Subaru FB25 Engine Problems, Reliability, Specs

The 3 Most Common Subaru FB25 Engine Problems

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The Subaru FB25 engine was introduced in the 2011 Subaru Forester. It’s also in the Legacy, Outback, and Crosstrek models in North America. The FB25 is a 2.5L flat-4 engine delivers 170-182 horsepower and 174-176 lb-ft. It may not offer incredible performance, but its plenty for most. The engine also offers a good balance of reliability and fuel economy. However, no engine is perfect and that applies here. In this guide, I discuss Subaru FB25 engine problems and reliability.

Subaru FB25 Engine Problems, Reliability, Specs

Common Subaru FB25 Engine Problems

  • Carbon build-up
  • Oil consumption
  • Cam carrier seal leak

In the rest of this article we address the above Subaru FB25 engine problems in-depth. So far, the FB25 is looking to be a reliable engine. Again, no engine is perfect so don’t let the few possible issues scare you away. Anyway, at the end of the article I circle back around to overall reliability.

1) Carbon Build-Up Issues

First up is an issue unique to the direct injection FA25D engine in 2019-2020+ models. It’s a topic that comes up often for DI engines. Engines produce some oil blow-by that makes its way onto intake ports and valves. Traditional port fuel injection sprays fuel into these intakes ports. Detergents then wipe away any oil deposits.

Direct injection doesn’t have that same benefit. Fuel is sprayed directly into the cylinders, so there is no fuel flow to clean intake valves. Over time, carbon deposits form on FB25 intake valves. Yes, it is true modern engines are better at managing blow-by into the intake valves. However, without fuel flowing over them it’s impossible to completely eliminate carbon build-up.

It’s likely many Subaru FB25D engines will go their whole lives without addressing this problem. It usually doesn’t pose any major threats to engine longevity. Although, we highly recommend cleaning the intake valves every 100,000 to 120,000 miles. Excess carbon deposits can cause significant power loss and other drivability problems.


  • Engine misfires
  • Stumbling / hesitant acceleration
  • Rough idling
  • Power / performance losses

As carbon deposits form they begin restricting air-flow into the cylinders. This can cause engine misfires, which then often lead to stumbling and rough idle. These symptoms usually begin once carbon build-up becomes pretty severe on the FB25 engine.

One of the biggest symptoms and concerns with carbon deposits is loss of power. It’s a really hard symptom to notice, though. Carbon build-up forms over the course of tens and tens of thousands of miles. As such, power loss is gradual rather than something you’ll notice immediately.

Walnut Blasting / Intake Valve Cleaning

One of the best ways to clean Subaru 2.5L flat-4 carbon deposits is walnut blasting. It’s a process many use since it’s very effective at cleaning valves and ports. Walnut blasting uses walnut media shells and a heavy-duty vac. The media shells are then blasted into the intake ports to knock off any deposits.

There are some other methods that are effective, but more intensive and time consuming. Of course, accessing intake valves means pulling the manifold so it’s usually at least 2-4 hours of labor. Expect about $350-600 for walnut blasting or similar services.

Using intake sprays can help slow the carbon build-up process. It’s usually not completely effective, but if used regularly it can buy extra time between full intake valve cleanings.

2) Excess Oil Consumption

Oil consumption is never an easy topic to write about. It’s always a bit vague since plenty of engines burn oil at excessive rates, but don’t experience any longevity or reliability issues due to it. There are some estimates that suggest high oil consumption affects about 4-7% of these engines. It also appears most common with the 6MT.

Subaru is aware of the FB25 oil consumption problems and addresses it in a couple ways. First, they were replacing piston rings or entire pistons. It didn’t seem very effective. As such, Subaru began actually replacing short-blocks to fix the issue. It seems to work for some, but doesn’t work for others.

Ultimately, this issue may simply boil down to QC. Piston and piston ring clearances can vary, and some may simply be too far off and allow excess oil past the piston rings. There isn’t really any solid evidence or proof as to what exactly causes FB25 Forester, Legacy, and Outback oil consumption. It’s something to consider and keep an eye on, though.

Whether or not it affects engine longevity is also up in the air. That said, if your engine consumes excess oil it’s important to keep an eye on oil levels. Running the engine on low oil can have negative impacts on long-term reliability and longevity.

FB25 Oil Consumption Symptoms

Symptoms of excess oil consumption are generally pretty simple. Often, the consumption itself is the only symptom. Nonetheless, look out for the following signs the FB25 is excessively consuming oil:

  • Losing ~0.75 or more quarts per 1,000 miles
  • Smoke from exhaust
  • Burning oil smells

What exactly classifies high oil consumption can be debated, too. Many manufacturers – including Subaru – state up to 1 quart per 1,000 miles is within tolerance. If you’re somewhere in that ballpark it’s definitely a considerable amount of oil loss. Around 0.75 quarts per 1,000 miles is where we would start looking further into the issue.

