Subaru’s popular ‘EJ25’ 2.5-liter Boxer engine is widely used across models such as the Outback, Forester, and Legacy. However, this engine is not without its issues. From blown head gaskets to excessive oil consumption, they have a strong reputation for reliability concerns. So, here’s what you need to know about the most common Subaru 2.5 Boxer engine problems.
What Are The Most Common Subaru Engine Problems?
Subaru’s flat-piston Boxer engines were once considered among the most durable, often surpassing 200,000 miles with minimal issues. However, over the years, with many design changes made and material choices being updated, these have led to a reputation for reliability concerns, particularly with the EJ25 2.5-liter engine.
When they’re working, a Boxer engine’s pistons look like they’re boxing each other, similar to sparring partners. Hence, that’s where the “boxer” name came from. Alas, while it makes Subaru cars quite special, they’re not immune to systemic faults. These include head gasket failures, excessive oil consumption, and oil mixing in with the coolant, among others.
The EJ25 came in two main naturally aspirated configurations: a DOHC version with a 9.5:1 compression ratio and an SOHC variant with a higher 10:1 compression ratio. Despite their differences, both engines share the same 2.5-liter displacement and similar reliability concerns, as we’ll get into later.
Either way, both engines are found in pretty much all of Subaru’s more popular models over the past couple of decades. This includes the Impreza, Outback, Forester, Baja, Legacy, and more. Unfortunately, due to inherent design flaws and engineering faults, Subaru 2.5-liter engines suffer problems like oil burning, oil starvation, and head gasket issues.
With that in mind, here are some of the most common Subaru 2.5-liter engine problems (mainly focusing on the EJ25 and its variants) that you might experience:
Subaru 2.5 Boxer Engine Problems (1) – Oil Mixing With The Coolant
The most common problem with Subaru’s 2.5-liter engines revolves around the head gaskets. However, this is way more common with some of the earlier models, as Subaru used low-quality gasket materials that were made out of composites. One of the side effects was repeated head gasket failures, causing issues like motor oil and coolant mixing.
Thus, creating a big, sludgy mess inside your engine. When motor oil and coolant mix, neither can function properly. As such, the motor oil is no longer effective at lubricating your engine, and the coolant is not able to effectively cool the engine. If not fixed promptly, this will cause excessive wear and tear with your engine, in addition to overheating.
Other consequences of having your motor oil mix with the coolant include wear on your piston rings. Over time, this will cause premature piston ring failure. Or, the camshaft bearings can also wear out prematurely. This is a chronic issue with many Subaru models. So, make sure you double-check the head gasket if you’re buying a used Subaru car.
Repair/Replacement Cost:
- Head gasket replacement: $1,000 – $1,500 (this price includes labor, new head gaskets, and fluids)
- Full engine rebuild (if the damage is far too severe): $3,000 – $5,000
Prevention & Maintenance Tips:
- You should regularly check coolant levels for contamination (i.e. milky or discolored appearance).
- Make sure that you only use high-quality, multi-layer steel (MLS) head gaskets when replacing them.
- Otherwise, ensure that the proper torque settings on the cylinder heads are used to prevent premature gasket failure.
- Additionally, remember to change your engine oil and coolant at recommended intervals to minimize wear.
Subaru 2.5 Boxer Engine Problems (2) – Oil Burning Inside The Engine
This is an extension of the previous point, but when motor oil leaks through and gets into the combustion chamber, it will start to burn. This is more commonly known as “oil burning”. When you experience oil burning with your car (not just limited to Subarus), a tell-tale sign is clearly seeing thick clouds of black smoke coming out of the exhaust.
Otherwise, instead of your car’s motor oil, if it’s coolant that’s leaking into the engine’s combustion chamber and is burning, you’ll notice white plumes of smoke coming out of your exhaust. Either way, this isn’t ideal, as it causes excessive consumption of both motor oil and coolant. Remember, both fluids are vital in the smooth running of a car.
Repair/Replacement Cost:
- Piston ring replacement: $1,500 – $3,500 (this quote includes labor and new piston rings)
- Valve stem seal replacement: $800 – $1,500 (this process requires cylinder head removal and resealing)
- Full engine rebuild (if excessive wear is present): $3,000 – $5,000
Prevention & Maintenance Tips:
- Make sure that you always check your oil levels frequently – excessive oil loss may indicate early-stage burning.
