The Subaru WRX / STI is a turbo charged high-performance automobile, and for that reason they tend to be more “needy” in the maintenance department. Below we discuss common rod bearing failures as well as Mass Air Flow (MAF) failures and what you can do about them to increase reliability in a Subaru WRX STI. 

“Knock Knock” you hear this a lot in the Subaru community. The knocking sound from a Subaru engine comes from a failure in one of the rod bearings, which is a journal bearing that relies on a small but important oil film to keep the connecting rod from making metal to metal contact with the crank shaft (an extremely important function of an engine).  Let's discuss some common issues so you know what to expect when it comes to Subaru Service.

Why Do Rod Bearings Commonly Fail on Subarus?

Well… it all has to do with oil, the oiling system, and the general small surface area of Subaru rod bearing. 

Oil contamination or “dirty oil” causes little scratches or scoring on the bearings which impedes them from keeping a consistent oil film. Once the bearings get too scored and a consistent oil film is lost, metal to metal contact occurs, and catastrophic engine failure follows. 

Turbo chargers increase cylinder pressures thus increasing blow by (combustion gasses escaping around the piston rings and into the crank case and oil sump) This contaminates the oil with combustion byproducts sooner than on naturally aspirated engines. With the Subaru EJ engines having such a small oil capacity, the oil gets saturated with combustion by products and other engine material sooner. This dirty oil wreaks havoc on bearings and causes them to wear prematurely. With the bearings already being at a disadvantage due to their size, wear also occurs sooner. 

Moreover, increased blow by, increases crank case pressure, putting load on the PCV (positive crankcase ventilation) and as the venting system goes into overdrive under boost, Subaru engines tend to burn more oil than other engines. With the small oil capacity, you can run very low on oil and cause oil starvation to the bearings. 

Lack of good synthetic oil, and infrequent changes can cause a buildup of deposits around the oil control ring on the piston causing it to jam up. This will put oil consumption in overdrive, again running low on oil and causing oil starvation.

Remedy:

More frequent oil changes (we recommend 4000 KMS, 4 months) and the use of higher grade synthetic oils (we use MOTUL) 

Yes, we know Subaru recommends a longer interval, its too long, and for good reason we don’t agree with it. We cannot stress this enough, a turbocharged Subaru needs more frequent oil changes, using a higher grade oil such as Motul. 

Subaru Oil Starvation's Negative Effects on Rod Bearings

Other causes of the catastrophic rod bearing failure is when the oil pick up to the oil pump cracks, or oil sloshing under heavy cornering. 

Subaru Is known to have made poorly designed oil pick up tubes that crack, and in the case of the later FA style engines, pickup tubes that are clogged from factory with RTV. The oil pickup is the “sipping straw” that the oil pump uses to syphon oil from the oil pan. If it cracks or is clogged up, the oil pump cannot supply oil to the engine as it should. When oil pressure to the rod bearings drop, catastrophic failure occurs. 

Due to the design of the oil pan of the Subaru boxer motor, and under heavy g forces during cornering, the oil will slosh to one side of the pan causing the oil pick up to suck up air instead of oil. This will also lead to starvation and rod bearing failure. 

Remedy:

We suggest upgrading the factory pick up tube with one of IAG Performance oil pick-ups. While the pan is off to replace the oil pick up, we suggest installing the IAG Performance oil pan baffling kit that helps with keeping the oil from sloshing away from the pick-up tube. Furthermore, IAG Performance also makes higher capacity oil pans that also further help the oiling system we suggest that go back on in place of the factory oil pan. 

Subaru WRX STI and Mass Air Flow Vacuum Boost Leaks

Ensuring your Subaru is metering the incoming air correctly and not allowing your Subaru go into a lean condition. Lean condition, or a lean air/fuel mixture in the combustion chamber means higher combustion temperatures and a higher chance of pre-detonation.

Subaru WRX / STI are MAF (Mass Air Flow) cars. Meaning that all the air incoming is measured by the MAF sensor right after the air box. If the MAF sensor reading is off because its dirty or getting up in age it can cause serious issues. Moreover, because air never gets measured again past the MAF sensor, any vacuum leaks down the line will cause unmetered air to enter, this extra air isn’t met with extra fuel in the combustion chamber causing a lean condition. 

Lean condition in the combustion chamber will cause pre-detonation, knock, pre-ignition, all terms of the same thing, is an event when the air fuel mixture combusts before the timed spark occurs. This causes the engine to fight itself and break components, and in Subarus, the weak factory pistons are the first to incur damage. 

Once the piston is damaged one of two scenarios will occur, either compression is lost and the engine will run on less cylinders which is very noticeable. Or, the ring land for the oil scrubbing ring will fail and cause major oil consumption. Most of the time this goes unnoticed as the engine still runs fine, but, its only a matter of time until the broken ring land scores the bore enough to lose compression, or the excessive oil consumption runs the engine out of oil and then “knock knock” the rod bearings fail. 

The latter scenario is why buying a used STI / WRX is a gamble if a leak down and compression isn’t done prior to purchase. Some owners will notice the oil consumption and choose to sell the vehicle or trade it in at a dealer, or auction. Since the issue is very difficult to spot in a regular inspection or test-drive many new owners of used Subaru STI / WRX models find themselves in a very tough situation shortly after purchase. 

This is why we started our “Forged Series Program” where we buy WRX STI cars with bad engines but good bodies, and “bullet proof” them to then sell them with warranty. The upgrades we do to bullet proof these Subarus is an IAG Performance forged internal block along with oiling modifications for added reliability. The IAG Performance short block includes a stronger forged piston that can withstand pre-ignition better, and higher cylinder pressures where the factory piston fails. 

Remedy:

Ensure your MAF is clean and reading correctly. It’s also not a bad idea to replace it every 70,000 KMS before it starts causing noticeable issues. 

Find and fix any vacuum leaks right away, including in the PCV system. At Lakeview Automotive we recommend a boost leak / vacuum leak test frequently to stay on top of any vacuum and boost leaks in the system. We have special tooling to conduct these tests and do them daily.

Subaru can be a bit fussy to own and maintain especially the turbo ones. Being cautious when purchasing one will go a long way. Keep in mind that oiling, vacuum / boost leaks, and pre-detonation is an important matter.