P0304 on Subaru

The Subaru Baja's 2.5L EJ25 naturally-aspirated engine suffers from the same chronic head gasket failure that affects all EJ25-equipped Subarus. The composite gaskets degrade over time, typically between 80,000-150,000 miles, causing external oil and coolant leaks. The Baja's engine placement and bed-over-engine design can make leaks harder to spot until they become severe. The issue affects all 2003-2006 NA Bajas. Turbo models use a different gasket design and are less susceptible.

The FA20 engine in 2013-2016 BRZs was subject to a safety recall (WQG-66 / NHTSA 18V-845) for defective valve springs that can fracture due to insufficient hardness. A broken valve spring causes a cylinder misfire and can lead to engine stalling. In worst cases, the broken spring allows valve-to-piston contact, causing catastrophic engine damage. Subaru recalls affected VINs for free valve spring replacement. The recall also affects the Toyota 86/Scion FR-S which shares the same engine.

The 2.5L EJ253 naturally-aspirated engine in 1999-2011 Imprezas is notorious for external head gasket failure. The composite head gaskets degrade over time, allowing coolant and oil to seep externally at the gasket mating surfaces. Unlike a typical blown head gasket that causes coolant-oil mixing, Subaru EJ25 head gaskets typically leak externally first. The boxer engine design means gaskets are at the bottom of the heads, making leaks drip downward and often go unnoticed. If left unaddressed, external leaks can progress to internal coolant-oil mixing and overheating.

The 2.5L EJ25 engine in 2000-2009 Legacys suffers from the same head gasket failure that plagues all EJ25-equipped Subarus. The composite gaskets degrade, causing external oil and coolant leaks at the head-to-block mating surfaces. The Legacy's higher-mileage highway use means many owners encounter this between 80,000-150,000 miles. Phase 1 engines (2000-2005, single overhead cam) are more prone to coolant leaks, while Phase 2 engines (2006-2009, DOHC) tend to leak oil externally first.

The 2.5L non-turbo engine is known for external head gasket leaks. Oil seeps past the gasket and can contaminate the coolant. Subaru extended warranties and improved gasket design in later years.

While the FB25 has improved head gasket reliability compared to older EJ engines, some vehicles still experience head gasket failures. External oil leaks and coolant leaks can develop, particularly at higher mileage.

The 3.6L EZ36 flat-6 engine uses timing chains instead of belts, but the chains and guides are not maintenance-free as often assumed. The timing chain tensioner and plastic chain guides wear over time, typically after 120,000-180,000 miles. Worn chains stretch, causing rough idle, misfires, and timing-related codes. The EZ36 uses three timing chains (one for each bank plus one for the center) making replacement a complex and labor-intensive job. If a chain jumps timing due to severe stretch or guide failure, valve-to-piston contact can cause catastrophic engine damage.

The 2006-2007 Subaru Tribeca with the 3.0L EZ30 flat-6 engine can develop head gasket leaks, typically at higher mileages (130,000-200,000 miles). While the EZ30 is generally more resistant to head gasket issues than the smaller EJ25 boxer engines, the problem does occur. The flat-6 boxer design means the head gaskets are oriented horizontally, which allows coolant to seep past the gasket under gravity when the engine is off. Symptoms include external oil leaks between the head and block, coolant consumption, and in severe cases, coolant entering the combustion chamber causing white exhaust smoke and overheating. The repair is complex due to the flat-6 layout and cramped engine bay of the Tribeca.

The EJ257 engine in the STI is known for rod bearing failures, especially when modified or driven hard. The stock rod bearings have thin oil clearances and can spin under high load conditions. Oil starvation during hard cornering can accelerate bearing wear.

The STI's EJ257 2.5L turbocharged boxer engine is prone to piston ringland cracking, particularly on cylinder 4. Pre-detonation (knock) from heat soak, lean conditions, or aggressive tuning cracks the thin ringland bridges between piston ring grooves. This causes compression loss and can lead to catastrophic engine failure.