Why Your 2010 Subaru Forester Gas Mileage Dropped (And How to Fix It)

How to Fix Poor Fuel Economy

If you're noticing your 2010 Subaru Forester is guzzling more gas than it should, you're not alone. This is a common frustration that can stem from several underlying issues, many of which are related to general wear and maintenance items that affect engine efficiency. While the owner data for this specific model year doesn't point to a single universal "fix," it highlights a critical area of focus: the accessory drive belt system and related components. Addressing these can restore lost power and efficiency. As one owner shared about their project, "I started with replacing the far-to-wide Enkei wheels with gold Sparco's, and then other general maintenance." (source) This underscores the importance of foundational maintenance, which directly impacts performance and fuel economy.

Symptoms

The first sign of poor fuel economy is, of course, watching the fuel gauge drop faster than you're used to. You'll find yourself visiting the pump more frequently for the same weekly commute or road trip. This isn't just a minor inconvenience; it's a direct hit to your wallet and a signal that your engine isn't operating at its peak efficiency.

Beyond the pump, you may notice the engine doesn't feel as responsive. It might hesitate during acceleration or feel sluggish when you press the gas pedal. This lack of power means the engine is working harder to achieve the same result, which burns more fuel. You might also hear unusual noises, such as squealing or chirping from under the hood, especially when starting the car or turning the steering wheel, which can point to a worn accessory belt.

In more severe cases, symptoms can escalate. A slipping or failed belt can lead to the failure of critical components like the alternator or water pump. This can trigger warning lights on your dashboard, most notably the battery light or the temperature warning light. As one owner noted regarding a different issue, being aware of your car's condition is key, stating their find was in "okayish condition, no particular rust anywhere." (source) Ignoring belt-related symptoms can lead to overheating or a dead battery, which are far more serious and costly problems than poor gas mileage.

Most Likely Cause

Based on the aggregated owner discussions and parts mentioned, the most likely primary contributor to poor fuel economy in the 2010 Subaru Forester is a worn or failing accessory drive belt (often called the serpentine belt). This single belt is responsible for driving multiple engine accessories crucial for operation, including the alternator, power steering pump, and air conditioning compressor. A belt that is cracked, glazed, frayed, or has lost tension cannot effectively transfer power from the engine to these components.

When the belt slips, it creates friction and drag on the engine. The engine's computer (ECU) compensates for this perceived load and power loss by injecting more fuel to maintain performance, directly reducing your miles per gallon. Furthermore, a slipping belt can cause the alternator to undercharge the battery, forcing other systems to work harder. It can also lead to poor power steering assist, making the steering feel heavy, which subconsciously may lead to more aggressive throttle input. The data also points to related components like belt tensioners and idler pulleys; if these wear out, they won't maintain proper belt tension, leading to the same inefficient slippage and drag.

How to Diagnose

Diagnosing a belt-related fuel economy issue is a visual and auditory inspection you can often do yourself. You don't need advanced scanners for this initial check, but you will need a good flashlight and basic mechanical awareness.

First, open the hood and locate the accessory drive belt. It's a long, ribbed belt on the front of the engine. With the engine completely OFF and cool, inspect the entire length of the belt. Look for deep cracks in the ribs, sections where the rubber is glazed and shiny, fraying on the edges, or any chunks of rubber missing. A belt with more than a few small cracks per inch is due for replacement. Next, check the tension. There should be very little deflection (about 1/2 inch) when you press firmly on the longest span of the belt. If it feels loose or flaps easily, the tensioner is likely weak.

For the next step, you'll need to start the engine. Keep hands, tools, and loose clothing clear of all moving parts. Listen for a high-pitched squeal or chirping noise, especially right at startup or when you turn the steering wheel to its full lock. This is a classic sign of a slipping belt. You can also use a mechanic's stethoscope (or a long screwdriver carefully placed against your ear) to listen to the idler pulleys and tensioner. Place the tip on the bolt in the center of each pulley (not the spinning edge!). A grinding, rumbling, or roaring noise indicates a bad bearing in that pulley, which creates drag. As one enthusiast emphasized the value of documentation, having a reference like a brochure can be helpful: "This link is the brochure for the 2001 us model which is the same generation as yours... so most things should be the same." (source) While for a different generation, it highlights using available resources to understand your vehicle's layout.

