You have a Subaru, and someone asked if it had an interference motor, but you couldn’t answer them. What is an interference motor, and which Subarus have them?
With so many types of engines being put into vehicles today, it is easy to get confused. You know that Subaru makes a quality line of cars and SUVs, but you'd like to know more since you’ve got one sitting in your drive. What are some of the differences in motors, and why is it important? The last thing you want to have is someone asking you a question about your car you can’t answer. Knowing some details about your Subaru motor can also help you the next time you need to take the car into the shop and prevent you from overpaying for the work the mechanic tells you is needed.
All Subaru models from 1997 to today have interference engines (including the 1996 2.5L phase 1, non-interference piston to valve, but interference valve to valve). All prior models were non-interference engines.
Whether an engine is interference or non-interference sounds complicated, but it isn’t. The internal combustion engine is a series of internal pistons that move up and down in time with valves and a fuel injector. (While the injector squirts a tiny amount of gas into the chamber, the valves allow air in before the spark, and the exhaust is released after the combustion happens). When the piston moves up, it compresses the air mixed with gasoline, and then as a spark occurs, the plug ignites the mixture to create the combustion needed to drive the car. If the piston can “interfere” with the valve's regular operation, it is an interference engine.
After serving several years in the auto service industry, I found that most people did not know the difference (most didn’t care). Still, there is a reason that Subaru continues to use the specific design they have for their engines. To help explain why that is, let’s explore the world of Subaru engines together and see if we can’t educate ourselves a bit about which Subaru vehicles have interference engines and why that’s so important.
Which Subaru Engines are Interference Engines?
Every recent Subaru engine from 1997 to the latest model has interference engines. If you might have an older Subaru in your drive or are considering purchasing one, here is a list for easy reference. (Source: scoobyenthusiast.com)
Is a Subaru 2.5 an Interference Engine?
All Subaru 2.5 L engines are interference engines, except for 1996 2.5 L Phase 1 (interference valve to valve, but non-interference piston to valve).
Subaru has experimented with a 2.5 L 16 valve flat boxer-styled engine that produces 155 - 173 horsepower depending on the year. The engine has a unique design in that the pistons lay side by side and push towards one another, rather than up and down like most conventional engines. (Imagine two boxers punching each other- hence the name “boxer” engine).
The advantage of the boxer engine design is that it provides a smoother ride and better fuel economy. The pistons do not have to work as hard because they move from side to side. Perhaps the most significant benefit of the design of this engine is that the lower center of gravity allowed the engine to be set lower. In turn, the lower engine pushes the motor under the vehicle's passenger compartment when the vehicle is involved in an accident. This design significantly reduces the severity of injuries when the car is in a front-end collision.
The first vehicle to use the 2.5 L engine was the 1994 Legacy, which used the EJ25D for several years. Before long, the boxer-style engine was showing up in different models, including the Outback, Impreza, and Forester. The table below indicates just what models have 2.5 engines.
Is the 2.5 L a Reliable Engine?
The short answer is yes. Many owners of older Subarus find that their vehicles can go well above 200,000 miles with proper maintenance.
Over the years, there have been some commonly reported problems with the 2.5 engine on older model Subarus: head gasket leakage is the primary concern. The head gasket for these engines was graphite coated, which tended to disintegrate after 100,000 miles and then needed replacing. In addition, because the engine moves sideways rather than up and down, oil can leak out more easily. Subaru extended its powertrain warranty to 10yr/100 k miles in response to consumer complaints and now uses a head gasket not covered in graphite.
In addition, there were some complaints about the 2005 - 2015 models for excessive oil consumption. While no recall was ever issued, the company agreed to replace any excessively consuming oil engine in response to a class-action lawsuit. The lawsuit affected about 650,000 owners of the Subaru Outback, Forester, Legacy, Impreza, and Crosstek models.
What Kind of Engine Does Subaru Use Now?
Currently, Subaru is the only auto manufacturer to use the boxer-style engine in all of its vehicles. The Forester, Impreza, Crosstrek, Outback, WRX/STI, Legacy, and BRZ all have this engine type. The 2.0 Turbo boxer engine is one of the best engines today, an award they have won for multiple years. The boxer engine continues to be an important part of why Subaru vehicles have the reputation of being considered some of the most reliable cars on the road.
Subaru has introduced a hybrid version of the boxer-style engine on their Crosstek Hybrid in the last couple of years, which gets an astounding 90 MPGe. Subaru has plans to launch a hybrid version of their Outback with a 1.8 turbocharged engine in a joint venture with Toyota. Best estimates are that the vehicle will be introduced in 2025 when the Outback is due for a major redesign)
What are the Advantages of a Boxer Style Engine?