- Otherwise, only use high-quality synthetic oil and change it at the recommended intervals to reduce wear on piston rings and valve seals.
- Elsewhere, please avoid aggressive driving, as high-RPM operation can worsen oil consumption over time.
- If oil consumption becomes excessive, consider using oil additives designed to help recondition seals and rings.
Subaru 2.5 Boxer Engine Problems (3) – Coolant Leaks Or Heavy Oil Consumption
Head gasket failures with these Subaru 2.5 engines can also cause coolant leaks elsewhere. You’ll mostly notice these coolant leaks from outside the engine. This leakage will cause your car’s coolant levels to diminish. Ignore it for too long, and your car will start to overheat. And, even suffer catastrophic terminal damage, due to low coolant and poor cooling.
Additionally, besides coolant leaks, excessive oil consumption is another common fault with Subaru’s 2.5 engines. Some Subaru vehicles tend to consume motor oil at a much faster rate than a lot of other vehicles. This problem usually occurs and is exacerbated in vehicles with higher mileage. However, newer Subaru models aren’t immune to these, either.
In Subaru vehicles, this excessive oil consumption is caused by several factors. For example, worn piston rings or valve seals can cause engine oil to leak into the combustion chamber and burn off, as mentioned previously. Therefore, if you own a Subaru vehicle with an EJ25 engine, it’s wise to regularly check your motor oil levels and top it up as needed.
Repair/Replacement Cost:
- Coolant leak repair: $150 – $800 (this depends on whether it’s a hose, radiator, or gasket issue)
- Head gasket replacement: $1,000 – $1,500 (if the leak is due to gasket failure)
- Piston ring replacement: $1,500 – $3,500 (if excessive oil consumption is caused by worn rings)
- Engine oil consumption repair (valve seals, PCV system, etc.): $500 – $2,000
Prevention & Maintenance Tips:
- Make sure that you regularly inspect coolant levels and look for leaks – a drop in coolant without visible leaks may indicate internal head gasket failure.
- Use only a high-quality head gasket (MLS type) and ensure proper torque specs during replacement to prevent future failures.
- For oil consumption, use synthetic oil and change it at shorter intervals if the oil burn is excessive.
- In addition, make sure that you check and replace PCV valves if clogged, as they can contribute to oil burning issues.
- Lastly, please keep an eye on engine temperature – if overheating occurs, stop driving immediately to avoid severe engine damage.
Subaru 2.5 Boxer Engine Problems (4) – Blown Head Gaskets
While we’re on the subject of head gaskets, among the most frequently reported Subaru engine problems is blown head gaskets. You can easily spot early warning signs of this occurring. This is usually cases like engine overheating, poor performance, engine stalling, and more. You might also notice a sludgy residue when you pull out your motor oil dipstick.
This is due to, as we mentioned earlier, motor oil and coolant mixing together. If not solved promptly, this problem can eat into your bearings and your crankshaft. Ultimately, it could even cause your engine to seize up, and even scrapping the engine altogether. You should also pay close attention to the color of your exhaust smoke to pinpoint the cause.
Blue or black smoke means that your engine is burning oil, whereas white smoke means that it’s burning coolant. Unfortunately, replacing the head gasket isn’t cheap – it’ll cost anywhere from $1,000 to $1,500. While the cost of the actual head gasket material itself isn’t too expensive, your Subaru’s engine needs to be disassembled. This costs a lot in labor hours.
Repair/Replacement Cost:
- Head gasket replacement: $1,000 – $1,500 (this quote includes labor, new gaskets, and fluids)
- Engine rebuild (if the head gasket failure causes internal damage): $3,000 – $5,000
- Coolant flush and oil change (if contamination occurs): $100 – $300
- Radiator replacement (if overheating damages it): $400 – $900
Prevention & Maintenance Tips:
- Ensure that you monitor your coolant and oil levels regularly, as sudden drops can indicate early head gasket failure.