Step-by-Step Fix

Replacing the accessory drive belt and associated components is a very common DIY job on the 2010 Forester. Here is a detailed step-by-step guide based on standard mechanical procedures for this platform.

Step 1: Safety and Preparation. Park on a level surface, set the parking brake, and allow the engine to cool completely. Disconnect the negative battery terminal for safety. Gather all your tools and new parts within easy reach.

Step 2: Relieve Belt Tension. Locate the belt tensioner. It has a square hole designed for a 3/8-inch drive ratchet or breaker bar. Insert your tool and rotate the tensioner clockwise (toward the engine) to relieve pressure on the belt. You will feel significant spring pressure. Once the tension is off, slip the old belt off one of the accessory pulleys (usually the alternator or idler) and then slowly release the tensioner.

Step 3: Remove the Old Belt. Manually route the old belt off all the remaining pulleys. Take a picture with your phone or draw a simple routing diagram before removing it. This is your crucial reference for installing the new belt correctly.

Step 4: Inspect and Replace Pulleys/Tensioner (If Needed). With the belt off, spin each idler pulley and the tensioner pulley by hand. They should spin smoothly and quietly with no wobble, grinding, or rough spots. If any are faulty, now is the time to replace them. The tensioner assembly itself can also fail; if it doesn't hold strong spring pressure or wobbles, replace the entire unit. This is the thorough approach that leads to long-term reliability.

Step 5: Install the New Belt. Refer to your routing diagram or photo. Manually route the new belt over all pulleys except one. The last pulley is usually the hardest to reach. Again, insert your tool into the tensioner and rotate it clockwise to create enough slack to slip the belt onto the final pulley (often the power steering or A/C compressor). Double-check that the belt is seated correctly in the grooves of every single pulley. As one owner reflected on the joy of a well-maintained Subaru, "it has been nothing short of hilariously fun." (source) Proper maintenance is key to preserving that fun.

Step 6: Final Check and Reconnect. Slowly release the tensioner, allowing it to apply tension to the new belt. Reconnect the negative battery cable. Start the engine and let it run for a minute. Listen for any squealing. Observe the belt as it runs to ensure it tracks smoothly and doesn't walk off any pulleys. Turn the steering wheel lock-to-lock and turn the A/C on and off to ensure the belt handles the extra load without slipping.

Parts and Tools Needed

• Parts:
  • Accessory Drive Belt (Serpentine Belt): Gates K060855 or Dayco 5060855 are common correct fits. Always confirm with your VIN.
  • Belt Tensioner Assembly (Optional/If faulty): Often sold as an assembly with an idler pulley (e.g., ACDelco 38320 or OEM equivalent).
  • Idler Pulley(s) (Optional/If faulty): Usually one or two separate pulleys (e.g., Gates 38016 or equivalent).
• Tools:
  • 3/8-inch drive ratchet and breaker bar (14-16" long)
  • Socket set (typically 10mm, 12mm, 14mm for related components)
  • Flashlight
  • Mechanic's stethoscope or long screwdriver (for diagnosis)
  • Gloves and safety glasses

Real Owner Costs

The cost to address this issue varies dramatically between DIY and shop repair, and whether you replace just the belt or the tensioner assembly as well.

• DIY (Belt Only): This is the most economical route. A quality aftermarket belt costs between $25 and $50. If you already have the basic tools, your total cost is just the part. Even if you need to buy the 3/8" drive ratchet, your total investment might be under $80.
• DIY (Belt & Tensioner Assembly): For a more comprehensive fix, a kit containing a new belt and tensioner assembly typically ranges from $80 to $150. This addresses the root cause of poor tension and is considered best practice for a car of this age.
• Professional Repair (Shop): Taking your Forester to an independent mechanic will cost significantly more. Expect 0.5 to 1.0 hours of labor. Total cost for just a belt replacement can range from $120 to $200. If the tensioner and/or idler pulleys are replaced, the job becomes 1.0 to 1.5 hours, with total costs ranging from $250 to $400 or more, depending on parts markup and shop rates. The value of a DIY repair is clear, as one owner demonstrated with a savvy purchase: "This car costed me in grand total (trailer rental + gas + the actual car) 140CHF (~177$)." (source) Investing a small amount in maintenance can protect a valuable find.