There are several reasons why Subaru continues to use the flat boxer-styled engine.
The Comfort of Ride
Because the engine is lying across the engine compartment, the lower and flatter style provides more balance to the engine. Steering performance is better because the engine provides a lower center of gravity, so it handles better around a corner (less engine lean). Other internal combustion engines endure more forces during a turn because they are taller (think of the wind forces a taller building endures vs. a single-story home).
Longer Life for the Components of the Engine
The engine uses less energy to accomplish its task in a boxer-style engine because it pushes horizontally rather than vertically. The easier an engine’s components can perform their routine, the longer those components last (all other things being equal). This design is a reason many Subaru engines last for a long time.
Better Fuel Economy
The boxer-style engine design increases fuel economy because the engine and transmission do not have to work as hard to accomplish their responsibilities. In short, the Subaru burns less fuel than many other models.
Due to the lower center of gravity, when something (like another car) impacts the front of the vehicle, the boxer engine tends to be pushed below the passenger compartment. Think of having a flat crate versus a tall box being pushed back toward the car's firewall, and you will understand how the crate could do less damage). The flat design of the boxer engine means there is less likelihood of that happening because there is less mass contact with the cabin.
What are the Disadvantages of the 2.5 Interference Engine?
As its name implies, the interference engine has the possibility of the pistons interfering with the valve’s regular performance. This event rarely happens because everything depends on the proper timing of the movement of the pistons vs. valves. Your car’s engine depends on a timing belt or chain to get the movement of the pistons just right, and if the belt breaks, then the failure could result in bent valves and cause severe damage to the engine.
In addition, the flatter style engine made changing sparkplugs very difficult, particularly in older models. While the company attempted to address the issue by using extended life plugs, Subaru owners have constantly complained about this over the years. Subaru currently recommends an NGK plug (iridium spark plugs) that are more expensive than normal sparkplugs you might find elsewhere.
Are There Recall Issues for Some Subaru Engines?
There was a time when Subaru engines were known for their quality and performance. Many older engines are pushing over 200,000 miles, and many owners have had relatively minor problems with their vehicles. While older models do sometimes have head gasket issues, for the most part, they have stood up well over the years.
However, even the best car manufacturers can move too quickly into production to boost slagging sales. Attempting to gain market share, Subaru decided to introduce a motor that is encountering severe issues. In 2021, Subaru issued a recall for 165,000 vehicles for a faulty fuel pump, which could cause the car to stall and contribute to an accident. The following vehicles listed took part in the recall.
- 2018 - 2019 Subaru BRZ
- 2019 - 2020 Subaru Ascent
- 2018 Subaru Forester
- 2018 - 2020 Subaru Impreza
- 2018 - 2020 Subaru Outback
- 2018 - 2019 Subaru WRX
How Safe Are Subaru Vehicles?
Subaru has a history of making very safe vehicles. The company has received more Top Safety Picks since 2013 than any other manufacturer. During that time, 57 vehicles have been recognized for this award by the IIHS. The list below indicates the safety rating for their current lineup of vehicles (2021 models rated).
- Subaru Crosstrek Hybrid (TSP+ for 3 consecutive years)
- Subaru Forester (TSP for 15 consecutive years)
- Subaru Legacy (TSP for 16 consecutive years)
- Subaru Outback (TSP for 13 consecutive years)
- Subaru Ascent (TSP for 4 consecutive years)
- Subaru Crosstrek (TSP for 10 consecutive years)
- Subaru Impreza (TSP for 14 consecutive years)
- Subaru WRX (TSP for 8 consecutive years)
How Reliable are Subaru Models?
Based on statistics from Reparipal.com, Subaru has a 3.5 out of 5 reliability score. The average maintenance cost is $642 per year, slightly above average. Edmunds.com has a similar figure of just over $800 per year for the first five years of ownership.
The Subaru Ascent rates as one of the most unreliable vehicles by Consumer Reports in 2022. While the SUV did very well in the road test, the mid-sized SUV received a one out of five reliability rating, with a 60 out of 100 score. The testers at CR project owner satisfaction to be three out of five. Problem areas for the Ascent are transmission issues, braking system failure, and climate control systems. The 2019 Ascent, Outback, and Legacy models have multiple recalls on transmission issues and a faulty PCV valve, and a second recall for driveshaft bolts that tend to loosen when they shouldn’t.
About THE AUTHOR
I've spent many years selling cars, working with auto detailers, mechanics, dealership service teams, quoting and researching car insurance, modding my own cars, and much more.Read More About Charles Redding