The Three Most Frequent Subaru FB25 Engine Issues

The Three Most Frequent Subaru FB25 Engine Issues. The Subaru FB25 motor first appeared in the Subaru Forester in 2011. In North America, it is also available in the Legacy, Outback, and Crosstrek versions. The FB25’s 2.5L flat-4 motor produces 170-182 horsepower and 174-176 lb-ft of torque. It may not provide exceptional speed, but it will suffice for the majority of users. FB25 2.5 engines also provide an excellent mix of dependability and fuel economy. However, no engine is flawless, and this is true here as well. This guide covers Subaru FB25 engine specifications, problems, reliability, and other topics.

The Three Most Frequent Subaru FB25 Engine Issues

What Vehicles Make Use Of The FB25?

2.5L Subaru engines in the FB engine family are available in the years and variants listed below:

  • Subaru Forester (2011-present)
  • 2013-present Subaru Legacy
  • From 2013 to the present, Subaru Outback
  • Subaru Crosstrek from 2021 to the present

This motor is only available in the North American market. As a result, these models are not equipped with the same FB25 2.5L flat-4 engine in other countries. There are also two engine variants, with the most recent FB25D finding its way into the Forester in 2019. In 2020, it fully replaced the original FB25B in all models. More on this following engine specifications for the 2.5 engine.

Subaru FB25 Engine Specifications

The 2.5L FB25 motor specifications are as follows:

The Three Most Frequent Subaru FB25 Engine Issues

The FB25, unlike many other Subaru engines, lacks a turbine. It has a 2.5L flat-4 boxer motor that is naturally aspirated. An over-square design with a 94mm x 90mm bore x stroke is used to accomplish the displacement. The 2.5L boxer engine’s aluminum head and cylinder contribute to its light weight.

The compression ratio on original FB25B motors is 10:0:1. Later direct injection versions have a higher 12:0:1 ratio. All of this adds up to 170-182 horsepower and 174-176 pound-feet of torque. Again, due to a few updates, 2019+ models with the FB25D engine have the more powerful engine.

FB25B versus FB25D

The original variant of the engine, the FB25B, was introduced by Subaru in 2011. The initial engine was phased out of the FB25D starting in 2019. The most noticeable change is the use of direct fuel injection rather than the older-style port fuel injection.

Direct injection has numerous advantages, and we are big supporters of the technology. Direct injection of gasoline into the cylinders improves performance, fuel economy, and emissions. It’s a win-win situation in almost every way.

However, there is one possible issue with the FB25D direct injection engine: carbon build-up. We’ll talk about this as a “common issue” later, so don’t worry about spoilers. The major point here is that Subaru switched to direct injection with the FB25D engine, allowing for a higher compression ratio of 12.0:1.

The Three Most Frequent Subaru FB25 Engine Issues

Typical Subaru FB25 Motor Issues

Among the most prevalent issues with the FB25B and FB25D flat-4 engines are:

  • Carbon accumulation
  • Consumption of oil
  • Cam carrier gasket failure

The following sections of this essay go over the aforementioned Subaru FB25 engine issues in detail. But first, a few brief observations. These are the most prevalent, but that doesn’t mean they affect a large percentage of engines. We’re merely stating that these are some of the most common areas where problems arise.

So far, the FB25 appears to be a dependable motor. Again, no engine is flawless, so don’t be put off by the few potential flaws. Anyway, at the conclusion of the article, we return to the Subaru FB25 reliability issue.

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1) 2.5 FB25D Carbon Deposit Problems

The first is a problem that is specific to the direct injection (DI) FA25D engine in 2019-2020+ models. It’s a frequently asked question about DI engines. Oil blow-by is produced by engines and finds its way onto intake openings and valves. In traditional port fuel injection, gasoline is sprayed into these intake ports. Detergents are then used to remove any grease deposits.

Direct infusion does not have the same advantage. There is no fuel flow to clean intake valves because fuel is sprayed straight into the cylinders. Carbon crystals accumulate on the FB25 intake valves over time. True, modern engines are superior at controlling blow-by into the intake valves. However, it is impossible to fully eliminate carbon buildup without fuel flowing over them.

Many Subaru FB25D engines will most likely go their entire lifetimes without being serviced. It generally does not represent a significant threat to engine longevity. However, we strongly advise cleansing the intake valves every 100,000 to 120,000 miles. Excess carbon deposits can result in substantial power loss as well as other drivability issues.

Signs of Subaru 2.5 DI Carbon Buildup

Excess carbon build-up on the FB25D motor can cause the following symptoms:

  • Misfires in the engine
  • Stumbling / cautious pace
  • Idling in a rough manner
  • Losses in power and efficiency

As carbon deposits accumulate, they begin to limit airflow into the cylinders. This can result in engine misfires, which can contribute to stumbling and rough idle. These symptoms typically appear when carbon buildup on the FB25 engine becomes severe.

Loss of power is one of the most serious symptoms and worries associated with carbon deposits. However, it is a difficult sign to detect. Carbon buildup occurs over tens of thousands of kilometers of travel. As a result, power reduction is gradual rather than noticeable right away.

FB25D Walnut Blasting / Cleansing of Intake Valve

Walnut blasting is an excellent method for removing carbon residues from Subaru 2.5L flat-4 engines. It’s a popular method for cleaning valves and ports because it’s very efficient. Walnut blasting is accomplished with walnut media fragments and a heavy-duty vacuum. After that, the media shells are blasted into the intake openings to remove any deposits.

Other effective techniques exist, but they are more intensive and time consuming. Of course, reaching intake valves requires pulling the manifold, which takes at least 2-4 hours. Walnut blasting or comparable services should cost around $350-600.

