P0308 on 2006-2011 Buick Lucerne: Cylinder 8 Misfire Causes and Fixes

What's Unique About the 2006-2011 Buick LUCERNE

The Buick Lucerne was the last Buick sedan to offer V8 power, using the transverse (front-wheel-drive) 4.6L Northstar LD8 V8. Unlike the V6 Lucerne models which use a waste-spark system with ignition wires and coil packs, the 2006-2011 V8 features a more modern Coil-on-Plug (COP) ignition system. This means each cylinder has its own dedicated ignition coil mounted directly on the spark plug, making diagnosis of a single-cylinder misfire like P0308 very straightforward through component swapping.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this vehicle.

Symptoms You May Notice

• Rough or vibrating idle
• Engine hesitation or stumbling during acceleration
• Noticeable loss of engine power
🎬 Watch: Understanding the common causes and fixes for P0308 codes.
• Flashing or solid Check Engine Light
• Reduced fuel economy
• A smell of raw fuel from the exhaust

Most Likely Causes

1. Faulty Ignition Coil 🔴 High Probability → Shop Ignition Coil The Coil-on-Plug design exposes coils to significant heat cycling, and they are a common failure item over time on many engines, including the Northstar V8. The plastic housings can crack and internal windings can short out.
   How to confirm: Swap the ignition coil from cylinder #8 with another cylinder (e.g., cylinder #6). Clear the codes and run the engine. If the trouble code changes to P0306 (Cylinder 6 Misfire), the coil is faulty. Cylinder #8 is the last cylinder on the engine bank closer to the radiator, on the passenger side of the vehicle.
   Typical fix: Replace the faulty ignition coil. It is held in by a single 10mm bolt and can be replaced in minutes.
   Est. part cost: $40-$70
2. Worn or Fouled Spark Plug 🟡 Medium Probability → Shop Spark Plug Spark plugs are a routine maintenance item. The original ACDelco iridium plugs have a service interval of 100,000 miles. If they have not been replaced, they can wear out, leading to a weak spark that is insufficient to ignite the fuel mixture under load.
   How to confirm: After confirming the coil is good, swap the spark plug from cylinder #8 with another cylinder. If the misfire code follows the plug, the spark plug is the issue. Visually inspect the plug for heavy carbon buildup, oil fouling, a worn electrode, or a cracked porcelain insulator.
   Typical fix: Replace the spark plug. It is highly recommended to replace all 8 spark plugs at the same time 🎬 See this in-depth tutorial on changing Northstar V8 spark plugs. for balanced engine performance and to prevent future misfires.
   Est. part cost: $8-$15 per plug
3. Defective Fuel Injector ⚪ Low Probability → Shop Fuel Injector Fuel injectors can become clogged with carbon deposits over time or fail electrically, preventing the correct amount of fuel from being delivered to the cylinder. A stuck-open injector can dump raw fuel, while a clogged one can cause a lean misfire.
   How to confirm: Listen to the injector with a mechanic's stethoscope; it should make a consistent clicking sound. You can also test the injector's resistance with a multimeter or swap it with another cylinder's injector to see if the misfire follows. This is a more involved process requiring removal of the fuel rail.
   Typical fix: Replace the faulty fuel injector. This requires depressurizing the fuel system and removing the fuel rail. It's recommended to replace the O-rings on all injectors that are disturbed during the process.
   Est. part cost: $50-$100

Rare But Worth Checking

• Vacuum Leak: A leak in the intake manifold gasket near cylinder #8 can lean out the air/fuel mixture and cause a misfire. A common failure point on Northstar engines is the rubber boot connecting the throttle body to the intake plenum 🎬 Watch: Diagnosing Northstar idle misses and vacuum leaks without coolant loss., which can crack and cause unmetered air to enter, sometimes triggering lean codes (P0171, P0174) along with misfire codes. This can be detected by spraying brake cleaner near the gasket/boot with the engine running and listening for a change in idle speed.
• Low Cylinder Compression: This indicates an internal engine mechanical problem, such as a bad valve, worn piston rings, or a jumped timing chain. A worst-case scenario for a Northstar engine is a head gasket leak allowing coolant into the cylinder, though this is less common on the 2006+ models with improved head bolt design. A compression test is needed to confirm this.
• Damaged Wiring or Connector: The wiring harness or connector for the ignition coil or fuel injector on cylinder #8 could be damaged, loose, or corroded, causing an intermittent connection. A visual inspection of the harness for chafing or broken clips is a key diagnostic step.

Diagnosis Steps

1. Confirm the P0308 code is present using an OBD-II scanner. Note any other codes, such as P0300, P0171, or P0174.
2. Identify the location of cylinder #8. On the front-wheel-drive Lucerne, the engine is mounted transversely. The cylinders are numbered 1-3-5-7 on the rear bank (near the firewall) and 2-4-6-8 on the front bank (near the radiator). Cylinder #8 is the furthest to the passenger side on the front bank.
3. Perform a visual inspection of the engine bay. Look for obvious issues like a disconnected vacuum hose, damaged wiring to the cylinder 8 coil, or signs of fluid leaks.
4. Swap the ignition coil from cylinder #8 with the coil from cylinder #6 (the adjacent, easier-to-access coil on the same bank). Clear the codes, start the engine, and see if the code changes to P0306. If it does, the ignition coil is bad.
5. If the code remains P0308, swap the spark plug from cylinder #8 with the plug from cylinder #6. Clear codes and re-test. If the code changes to P0306, the spark plug is bad.
6. If the misfire remains on cylinder #8, inspect the wiring and connector to the ignition coil and fuel injector for any visible damage or corrosion. Ensure the connector is fully seated.
7. Listen to the fuel injector on cylinder #8 with a mechanic's stethoscope or a long screwdriver. It should have a regular, audible clicking sound. Compare its sound to an adjacent injector.
8. If ignition and fuel delivery seem okay, check for a vacuum leak, especially around the intake manifold gasket for cylinder #8 and the throttle body boot.
9. If all else fails, perform a cylinder compression test to check for mechanical engine problems like bad valves or piston rings.