As oil makes it past piston rings it burns in the combustion chamber. This may cause smoke from the exhaust and/or burning oil smells. They’re not totally common symptoms of Subaru FB25 oil consumption, however.

Oil Consumption Fix

Owners with excess oil consumption should check with a local Subaru dealer. This is an issue they’re aware of, and can help determine the best steps to take. Piston ring or short-block replacement may be necessary.

There are some other ways to potentially mitigate FB25 oil consumption. Try different oil weights or brands of oil (ensure it’s within manufacturer spec). Avoid excessive idling and keep throttle input and RPM’s modest until the engine is warm. Clearances tend to become smaller on a warm engine since metals expand with heat, which allows less room for oil to make it by.

3) Cam Carrier Seal Oil Leaks

Last up for common Subaru FB25 engine problems is cam carrier seal oil leaks. You can find the TSB from Subaru here. It applies to the 2012-2019 Forester, Impreza, and 2015-2019 WRX. Whether or not this is still an issue on the newest FB25D engine is up in the air. The engine is simply too new to tell.

It is a pretty common problem on many of the earlier engines, though. Cam carrier seal leaks are also very expensive. Some people even get quotes of $3,000+ to fix the oil leak. Fortunately, it’s not absolutely required to repair until it becomes a pretty serious oil leak. Don’t want too long, though. Due to the TSB Subaru make work with owners to lower repair bills, but it all depends on the age and mileage.

Cam carrier seals are not the only oil leak on FB25 engines. If you check the service bulletin you’ll also notice mention of the timing chain cover, cam cap, and upper oil pan. Another oil leak that’s common on many engines above 100,000 miles is the valve cover gaskets.

Cam Carrier Leak Symptoms

  • Visible leak
  • Burning oil smells
  • Light smoke from engine bay

A visible leak is a dead-give away that oil is leaking somewhere. Be cautious with the cam carrier seal leak on the FB25. It can often be confused with other possible leaks – such as valve cover gaskets. A small leak can often be hard to notice visibly.

It’s possible for engine oil to drip onto hot parts and burn off. This can produce light smoke from the engine bay or burning oil smells.

Cam Carrier Oil Leak Fix

The cam carrier seal fix is very labor intensive. Earlier we stated some receive quotes of $3,000+ for this repair. It’s a little excessive, but $1,000+ is very standard. As with most oil leaks the actual parts and repairs are inexpensive. The FB25 labor to access cam carrier seals is just expensive and lengthy.

Again, this is under a TSB from Subaru. If you’re over 100,000 miles and well out of warranty they likely won’t offer much help, if any. However, if your car is on the newer side and lower mileage they may offer discounted repairs or cover it even outside warranty periods.

Subaru FB25 Reliability

Is the Subaru FB25 2.5L boxer engine reliable? Yes, we believe this engine earns average remarks for reliability. You could even make a strong argument for slightly above average reliability. However, the costly cam carrier seal oil leaks and possible oil consumption problems bring the FB25 reliability down a bit.

Fortunately, oil consumption seems to only affect roughly 5% of engines. Subaru also has a TSB to address these problems with new rings or short-blocks if oil consumption is that severe. We won’t hold carbon build-up against the FB25. Many modern engines use DI and it’s truly awesome technology. Carbon build-up is simply a small drawback.

A lot of reliability simply comes down to maintenance and luck of the draw. All engines have the few bad apples, and we just can’t control that aspect. However, good maintenance is an easy way to increase the likelihood of having a reliable experience with the flat-4 boxer engine.

What’s your experience with the Subaru FB25 engine? Are you considering one?

Drop a comment & let us know!

Or check out our FA20 common problems article

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  1. Fb25 issue… getting engine oil into the coolant. Runs fine no smoke . Oil is fine no water. Was thinking about the upper pan seals would that be a possibility?

    1. I have the same problem, thinking it was a head gasket issue, I removed the engine resurfaced the heads and replaced the gaskets, also replaced the radiator (it started leaking) could not see an obvious cause. My car has about 125 K. After all the expense and stress, I noticed the same again. Car runs good other than I have to add some oil more often than I would like. I gave up and just do a coolant flush every other oil change (every 3000 miles)

  2. Purchased my 2011 Forester used from dealer in 2016 with ~82000 miles.
    Very happy with vehicle with exception of oil consumption from the get go. I contacted dealer and they agreed to perform a road test measuring the consumption. Low and behold the oil consumption under normal driving conditions was higher than average (~1 quart/800miles).
    They concurred that Subaru would correct the problem at no charge to me by installing a new short-block.
    That was in mid 2016 and now early 2022 with ~193000 miles my Forester is still performing great with very little as expected oil consumption due to the recommended 0-20W full synthetic oil.
    Bottom line – the upgraded FB25B short block was an excellent fix, Thank You Subaru and the Dealer that installed it.
    I expect many more enjoyable miles from my Forester.


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