Carbon buildup can be slowed by using intake mists. It is rarely entirely effective, but if used on a regular basis, it can extend the time between full intake valve cleanings.

2) Excessive Oil Usage in the Subaru FB25

It’s never simple to write about oil consumption. It’s always a bit hazy because many engines burn oil at excessive rates but have no problems with longevity or dependability as a result. According to some figures, high oil consumption affects about 4-7% of FB25 engines. It is also most prevalent with the 6MT.

Subaru is conscious of the FB25 oil consumption issues and is addressing them in a number of ways. They began by replacing piston rings or complete pistons. It didn’t appear to be very successful. As a result, Subaru started replacing short-blocks to address the problem. It appears to work for some people but not for others.

Finally, this may come down to quality control. Piston and piston ring clearances can differ, and some may be too far apart, allowing excess oil to pass through the piston rings. There is no concrete evidence or confirmation as to what causes FB25 Forester, Legacy, and Outback oil consumption. But it’s something to think about and keep a watch on.

It’s also unclear whether it impacts engine longevity. However, if your FB25 engine consumes a lot of oil, you should keep a watch on the oil level. Running the engine with insufficient oil can have a negative effect on its long-term reliability and longevity.

Signs of FB25 2.5L Oil Consumption

The symptoms of excessive oil consumption are usually straightforward. The consumption is frequently the only sign. Nonetheless, keep an eye out for the following indications that the FB25 is consuming too much oil:

  • Loss of 0.75 quarts or more per 1,000 kilometers
  • The smoke produced by emissions
  • The odor of burning gasoline

It is also debatable what constitutes excessive oil consumption. Many manufacturers, including Subaru, say that up to 1 quart per 1,000 miles is acceptable. If you’re in that range, you’re certainly losing a significant amount of oil. We’d start checking into the FB25 issue at around 0.75 quarts per 1,000 miles.

Oil burns in the combustion area as it passes through the piston rings. This may result in exhaust fumes and/or burning oil odors. They are, however, not universal signs of Subaru FB25 oil consumption.

Subaru FB Oil Usage Correction

Owners who are experiencing excessive oil usage should contact their local Subaru dealer. They are conscious of the problem and can advise on the best course of action. It is possible that a piston ring or short-block repair is required.

Other methods of possibly reducing FB25 oil consumption exist. Experiment with various oil weights or brands (ensure they are within the manufacturer’s specifications). Avoid excessive idling and keep throttle input and RPMs low until the motor reaches operating temperature. Clearances tend to shrink on a warm engine because metals expand with heat, leaving less space for oil to pass through.

3) Leaks in the 2.5L Flat-4 Cam Carrier Seal Grease

Cam carrier seal oil leaks are the final frequent Subaru FB25 engine problem. The Subaru Fix can be found here. It is applicable to the Forester, Impreza, and WRX from 2012 to 2019. It’s unclear whether this is still a problem on the newest FB25D engine. Simply put, the engine is too new to discern.

However, it is a fairly prevalent issue on many of the earlier Subaru FB25 engines. Cam carrier seal breaches are also extremely costly. Some individuals have received quotes of $3,000 or more to repair the oil leak. Fortunately, it is not strictly necessary to repair until there is a significant oil leak. But I don’t want to wait too long. Subaru will work with owners to reduce repair costs as a result of the TSB, but it all relies on the age and mileage of the vehicle.

The only source of oil leakage on FB25 motors is the cam carrier seals. The timing chain cover, cam cap, and upper oil pan are also mentioned in the repair bulletin. The valve cover gaskets are another source of oil leakage on many engines with over 100,000 kilometers.

Subaru FB25 Cam Carrier Leak Signs

As with any oil leak on the FB25 motor, the symptoms are fairly standard. Check for the following indications that the cam carriage is leaking:

  • Obvious dripping
  • The odor of burning gasoline
  • Light smoke is coming from the engine compartment.

A noticeable leak indicates that oil is leaking somewhere. Be wary of the FB25’s cam carrier seal leak. It is frequently mistaken with other potential leaks, such as valve cover gaskets. A tiny leak is often difficult to detect visually.

Engine oil has the potential to drip onto hot components and burn off. This can result in light smoke from the engine bay or the scent of burning oil.

Repair 2.5L Boxer Engine Oil Overflow

The cam carrier gasket repair is a time-consuming process. We previously mentioned that some people have received quotes of $3,000 or more for this repair. It’s a little extravagant, but $1,000+ is quite common. As with most oil leaks, the components and repairs are cheap. The FB25 work to access cam carrier seals is simply expensive and time-consuming.

Again, this is covered by a Subaru Warranty. If you’ve driven over 100,000 miles and are out of warranty, they’re unlikely to give much assistance, if any. However, if your vehicle is newer and has fewer miles, they may offer discounted repairs or cover it even after the warranty period has expired.

FB25 Subaru Dependability

Is the 2.5L boxer motor in the Subaru FB25 reliable? Yes, we think this engine receives average reliability ratings. You could even make a compelling case for slightly above-average dependability. However, the FB25’s dependability is hampered by costly cam carrier seal oil leaks and potential oil consumption issues.

Fortunately, oil consumption appears to impact only about 5% of engines. If the oil consumption is serious enough, Subaru has a TSB to address the issue with new rings or short-blocks. We will not hold carbon accumulation against the FB25. DI is a fantastic technology that is used in many contemporary engines. Carbon buildup is merely a minor disadvantage.

Much of the Subaru FB25’s dependability is determined by maintenance and chance of the draw. Every engine has a few bad apples, and we have no influence over that. Good maintenance, on the other hand, is a simple method to increase the likelihood of having a reliable experience with the flat-4 boxer engine.