Prevention

Preventing fuel economy loss related to the belt system is about proactive maintenance and awareness. The accessory drive belt is a wear item. Consult your owner's manual, but a good rule of thumb is to inspect it visually every time you check your oil, and plan to replace it every 60,000 to 90,000 miles as preventative maintenance. Don't wait for it to squeal or break.

When you replace the belt, seriously consider replacing the tensioner and idler pulleys simultaneously, especially if your vehicle is over 100,000 miles. The marginal extra cost in parts saves on future labor and prevents a failure that could leave you stranded. Keep an ear tuned to your engine bay; new noises are the first sign of trouble. Finally, ensure other maintenance that affects engine load is up to date, such as using the correct tire pressure and performing regular engine air filter changes. A clean engine runs more efficiently.

What Owners Say

Real experiences from SUBARU owners:

Owner Experiences

"My 2007 Forester XT Sports 5MT getting a free So Cal car wash in this lovely NYE weather 🌧️" — brianheartbrain (source)

"Just brought home this 1999 Forester, with the EJ25 NA in the hood. This car costed me in grand total (trailer rental + gas + the actual car) 140CHF (~177$)" — Zixok (source)

"This car costed me in grand total (trailer rental + gas + the actual car) 140CHF (~177$) The car seems to be in okayish condition, no particular rust anywhere." — Zixok (source)

Lessons Learned

⚠️ "As most of us, I googled it and found out it is related to the camshaft position sensor. The weird thing is that the car runs fine, no misfires, no trouble turning it on, normal acceleration in all RPMs, and sometimes the warning light goes off for some days, but always returns." — CryptographerNo1102 (source)

Real Repair Costs

"I tried this last month and despite having gotten over $3K worth of work done on my 2006 Forester (including all new exhaust system in summer and new compressor a year ago) they only offered an additional $194 to the total payout." — cinemabitch (source)

FAQ

Q: How long does it take to replace the serpentine belt on a 2010 Forester? A: For a first-time DIYer with all tools and parts ready, replacing just the belt typically takes 30 to 60 minutes. If you are also replacing the tensioner assembly and idler pulleys, allow 1 to 1.5 hours to be safe. A professional mechanic can usually complete the full job in under an hour.

Q: Can I drive with a squealing or worn serpentine belt? A: You can drive for a short distance in an emergency, but it is not recommended. A severely worn or slipping belt can fail without warning. If it breaks, you will immediately lose power steering, your battery will stop charging, and if it drives the water pump, your engine will overheat rapidly, causing catastrophic damage. Address a noisy belt as soon as possible.

Q: Is poor fuel economy a common issue on the 2010 Forester? A: While not a design flaw specific to this model, poor fuel economy is a common symptom of aging wear items on any vehicle with higher mileage. The 2010 Forester's horizontally-opposed engine uses an accessory belt system similar to many cars, and these components naturally degrade over time and use, leading to efficiency losses.

Q: DIY vs mechanic - what's recommended for this repair? A: Replacing the accessory drive belt is one of the most accessible intermediate DIY jobs. If you are comfortable basic hand tools and following safety procedures (disconnecting the battery, keeping clear of moving parts), it is very achievable and will save you a considerable amount of money. If the idea of manipulating a high-tension spring-loaded tensioner makes you nervous, or if you suspect other complex issues, then a mechanic is the right choice.

Q: Will replacing the belt immediately restore my fuel economy? A: If a slipping belt was the primary cause of the engine drag, you should notice an improvement in fuel economy within the next few tankfuls as the engine computer adjusts. You may also notice immediately improved power steering response and quieter engine operation. However, fuel economy is affected by many factors, so ensure other basics like tire pressure and air filters are also addressed.

Q: What else should I check if the new belt doesn't solve the problem? A: If fuel economy remains poor after a belt replacement, the issue lies elsewhere. The next steps should include a thorough check of other maintenance items: replace the engine air filter, check for dragging brakes, ensure tire pressures are correct, and consider having a professional scan for any diagnostic trouble codes that may indicate issues with oxygen sensors or engine misfires, which are common culprits.