P0308 on 2006-2010 Volvo S80 4.4L V8: Cylinder 8 Misfire Causes and Fixes

What's Unique About the 2006-2010 Volvo S80

This generation of S80 featured a unique, transversely mounted 4.4L V8 engine (B8444S) designed by Volvo and manufactured by Yamaha in Japan. Due to its compact, 60-degree V-angle design to fit in an engine bay intended for smaller engines, components are tightly packed. The firing order is 1-8-4-3-6-5-7-2. Cylinder 8 is located on the bank closer to the radiator, at the rearmost position (transmission side). Owner experiences frequently cite that oil leaks from the valve cover gasket can seep into the spark plug tubes and cause misfires on this engine by saturating the ignition coil boot. This is a very common issue and should be a primary check.

Symptoms You May Notice

• Rough or vibrating idle
• Engine hesitation or stumbling during acceleration.
• Noticeable loss of power
• Flashing or solid Check Engine Light.
• Reduced fuel economy
• A smell of unburned gasoline from the exhaust

Most Likely Causes

1. Faulty Ignition Coil 🔴 High Probability Coil-on-plug ignition systems are a common failure point on many modern engines as they age due to heat and vibration. It's the most frequent cause for a single-cylinder misfire. Oil from a leaking valve cover gasket can also saturate and destroy the coil.
   How to confirm: Swap the ignition coil from cylinder 8 with another cylinder (e.g., cylinder 6 on the same bank). Clear the codes and run the engine. If the code changes to P0306, the ignition coil is faulty.
   Typical fix: Replace the failed ignition coil. It is often recommended to replace all 8 coils if they are original and high-mileage, but replacing only the failed one is acceptable.
   Est. part cost: $40 - $90 per coil
2. Worn or Fouled Spark Plug 🔴 High Probability
   How to confirm: After ruling out the coil, remove the spark plug from cylinder 8. Inspect it for wear, carbon buildup, oil fouling, or a cracked insulator. Compare it to a plug from a working cylinder. You can also swap the plug with another cylinder to see if the misfire code follows.
   Typical fix: Replace the spark plug. Volvo specifies Iridium plugs for this engine. It is best practice to replace all 8 spark plugs at the same time to ensure even performance. 🎬 See this DIY walkthrough for a full spark plug service.
   Est. part cost: $80 - $150 for a set of 8
3. Leaking Valve Cover Gasket 🟡 Medium Probability The spark plug tube seals, which are part of the valve cover gasket assembly, can become hard and brittle with age, allowing engine oil to leak into the spark plug wells. This is a well-documented issue on the B8444S.
   How to confirm: When you remove the ignition coil from cylinder 8, inspect the coil boot and the spark plug well for the presence of liquid engine oil. This is a very strong indicator.
   Typical fix: Replace the valve cover gasket and the integrated spark plug tube seals for the affected cylinder bank. Thoroughly clean all oil out of the spark plug well before installing the new plug and coil. This is a labor-intensive job due to the tight engine bay. 🎬 Watch: A detailed guide to the V8 valve cover job.
   Est. part cost: $50 - $100 for the gasket set
4. Clogged or Faulty Fuel Injector ⚪ Low Probability
   How to confirm: If ignition parts are confirmed good, the fuel injector is the next suspect. A professional can perform a fuel injector balance test. A DIY method is to listen to the injector with a mechanic's stethoscope to ensure it's clicking like the others. Swapping injectors is also possible but is more labor-intensive.
   Typical fix: Replace the faulty fuel injector. It's recommended to replace the O-rings and seals when replacing an injector.
   Est. part cost: $70 - $150 per injector

Rare But Worth Checking

• Low Engine Compression: This indicates a more serious internal engine problem, such as a burnt valve or worn piston rings. A compression test is needed to diagnose this. This is uncommon but possible on higher-mileage engines.
• Damaged Coil or Injector Wiring: The wiring harness leading to the cylinder 8 ignition coil or fuel injector could be damaged or have a corroded connector. A visual inspection and continuity test can confirm this.

