P0308 on 2008-2009 Buick Allure: Cylinder 8 Misfire Causes and Fixes

What's Unique About the 2006-2009 Buick ALLURE

The Buick Allure Super features a unique transverse (sideways) mounted 5.3L LS4 V8 engine to fit its front-wheel-drive platform. While it suffers from common misfire causes like bad coils and plugs, this engine family is also subject to a specific, severe mechanical failure related to its Active Fuel Management (AFM) system. As noted in Technical Service Bulletin #PIP4138R, a persistent misfire on any cylinder, including #8, can be caused by a worn camshaft lobe and/or a failed lifter. This failure is often preceded by a distinct 'chirp, squeak, squeal, or tick noise' at half of engine speed. A standard tune-up will not fix this mechanical issue.

Diagnostic Flowchart

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

Symptoms You May Notice

• Solid or flashing Check Engine Light
• Rough or vibrating idle
• Engine hesitation or stumbling during acceleration
• Noticeable loss of power
• Engine noise, such as a chirp, squeak, or tick, which could indicate a mechanical issue like a worn camshaft
• Shaking feeling from the engine bay

Most Likely Causes

1. Faulty Ignition Coil 🔴 High Probability → Shop Ignition Coil Coil-on-plug systems are common failure points due to constant heat exposure. A single coil failing is a very frequent cause of a single-cylinder misfire on the LS4 engine.
   How to confirm: Swap the ignition coil from cylinder 8 with another cylinder (e.g., cylinder 6). Clear the codes and drive the vehicle. If the code changes to P0306, the coil is bad. This is the most effective initial diagnostic step. 🎬 Watch: How to properly install a new ignition coil.
   Typical fix: Replace the faulty ignition coil. It's often recommended to replace the spark plug at the same time. Use a quality OEM brand like ACDelco.
   Est. part cost: $40-$80
2. Worn Spark Plug 🔴 High Probability → Shop Spark Plug Spark plugs are regular maintenance items. A worn electrode, cracked porcelain, or oil fouling can prevent a strong spark, causing a misfire. Oil fouling can be a symptom of AFM-related oil consumption issues, even on non-AFM cylinders like #8.
   How to confirm: After confirming the coil is good, remove the spark plug from cylinder 8. Inspect it for wear, fouling, or damage. Compare it to a new plug or a plug from a working cylinder.
   Typical fix: Replace the spark plug. It is best practice to replace all 8 spark plugs at the same time for even performance, using the recommended Iridium plugs for this engine.
   Est. part cost: $10-$20 per plug
3. Faulty Fuel Injector 🟡 Medium Probability → Shop Fuel Injector
   How to confirm: Use a mechanic's stethoscope or a long screwdriver to listen to the injector; it should make a consistent clicking sound. You can also test the injector's resistance with a multimeter to see if it's within specification. Swapping the injector with another cylinder is also a definitive test, though more labor-intensive than a coil swap.
   Typical fix: Replace the faulty fuel injector. It is often recommended to replace them as a set if they are high-mileage.
   Est. part cost: $50-$100
4. Worn Camshaft Lobe and/or Lifter ⚪ Low Probability → Shop Engine Camshaft As documented in GM TSB #PIP4138R, the 5.3L LS4 engine can experience premature wear on camshaft lobes and lifters. This is a well-known failure pattern often initiated by a collapsed Active Fuel Management (AFM) lifter on cylinders 1, 4, 6, or 7, which can disrupt oil pressure and valvetrain stability, leading to wear on other components, including the lifter and cam lobe for cylinder 8.
   How to confirm: This is an advanced diagnosis. If ignition and fuel system checks do not reveal the cause, a compression test should be performed. If compression is low, or if a distinct valvetrain tick is present, the valve cover and potentially the cylinder head must be removed to visually inspect the camshaft and lifters for wear.
   Typical fix: Requires replacement of the camshaft, all 16 lifters (both AFM and standard), and associated gaskets. Many owners opt for an 'AFM delete kit' during this repair to prevent recurrence. This is a significant and costly repair.
   Est. part cost: $500-$1200

Rare But Worth Checking

• Damaged Spark Plug Wire: → Shop Spark Plug Inspect the wire leading to the cylinder 8 coil for any signs of damage, cracking, or arcing. The OEM part number for a set is ACDelco 9748RR.
🎬 Watch: Step-by-step guide to replacing plugs, coils, and wires.
• Intake Manifold Gasket Leak: → Shop Engine Intake Manifold A vacuum leak near the cylinder 8 intake runner can lean out the air/fuel mixture and cause a misfire. This would often be accompanied by a P0300 random misfire code and can be tested by spraying brake cleaner near the runner and listening for an idle change.
• AFM-Related Oil Consumption: A known issue with the AFM pressure relief valve can cause oil spray within the crankcase, leading to carbon buildup and oil fouling on spark plugs on any cylinder, including #8. This can cause a misfire even if the plug itself is not worn out.

