Key Takeaways

  • Subaru's EE20 diesel boxer engine was the world's first flat-four diesel engine in a passenger car, but it had reliability issues, including crankshaft and block failures.
  • Subaru had plans to develop a hybrid version of the EE20 engine, but due to increasingly stringent emissions standards, development of the engine was halted in 2016.
  • Selling diesel-powered cars in the US would have required substantial investment to meet strict EPA regulations, making it a wise decision for Subaru not to launch the diesel cars in America.

As far as engines go, Subaru's EJ and FA series are among the most recognized in the automotive space. You have the EJ20, EJ25, and FA20 as the flag bearers in the boxer-four lineup. That said, Subaru, in other parts of the globe, was known for making "non-performancy" daily runabouts, albeit in diesel form. In addition to some of the most throaty and emotive flat-four engines, the Japanese marque also introduced the world's first flat-four diesel in a passenger car, dubbed the EE20.

Subaru unveiled the world's first boxer diesel engine at the 2007 Geneva Auto Show. Per the announcement in Geneva, the EE20 was sold initially in the Subaru Legacy (wagon and sedan) and the Subaru Outback for the European market. Subaru also had grand plans for its diesel boxer engine as a 2013 concept came with a hybrid version of the EE20 boxer engine.

However, the boxer engine wasn't particularly robust. As is popular with many Subaru engines, reliability isn't their forte, and the EE20 was no exception. Admittedly, in Subaru's defense, some engines have surpassed hundreds of thousands of miles with just basic maintenance and no serious fettling. Earlier EE20s, however, do not fall in that category.

Updated August 2023: We have updated this article with more Subaru EE20 diesel boxer engine trivia. More specifically, about its reliability, problems, and why it wasn't necessarily a masterstroke from the folks at Subaru. We also give you an insight into Subura's now-shelved plan for a hybrid EE20 diesel powertrain.

RELATED: Here's How Porsche And Subaru Perfected The Boxer Engine

Everything You Need To Know About Subaru's EE20 Diesel Boxer Engine

Subaru-Impreza_XV-2010-boxer diesel
Subaru

Engine Specs

Manufacturer

Subaru

Production Years

2008-2020

Configuration

Flat-Four turbocharged

Displacement

2.0-Liter

Power

145-150 hp

Torque

260- lb-ft

Fuel

Diesel

Noteworthy Applications

2008 Subaru Legacy, 2008 Subaru Outback, 2008 Subaru Forester, 2008 Subaru Impreza, 2010 Subaru Impreza XV, 2013 Subaru VIZIV Diesel Boxer Hybrid Concept

The EE20 engine was conceptualized for Subaru's entry into Europe's increasingly popular diesel segment. Fuji Heavy Industries (FHI)—Subaru's parent company—spent three years developing the EE20. After a decent reception following its 2007 reveal, Subaru, late next year, extended the diesel lineup by offering a slightly detuned version for the Forester and Impreza models sold in the European market.

The engine in question is a 2.0L DOHC turbo flat-four, producing 150 horsepower (145 hp in some versions) and 260 lb-ft of torque. In terms of technical specifications, you get a 1,998cc engine, four valves per cylinder with 86 mm bore and 86 mm stroke. The Subaru diesel engine had a very square design, ran 16.3:1 compression, and redlined at 4,750 rpm. EE20 engines were initially introduced with Euro-4 compliance. However, later stages saw the engine family becoming Euro-5 and Euro-6 compliant.

In 2016, citing increasingly stringent emissions standards, the top brass at Subaru halted further development of the EE20. In March 2018, Subaru's parent company discontinued diesel engine production, shifting its focus to plug-in hybrids and electric vehicles.

Subaru Envisioned A Future With Hybrid EE20 Boxer Diesel Engines

2013 Subaru Viziv Concept with a hybrid diesel boxer engine front third quarter ariel view
Subaru

The VIZIV Concepts were a series of futuristic Subarus showcased from 2013 through 2019. It started with the sleek-looking VIZIV SUV Concept shown above and ended with the 2019 VIZIV Adrenaline Concept that looked like a Subaru Crosstrek from the future. The 2017 Subaru VIZIV Performance Concept also rekindled our hope of an all-new Impreza WRX STi.