Diagnosis Steps

1. Read the code with an OBD-II scanner to confirm P0308 is the primary code present.
2. Note if the Check Engine Light is steady or flashing. A flashing light indicates a severe, catalyst-damaging misfire.
3. Locate cylinder 8. On the transverse B8444S V8, the front of the engine is the belt side. The cylinder bank closer to the radiator contains cylinders 2, 4, 6, and 8. Cylinder 8 is the one closest to the transmission on that bank.
4. Remove the ignition coil from cylinder 8. Inspect the spark plug well for any oil. If oil is present, the valve cover gasket has failed and is the likely root cause.
5. If no oil is present, swap the ignition coil from cylinder 8 with the coil from cylinder 6 (the next one forward on the same bank).
6. Clear the codes with the scanner, start the engine, and let it run for a few minutes.
7. Re-scan for codes. If the code is now P0306, the ignition coil you moved is faulty and needs replacement.
8. If the code remains P0308, swap the spark plug from cylinder 8 with the one from cylinder 6. Clear codes and re-test.
9. If the code is now P0306, the spark plug is the culprit.
10. If the code remains P0308 after swapping both the coil and plug, the issue is likely with the fuel injector, wiring, or a mechanical engine problem. Inspect the wiring and connector to cylinder 8's coil and injector for any visible damage. At this point, a professional diagnosis involving a compression test or fuel injector test is recommended.

Parts You'll Likely Need

• Ignition Coil (OEM #8687939) — This is the most common failure item causing a single-cylinder misfire on this engine.
  Trusted brands: Volvo (Genuine), Bosch, Denso, Pro Parts Sweden
  OEM price range: $70-$110
  Aftermarket price range: $40-$75
• Iridium Spark Plug (OEM #30731383) — Spark plugs are a regular maintenance item and a frequent cause of misfires. It's recommended to replace them as a set.
  Trusted brands: Volvo (Genuine), Bosch, NGK, Denso
  OEM price range: $120-$160 (set of 8)
  Aftermarket price range: $80-$120 (set of 8)
• Valve Cover Gasket Kit (Front Bank) — A common source of oil leaks into the spark plug wells, causing misfires. Cylinder 8 is on the front bank (radiator side).
  Trusted brands: Victor Reinz, Elring, Genuine Volvo
  OEM price range: $80-$120
  Aftermarket price range: $40-$70

Related Codes That Often Appear With This One

• P0300 — If the misfire is intermittent or begins to affect other cylinders, a P0300 (Random/Multiple Cylinder Misfire) code may appear alongside P0308.

Platform-Specific Known Issues

• Oil in Spark Plug Well: A user on YouTube performing a valve cover gasket replacement on a 2010 S80 V8 specifically notes that the job was necessary because oil was weeping and dripping, which is a common failure mode leading to misfires. Another detailed DIY guide for the V8 valve cover job notes that oil leaking from the 2-4-6-8 bank cover drips directly on the exhaust manifold, while leaks on the 1-3-5-7 bank can saturate the alternator and A/C compressor.

Scan Tool Commands That Help

• Volvo VIDA: Misfire Counter — To monitor live misfire counts on each cylinder to confirm the misfire is isolated to cylinder 8 and to verify the repair after a fix has been attempted.
• Volvo VIDA: Injector Test / Fuel Correction — After ruling out ignition components, this test can be used to check the fuel quantity correction for each injector. A significant deviation on cylinder 8 (e.g., outside +/- 2.3 mg/cycle on some models) would indicate a faulty injector.
• Volvo VIDA: Read / Erase All Codes — Used to pull manufacturer-specific codes and to clear all fault codes from modules after a repair is completed. This is necessary to turn off the check engine light and confirm the issue is resolved.

Wiring & Ground Locations

• Engine Control Module (ECM) — Located in the left rear of the engine compartment.. The ECM is the source of the P0308 code. All sensor inputs, coil firing signals, and injector pulse signals route through it. A poor connection at the ECM could theoretically cause a misfire, though it's rare.
• G89 / G90 / G100 — G89 is on the left front of the engine compartment, G90 is on the left side of the engine, and G100 is on the top left rear of the engine.. These are primary engine ground points. A corroded or loose ground can cause a variety of electrical issues, including weak spark or erratic sensor readings that could lead to a misfire code.

