave deposits in the engine oil passages over time that can build up and create bigger problems. As the oil passages become clogged it becomes difficult for oil to drain from the top of the engine down into the crankcase, cause oil to build up and pool inside the top of the cylinder head. Extremely clogged passages will cause pressure to build up and will open the opportunity for oil to push through the PCV valve and enter the air intake tube where it will drain into the air filter and filter housing. Purchase an engine flush formula from your local parts store and follow the directions. In general you will pour the formula into the oil and run the engine for a period of time. Once the engine has been flushed drain the oil and fill it with fresh engine oil. Replace the filter and see if the problem persists. In some cases you may need to flush the engine multiple times to throughly clear all the oil passages and completely solve the problem.

Worn Piston Rings

Each piston has an oil ring that allows the piston to travel up and down without oil passing the top of the piston and entering the combustion chamber. Once the oil is in the combustion chamber it will be burned off during combustion or pushed into the intake and exhaust manifolds resulting in oil coated exhaust or oil working its way to the air filter box. The classic tale tale sign of worn or leaking piston rings is a cloud of blue smoke when the engine starts or smoking while the engine is under high load. Worn piston rings can be detected by running a compression test on each cylinder. Remove the fuel pump fuse and the wire from the ignition coil to the distributor; unplug the ignition packs or individual coils for a distributor-less ignition. Install a compression tester into each individual spark plug fitting . When the tester is installed have a helper crank the engine; an accurate reading will be attained after the engine has cranked 6 times. Write down the number and repeat for each cylinder and compare them when the process is complete. Each cylinder in general should read between 140 and 160 Psi. If any cylinder tests low squirt a small amount of oil or transmission fluid into the corresponding spark plug hole and perform the test again. If the compression raises after applying oil then the piston ring for that cylinder is bad and is likely the cause of the oil blow-by.

Repairing Internal Problems

Replacing a piston ring is an in-depth process that will require that you remove the engine from the vehicle and tear it down completely. Replacing a piston ring will result in a full rebuild of the engine and should be left to a mechanic that has extensive internal engine experience if you are not comfortable performing such a repair. Repairing an internal engine problem such as a piston ring is time consuming and will tack on a hefty charge for labor, so if you cannot have the repair performed right away check the oil daily and replace the air filter anytime it becomes saturated to help keep the engine running as smooth as possible and prevent further damage until the repair can be performed.

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