Available in multiple configurations and versions, Subaru’s 2.5L four-cylinder ‘boxer’ engine has been a mainstay in the brand’s lineup for many years. We review its best aspects.
Of the main Japanese car companies, Subaru alone has a reputation for being a bit quirky. Standard all-wheel drive on most of their cars, unique styling, and a boxer engine help to separate them from the Toyotas of the world.
When it comes to these unique boxer engines, the heavyweight of the group is the 2.5-liter four-cylinder. Available in a majority of the lineup, the “EJ” and newer “FB” have been under the hood of Subarus for a combined 32 years.
Let’s take a look at what the keys to success have been for this unorthodox engine design.
Subaru’s 2.5L EJ Engine: Why Begin with a Boxer?
We start with the basics: All EJ-series engines share a 16-valve, flat-four horizontal configuration with 1.5- to 2.5-liter displacements. But why a boxer to begin with?
Karl Benz (yes, of Mercedes-Benz fame) patented the flat-four engine in 1896, and Subaru introduced it in their lineup in 1971.
Motor Trend explains, “the engine was configured to be inline with the transmission, and it had a lower center of gravity to help minimize body roll in comparison to inline-four and V-type engines, which have a higher center of gravity and are prone to side-to-side vibration.”
The design was also lighter and compact, which is important for a young car company that was trying to get its feet on the ground.
EJ Engines: Naturally Aspirated or Turbocharged
As you’d expect from an engine that has a 30-year history, the EJ could be had in a variety of configurations. There is a less complex Single Overhead Cam (SOHC) or more efficient and powerful Dual Overhead Cam (DOHC) design.
The DOHC engines are more difficult to service when it’s time for a spark plug change. So, those engines get platinum-tipped plugs, which extends service intervals to 80,000 miles. You can have your EJ in naturally aspirated or turbocharged form.
All DOHC and 1998 or newer SOHC EJ engines are interference engines, so if the timing belt fails, the valves will likely be damaged. It’s recommended to replace the timing belt every 60,000 miles to prevent this from happening.
We’ll focus on the 2.5-liter engine specifically, which was introduced in the Legacy 2.5GT and Outback in 1996. Turbocharging was introduced on the 2005 Legacy and the 2005 revision of the WRX.
Car and Driver noted the difference from the 2.0-liter version in a 2006 WRX. “The newfound power and torque are available at lower rpm, eliminating much of the lifeless feel the WRX had before the turbo came bursting into the room around 4,000 rpm.”
The EJ25 will soon be discontinued, with the current WRX STI being the last model to receive it.
EJ25 Engine Problems & Known Issues
Subaru has an excellent reputation for reliability, but there are always issues to be aware of, and the EJ is no different. Some problems experienced with the EJ engine include a faulty fuel pump that could lead to engine stalling, and an engine computer that has the potential to keep powering the ignition coil even after the motor’s shut off.
FB25 Engine: Lighter Equals More EfficiencyThe FB25 was introduced in 2011 with the Forester, and though the displacement of both the EJ25 and the FB25 round to 2.5 liters, the FB is slightly larger, at 2,498 ci versus 2,457. There are again non-turbo and turbo versions.
Subaru says the main motivation for the new engine was improved efficiency, and they claim a 28% reduction in friction losses, with the biggest contributors being lighter pistons and connecting rods.
All of this adds up to just one additional mpg of range, but that’s actually a 5% increase, and it’s deceiving because the FB engine was initially paired with older transmission designs.
With standard all-wheel drive, Subaru’s vehicle offerings will often have less range when compared to a front-wheel-drive competitor.
This new engine is also chain-driven (as opposed to the EJ’s belt), which is a more robust setup that allows for a new block design with fewer emissions. It also has variable-valve timing to improve power and efficiency.
“Performing these same upgrades to the EJ would have resulted in a much heavier engine,” reports Car and Driver.
Jalopnik loved the boxer/AWD combination. “To Subaru’s credit, the Boxer/AWD formula remains brilliant,” they said. “While carmakers are throwing all the technology they can at their all-wheel-drive systems to make sure young families don’t fall off a cliff in their crossover, none of them will ever match the low center of gravity of a horizontally opposed engine.”
FB25 Engine Problems & Known Issues
Some of the FB series engines consume oil, with owners needing to top off the oil level every few thousand miles. YouTuber mechanic Mr Subaru 1387 tells us that, “this raises some concern when it comes to longevity,” and that “as they wear, oil consumption will rise.” This could lead to deposits on your pistons, spark plugs, and oxygen sensors. Not helping matters much, is the oil spec for this engine — a very light 0W-20.
MDH Motors took a failed FB25 apart to find, “This engine burned oil so badly that it damaged the catalytic converter and completely burned up the exhaust valves.” To solve this, Subaru has issued a Technical Service Bulletin that replaces toe piston rings after inspection, if they determine the engine is consuming too much oil.
There was also a recall for engine valves that can fail, according to Consumer Reports in 2019. “Subaru is recalling certain Crosstrek, Forester, and Ascent SUVs and Impreza hatchbacks and sedans because they can suddenly lose power while driving, which could cause a crash,” says Consumer Reports. “A faulty part in the engine could allow oil to enter places it shouldn’t. Pieces of the part could also separate and damage other engine components, which could cause the engine to suddenly lose power.”
Differences Between the EJ and FB Engines
The EJ was phased out in the late 2010s because of limitations in its design, such as port fuel injection verses newer direct injection in the FB engine.
Over at EngineBuilder, Justin Anning of Nemesis Performance explains: “It is a completely different design than the EJ, although it still remains an opposing four-cylinder. It has direct injection technology, a mechanical fuel pump, and asymmetrical connecting rods.”
Engine Works states: “In comparison with the EJ25, the FB25 is more responsive at low speeds. Having an identical output, it achieves lower rpm and higher torque at lower rpm. Moreover, it consumes less fuel.”
Should You Buy a Used Subaru with Either the FB25 or EJ25 Engine?
We’ll use the Forrester as our example, as it’s Subaru’s most popular model.
Autotrader gives an important tip, “Those seeking to spend a bit more should jump to the fifth-generation Forester, preferably a 2016 or newer, as these models don’t suffer the oil consumption issues that plagued some 2014 and 2015 models.”
Subaru offers certified vehicles that are subjected to a 152-point inspection and a seven-year/100,000-mile warranty on the powertrain, so that should offer some peace of mind in regards to the longevity of your Subaru engine.
A Subaru offers a lot of advantages over its competition, and with an overall dependable reputation and the safety of standard all-wheel drive and, you’ll be able to enjoy many trouble-free miles with your Subaru 2.5-liter engine, regardless of the model you choose.
Photos: Subaru