But the most ambitious part of the VIZIV concepts that many missed was Subaru's idea of plinking in a hybrid diesel powertrain. The 2013 VIZIV SUV concept was envisioned as a plug-in hybrid model. It uses the 2.0-liter EE20 boxer diesel engine with three electric motors (one for the front axle and two for the rear). The gearbox of choice is Subaru's Lineartronic (CVT) gearbox, which is attached to the front axle motor. The gearbox worked in perfect harmony with the famed Subaru Symmetrical AWD system.

The VIZIV SUV concept was Subaru's vision of an off-road-ready, family-friendly, and highly efficient diesel-powered future crossover. Subaru's hybrid diesel powertrain was also imagined in cars like the 2014 VIZIV-2 Concept, the 2015 VIZIV Future Concept, and the VIZIV-7 three-row SUV Concept.

Are Subaru EE20 Diesel Boxer Engines Reliable?

Subaru-Impreza_XV-2010-boxer diesel
Subaru

Subaru is a bit behind in being a reliable transport compared to its adversaries. However, that's not necessarily the case with all Subarus. There are examples of Legacys and Outbacks registering 200k miles without serious issues. But that doesn't mean all Legacys, Outbacks, Foresters, and Imprezas with the EE20 are super reliable. It boils down to a few basic things, like routine maintenance, conservative driving, and engine design. The latter was a problem in the early Subaru EE20 diesel engine.

It's worth noting that mileage (in the context of old cars) isn't that accurate of a metric when it comes to reliability. The reason is that a high mileage car that's been cared for could be far more reliable than a beat-up car that's done 80k. It goes to show that if you take care of something, it will last.

Subaru EE20 Diesel Boxer Engine Problems: Crankshafts And Blocks Failing​​​​

2009 Subaru Forester Diesel front third quarter view
Subaru

Okay, so what about the EE20's reliability? Forum members have mixed reviews, with most owners claiming the engine is problematic. Some of the key issues were DPF (Diesel Particulate Filter) soot/regeneration, oil dilution, and associated emission control issues.

Perhaps the most commonly discussed problem with the EE20 is the crankshaft or bottom-end failure. This is a big no-no in any car, let alone a Subaru. However, it's common in most Euro-4 engines (and some Euro-5 versions) made between 2008 and 2013. Euro-6 EE20 owners haven't reported experiencing crankshaft breakages or block cracking.

  • Problem Owners Report: Knocking sounds from the engine, major power loss, rough gearbox, weak clutch, glow plug failure, leakage in the induction system
  • Highest Mileage Reported On Carsurvey: 211,000 miles (1995 Subaru Legacy GL 2.0 from Norway)
2010 Subaru Impreza XV front third quarter view

According to a Subaru Forester forum member, EE20 crankshafts are breaking due to the engine design. More specifically, the way the block is manufactured. By design, the EE20 diesel blocks have an aluminum casting with steel inserts in the crankshaft supporting bridges instead of one solid piece of aluminum. When the cylinders and undersides were machined, the machining tool left stress cracks in the metal, weakening the bridges' structural integrity.

The cracks develop further during regular usage, extending to the crankshaft and then to the steel insert, leading them to crack and fail. Failure mileage is typically between 80,000 and 130,000 kilometers (approximately 50,000 and 80,000 miles). That said, it's a matter of luck. Some users have experienced failure at extremely low odometer readings, while others were fortunate to have escaped without issues until 200,000 kilometers.

Subaru Not Selling Diesel Boxer Cars In The US Was A Good Idea

Subaru Legacy Diesel front third quarter view
Theaa

Currently, on average, in the US, one gallon of gasoline costs $3.98, and one gallon of diesel costs $4.38. Diesel has always been more expensive than gasoline in the US. In almost every other country where diesel cars are prevalent, diesel is cheaper than gasoline. This is one of the main reasons that makes diesel-engined cars more "economical".

But getting to sell diesel-powered cars in the US is no small feat. The US EPA regulations are much stricter than the already tight-knit Euro emission norms. The permissible NOx per mile emission as per EPA is under 0.10 g. The Euro emission standards are much more lenient, with a permissible rate of over 0.20 g of NOx per mile.

So, to cater to the stringent EPA norms, Subaru would have had to invest hundreds of millions of dollars to make its EE20 diesel boxer engine complaint for the US. Looking back at how the EE20 boxer diesel engine failed, it was wise for Subaru not to try and launch it in the US.