Parts You'll Likely Need

• Ignition Coil (OEM #12597745) — This is the most frequent cause of a single-cylinder misfire on the Northstar V8 due to heat and age. It is a simple and quick part to replace during diagnosis.
  Trusted brands: ACDelco, Delphi, NGK
  OEM price range: $50-$70
  Aftermarket price range: $30-$50
• Iridium Spark Plug (OEM #41-987) — As a primary ignition component and a regular wear item, a worn spark plug is the second most likely cause of a misfire. It's recommended to replace all 8 if they are near their 100,000-mile service life.
  Trusted brands: ACDelco, NGK
  OEM price range: $10-$15
  Aftermarket price range: $8-$12
• Fuel Injector (OEM #12596683) — Less common than ignition parts, but a clogged or failed injector is a known cause of persistent single-cylinder misfires that don't resolve with coil/plug swaps.
  Trusted brands: ACDelco, Standard Motor Products
  OEM price range: $70-$110
  Aftermarket price range: $40-$80

Related Codes That Often Appear With This One

• P0300 — If the misfire becomes more frequent or other cylinders begin to misfire (e.g., due to a large vacuum leak or failing fuel pump), the more generic P0300 (Random/Multiple Cylinder Misfire) code may appear alongside P0308.
• P0171 / P0174 — If the misfire on cylinder 8 is caused by a vacuum leak, the engine may run lean on one or both banks, triggering 'System Too Lean' codes for Bank 1 (P0171) and/or Bank 2 (P0174).

Technical Service Bulletins (TSBs) & Recalls

• PIP5628G: Communicates information needed to aid in diagnosing if a vehicle comes in with misfire and DTC Codes P0300 - P0308 and/or P050D. [⭐ MANUFACTURER TSB — highest authority]

Platform-Specific Known Issues

• The provided Technical Service Bulletin (TSB) #PIP5628G is a general document from GM that gives technicians information to aid in diagnosing misfire codes from P0300 to P0308, but it does not specify a single root cause. [⭐ MANUFACTURER TSB — highest authority]
• A known vacuum leak source on this platform is the rubber intake plenum-to-throttle body coupler, which can tear and cause lean conditions and misfires. While this often sets a P0300 random misfire code, it can manifest as a single cylinder misfire initially.

Mechanic-Grade Diagnostic Values

• Fuel Injector Resistance — expected: Approx. 12 Ohms. Failure: A significantly higher (open circuit) or lower (shorted) reading indicates a faulty injector coil.
• Cylinder Compression — expected: 120 - 150 PSI per cylinder, with minimal variation between cylinders.. Failure: A reading significantly below 100 PSI, or a variation of more than 15-20% from the other cylinders, points to a mechanical engine problem (valves, piston rings, head gasket).
• Mode $06 Misfire Counter — expected: Near zero for the test interval.. Failure: A high count of misfires logged under the Test ID (TID) for cylinder 8, even if a P0308 code has not yet been set. This can confirm an intermittent misfire.

Scan Tool Commands That Help

• Tech 2 / GDS2 (or equivalent bidirectional scanner): Cylinder Power Balance / Injector Balance Test — This is a highly effective test to perform after checking for spark. The command allows you to disable the fuel injector for each cylinder one by one while monitoring engine RPM. If you disable cylinder #8 and there is little or no drop in RPM, it confirms that cylinder #8 was not contributing power, isolating the problem to that cylinder.

Wiring & Ground Locations

• G112, G114, G115 — All are located on the lower left rear of the engine for the 4.6L V8.. These are primary engine grounds. A loose or corroded connection at these points can cause a variety of erratic electrical issues, including weak spark or incorrect sensor readings that could lead to a misfire code.
• S135 — In the even bank ignition coil harness, located between the main harness connector (C139) and the ignition coil for cylinder #8.. This is a splice pack where multiple wires are joined. Corrosion or a broken wire within this splice is a specific, hard-to-find failure point that would cut power or the control signal specifically to the cylinder 8 ignition coil, directly causing a P0308.
• ECM (Engine Control Module) — Located in the left side of the engine compartment.. This is the source of the command signals for both the ignition coil and the fuel injector. Knowing its location is critical for advanced diagnosis, such as testing for signal output directly at the module's pins.

Real Owner Repair Stories

• Quora user (2007 Buick Lucerne w/ Northstar V8, ~100,000 miles) — Blown head gasket.
  ✅ What actually fixed it The post implies a major engine repair or replacement was necessary. This serves as a real-world example of the most severe potential cause of a misfire on this engine.

OEM Part Supersession History

• 12597745 (Ignition Coil) → 19421260 — Standard part number evolution by GM.
  Heads up: ACDelco service part number is often D598A. These numbers should be cross-compatible.
• 12596683 (Fuel Injector) → 12597871, 12577869 — Standard part number evolution. Aftermarket equivalents like Bosch 0280158083 are also available.
  Heads up: These are all high-impedance, EV6/Uscar connector style injectors. Ensure the replacement matches this type.

Model Year Variations Within This Range

• 2006-2011: There were no significant mechanical or electronic changes to the 4.6L Northstar V8 within this specific year range that would affect the diagnosis of a P0308 code. The entire 2006-2011 FWD engine generation is distinct from the pre-2006 versions but is consistent within itself.