- Only use high-quality, multi-layer steel (MLS) head gaskets instead of OEM-style composite ones when replacing.
- Make sure that your cylinder heads are properly torqued to prevent uneven pressure on the gaskets.
- Avoid prolonged overheating – address any cooling system issues (radiator, thermostat, & water pump) immediately to reduce stress on the head gasket.
- Furthermore, only use Subaru-specific coolant and change it at the recommended intervals to prevent corrosion and leaks.
Subaru 2.5 Boxer Engine Problems (5) – Oil Starvation Problems
Oil starvation is when parts of your engine are left without motor oil and proper lubrication. For instance, if you’re driving a car on track and pull high-G maneuvers, your motor oil will float from one side to another. In this scenario, without proper mods, there are moments when parts of your engine are deprived of oil and aren’t properly lubricated.
Without an adequate amount of oil, the metal parts of your engine will start to rub and grind against each other. This excessive friction will start eating away at components such as cylinder walls, crankshaft bearings, and more. This is worse with Subaru’s EJ25, where the crankshaft bearings are known to wear out prematurely and rapidly if there isn’t enough oil.
Boxer engines, due to their low placement, also mean the motor oil sump can’t hold enough oil. And, some Subaru cars have inadequate oil pickups, which doesn’t help with the oil starvation issues. It’s wise to not ignore these issues, as running your engine with insufficient oil can cause serious problems. This includes engine seizure and in time, outright failure.
Repair/Replacement Cost:
- Crankshaft bearing replacement: $2,000 – $3,500 (note, this requires extensive engine work)
- Full engine rebuild (if internal damage is severe): $3,000 – $6,000
- Oil pump replacement (if low oil pressure is a factor): $300 – $800
- High-capacity oil pan and pickup upgrade (to reduce oil starvation in performance driving): $500 – $1,500
- Regular oil change (to maintain proper lubrication and avoid sludge buildup): $50 – $150
Prevention & Maintenance Tips:
- Only use high-quality synthetic oil with the correct viscosity to ensure proper lubrication under all driving conditions.
- Make sure that you regularly check oil levels and top up as needed – low oil levels significantly increase the risk of oil starvation.
- Elsewhere, consider upgrading to a high-capacity oil pan and an improved oil pickup if you track your Subaru or drive aggressively.
- Avoid excessive high-G cornering without modifications, as this can cause oil to move away from the pickup tube.
- You should also change your oil filter regularly to prevent oil starvation due to clogging or poor oil circulation.
- If you notice low oil pressure warning lights, address the issue immediately to prevent catastrophic engine failure.
Subaru 2.5 Boxer Engine Years To Avoid
With that in mind, if you’re considering buying a Subaru with the 2.5-liter Boxer engine, it’s vital to know which model and model years are more prone to engine problems. Now, it’s worth bearing in mind that Subaru has made a ton of improvements to the EJ25, with the 2013+ Outback, 2013+ Legacy, 2014+ Forester, and 2015+ Impreza featuring rather reliable engines.
However, do avoid the 1999 to 2011 models, unless they’ve had head gasket replacements and proper maintenance, due to their poor reliability. 2012 and newer models tend to be more reliable, though regular oil level checks and maintenance are still essential. This is because, from 2012 onwards, Subaru introduced an upgraded EJ25.
The 2012+ 2.5-liter Boxer engines come with better head gasket materials and improved piston ring designs, which significantly help with reducing oil consumption and head gasket failures. Still, if you are planning on buying a used Subaru with the 2.5-liter Boxer engine, avoid these ones like the plague for minimal problems down the line:
| Model | Problematic Years | Common Issues |
|---|---|---|
| Subaru Outback | 2000-2011 | Head gasket failures, oil leaks, coolant mixing with oil, and overheating |
| Subaru Forester | 1999-2010 | Head gasket failures (1999-2004), and excessive oil consumption (2005-2010) |
| Subaru Legacy | 2000-2009 | Blown head gaskets, coolant leaks, and oil burning issues |
| Subaru Impreza | 1998-2011 | Head gasket failures, and excessive oil consumption (non-turbo models) |
| Subaru Baja | 2003-2006 | Head gasket failures, and excessive oil consumption |