Real Owner Repair Stories

• Reddit user on r/Volvo (2008 Volvo XC90 V8 (B8444S engine)) — Random misfires, Check Engine Light, VIDA showed retarded intake and exhaust cam timing (P0012).
  ❌ Tried (didn't work) Replaced all spark plugs.
  ✅ What actually fixed it The variable valve timing (VVT) solenoids were dirty. An engine oil flush (using Liqui Moly engine clean) followed by a fresh oil change cleaned the solenoids and resolved the misfires and timing codes without replacing any parts.

Model Year Variations Within This Range

• 2005-early 2006: Engines with a serial number below 6833 are prone to balance shaft bearing failure due to water collecting in the engine valley. Later models (engine serial number 6833+) have a drain hole drilled in the block to prevent this. Failure of this bearing can cause the timing to jump, leading to misfires and catastrophic engine damage.

Diagnostic Flowchart

Other Known Issues on This Vehicle

Issues unrelated to this code that are worth knowing about as an owner of this generation:

• Balance Shaft Bearing Failure 🔴 High — Primarily affects 2005 and early 2006 models (engine serial numbers below 6833) without a water drainage hole in the engine block. Water collects in the V of the engine and destroys the unsealed bearing, leading to catastrophic engine failure. (Ref: A service campaign was issued by Volvo to drill a drain hole in affected vehicles, but it was not a formal recall.)
• Alternator Failure 🟠 Medium — The alternator is prone to failure, often exacerbated by oil leaks from the valve cover gasket dripping onto it. Replacement is labor-intensive due to tight packaging in the engine bay. Symptoms include 'Charging Failure' warnings and erratic electronics.
• Power Steering Pump/Hose Leaks 🟠 Medium — Leaks from the power steering pump and hoses are common. A recall (NHTSA #11V378000) was issued for the 2007 model year for an incorrectly routed return hose that could rupture. Owners report whining noises and fluid leaks. (Ref: Recall R246 (NHTSA 11V378000))
• Valve Cover Gasket Oil Leaks 🟠 Medium — Very common issue as the vehicle ages. Gaskets harden and leak oil down the engine block, onto the exhaust, and into the spark plug wells, causing misfires and burning oil smell.
• Transmission Issues (Aisin TF-80SC) 🟠 Medium — While generally robust, the 6-speed automatic can develop harsh shifting or shift flares, particularly if transmission fluid has not been changed regularly. Valve body issues are a known potential failure point.

Used vs. New Parts: Buying Guide for This Vehicle

When a used part is the smart pick: For this repair, buying used parts is generally not recommended. The primary causes are wear-and-tear items like ignition coils, spark plugs, and gaskets. The cost savings for a used ignition coil or fuel injector are minimal compared to the risk of receiving a part with limited remaining life.

Donor-vehicle mileage cap: roughly under 80000 miles for the part to have meaningful remaining life.

What to inspect on the donor part:

• For an ignition coil, check for cracks in the plastic housing or damage to the rubber boot.
• Verify the part number on the used component matches the OEM number exactly.
• Ask for the donor vehicle's mileage; avoid high-mileage donors for electrical components.

OEM-only on this vehicle (don't cheap out):

• Valve Cover Gasket: While aftermarket is available, an OEM gasket often provides the best fit and longest-lasting seal, which is critical on an engine known for this type of leak.

Aftermarket brands forum-validated for this vehicle:

• Ignition Coils: Bosch, Denso, Pro Parts Sweden
• Spark Plugs: NGK, Bosch, Denso
• Gaskets: Victor Reinz, Elring

Brands owners have reported issues with on this vehicle:

• Unbranded, no-name ignition coils from online marketplaces. These often have a high failure rate and may not perform to OEM specifications, causing persistent or new misfires.

Real Owner Stories

Aggregated from forums and TSBs cited above. Mileages and costs reflect what owners reported in those sources.

Related OBD-II Codes

• P0306 — This code will appear if you swap the ignition coil or spark plug from the misfiring cylinder 8 to cylinder 6 to confirm a component failure.
• P0300 — A general random/multiple cylinder misfire code that may trigger alongside P0308 if oil leaks from the valve cover gaskets are affecting multiple spark plug wells.
• P0302 — A misfire code for cylinder 2, which is located on the same 2-4-6-8 bank (closer to the radiator) as cylinder 8.
• P0304 — A misfire code for cylinder 4, located on the same bank as cylinder 8 and subject to the same valve cover gasket leak risks.

Frequently Asked Questions