Why Does My Subaru Use Oil?
For this article I am focusing on oil consumption and not external leaks from the engine, meaning if you check your oil, find it low, it’s not due for an oil change and there are no major visible leaks found with your engine, than this article applies.
If you’re a good car owner you DO periodically check your oil, I suggest every other tank of fuel, but it’s actually suggested by Subaru to do so every tank of fuel.
Why is this important? Over time the internal components of your engine such as the piston oil control rings, valve stem seals, valve guides and cylinder walls can all wear to the point where more oil is actually introduced into the combustion chamber than originally designed. This is a completely normal thing to happen in an internal combustion engine, and as long as the engine isn’t repeatedly ran low or worse out of oil, it won’t really affect the performance of the vehicle. As the engine ages it will reach a point where the piston oil control rings can no longer expand enough to slow the consumption of oil, the engine wears a little, every day you use it. Just like the more you walk in your shoes the quicker the bottoms wear out until one day your foot is wet, one day you will check your oil and it will be low where it hadn’t been before, this is the critical point in used vehicle ownership where staying on top of your maintenance aspects will avoid costly repairs, and not will mean a new engine. Some engines won’t use a drop of oil until the 2500 mile mark and then use a quart in a few hundred miles; this is mostly because of the dilution affect of the oil in the crankcase which I will address in a bit.
The key here is to know your car, and know the point at which it may use a little oil and stay on top of it! If you don’t check your oil how will you know if it’s low? Until the current era Outback limited models there has never been an oil level sensor on a Subaru. Prior to that Subaru used a low oil pressure light, but typically no oil pressure means the damage has in fact been done.
When I worked at the dealer we dealt with new Subaru vehicles that used some oil from day one, and unless it was more than a quart every 1000 miles there were never any warranty repairs authorized. As the internal combustion engine doesn’t achieve 100% combustion as its not 100% sealed it can never not use some oil as part of the combustion process.
Factors that can increase oil consumption are engines that don’t always reach operating temperature, vehicles that have “slow” front air fuel sensors or anything else affecting fuel trim allowing the car to have an excessively rich fuel mixture or problems with the 2 different crankcase ventilation systems. The flash point of your motor oil is one major factor in consumption the more diluted the oil is with unburned fuel the lower the flash point becomes, the easier it is for oil in the combustion chamber to be burned and head out the exhaust with the rest of the byproducts of combustion. What is supposed to happen is the un burnt fuel is designed to re enter the intake manifold and be part of the combustion process, but in order for that to happen the proper temperature must occur for oil vapor to separate from oil liquid. During the winter, stop and go driving and during short trips to the store this just doesn’t happen.
The key here is to be proactive and the more modern cars become the more detached we become of the simple everyday basic aspects of a car, the need to check the oil, the tires, the fluids and the bulbs will never really go away and while systems may be added to cars they are done with a price. An example of this is the low tire pressure monitoring system that adds about $500.00 to each new car now but was closer to $1000.00 just a few years ago.
You can have an engine that uses a quart of oil every 1000 miles and run like that for 300,000 miles as long as you don’t let it run too low repeatedly, or worse out of oil just once. Understanding your car is the only way to avoid costly repairs and life without your Subaru.
Pictured below is a piston from a 2.5l Subaru engine, the lower set of rings are the oil control rings and they are collapsed causing excessive oil consumption.
Thanks for reading, and check your oil!