Diagnosis Steps

1. Read the trouble codes with an OBD-II scanner to confirm P0308.
2. Note if the Check Engine Light is flashing, which indicates a severe misfire requiring immediate attention.
3. Locate cylinder 8. On the transverse 5.3L LS4 V8, the cylinders are numbered 1-3-5-7 on the rear bank (firewall side) and 2-4-6-8 on the front bank (radiator side). Cylinder 8 is the one closest to the passenger side headlight, making it relatively accessible.
4. Swap the ignition coil from cylinder 8 with the coil from cylinder 6. Clear the codes and run the engine to see if the misfire code moves to P0306. If it does, the ignition coil is faulty.
5. If the misfire remains on cylinder 8, inspect and replace the spark plug for cylinder 8. Check for excessive wear or oil fouling.
6. If the misfire persists, check the fuel injector. Listen for a steady clicking noise. Check the wiring harness for damage.
7. Inspect the spark plug wire and the wiring harness for the coil and injector for any visible damage.
8. If all ignition and fuel components check out, perform a compression test on cylinder 8 to check for mechanical engine problems like worn rings or valve issues.
9. If compression is low or a persistent ticking noise is present, refer to TSB #PIP4138R and investigate for a worn camshaft lobe or lifter, a known issue on this engine.

Parts You'll Likely Need

• Ignition Coil (OEM #ACDelco D585 (or UF413)) — This is the most common failure point for a single-cylinder misfire on this engine. The square-shaped Melco/Mitsubishi style coil is typically used.
  Trusted brands: ACDelco, Delphi
  OEM price range: $60-$90
  Aftermarket price range: $35-$60
• Spark Plug (OEM #ACDelco 41-110 (Iridium)) — A worn or fouled spark plug is a very common and simple-to-fix cause of a misfire.
  Trusted brands: ACDelco, NGK
  OEM price range: $8-$15
  Aftermarket price range: $6-$12
• Fuel Injector (OEM #GM 12580579 / ACDelco 217-2255) — A clogged or failed injector will prevent the proper amount of fuel from reaching cylinder 8.
  Trusted brands: ACDelco, Bosch
  OEM price range: $75-$100
  Aftermarket price range: $25-$50
• Camshaft and Lifter Kit — In cases where standard diagnostics fail, a worn camshaft is a known issue on this platform as per TSB PIP4138R, often requiring replacement of the cam and all 16 lifters.
  Trusted brands: ACDelco, Melling, Texas Speed & Performance (for AFM delete)
  OEM price range: $600-$1000
  Aftermarket price range: $400-$700

Related Codes That Often Appear With This One

• P0300 — If the condition causing the P0308 is intermittent or beginning to affect other cylinders, a P0300 (Random/Multiple Cylinder Misfire) code may also be stored.

Technical Service Bulletins (TSBs) & Recalls

• PIP4138R: Instructs technicians to check for a worn camshaft

Platform-Specific Known Issues

• The 5.3L LS4 V8 engine, particularly those with Active Fuel Management (AFM), has a documented history of lifter failure and subsequent camshaft lobe wear. This can cause persistent misfires that are often misdiagnosed as ignition or fuel system problems. TSB #PIP4138R specifically calls out checking for a worn camshaft lobe if standard diagnostics do not isolate the cause of P0300-P0308 codes. While cylinder 8 is not an AFM cylinder, the failure of an AFM lifter elsewhere can lead to oil pressure issues and valvetrain instability that damages non-AFM components.
• The transverse mounting of the V8 makes access to the rear bank of cylinders (1, 3, 5, 7) difficult. Fortunately, cylinder 8 is on the front, more accessible bank near the radiator.

Mechanic-Grade Diagnostic Values

• Fuel Pressure (Key On, Engine Off) — expected: 55-62 PSI. Failure: Pressure below 55 PSI can indicate a weak fuel pump or faulty regulator.
• Fuel Injector Resistance — expected: 11.0 - 16.0 Ohms (typically ~12.3 Ohms for stock LS injectors). Failure: A reading of 'OL' (open loop), 0.0 Ohms (short), or a value that deviates more than 0.5 Ohms from the other injectors indicates a faulty injector.
• Ignition Coil Primary Resistance (Square D585 'Truck' Style) — expected: 0.4 - 2.0 Ohms. Failure: A reading outside this range suggests a faulty primary winding in the coil.
• Ignition Coil Secondary Resistance (Square D585 'Truck' Style) — expected: 6,000 - 10,000 Ohms (6k-10k Ω). Failure: A reading outside this range suggests a faulty secondary winding.
• Ignition Coil Primary Resistance (Round D514A Style) — expected: ~0.820 Ohms. Failure: A significant deviation from this value indicates a fault.
• Ignition Coil Secondary Resistance (Round D514A Style) — expected: ~7,100 Ohms (7.1k Ω). Failure: A significant deviation from this value indicates a fault.

Hidden / Shadow Codes Worth Checking

• Misfire Data List / Graph: A GM Tech 2 or similar advanced scan tool can display 'Current Misfire' and 'Misfire History' counters for each individual cylinder. This allows a technician to see small numbers of misfires that are not yet significant enough to trigger a P0308 trouble code. (see via Requires a professional scan tool like a GM Tech 2 or GDS2. This data is typically found under the engine data or misfire data menu.)

Scan Tool Commands That Help

• GM Tech 2 / GDS2: Crankshaft Position (CKP) / Camshaft Position (CMP) Correlation Relearn — Use this after replacing a crankshaft or camshaft sensor, or if you suspect false misfire counts are being generated. It recalibrates the PCM's ability to accurately detect misfires, which can be affected by timing chain wear or sensor replacement.
• GM Tech 2 / GDS2: Injector Balance Test — To identify a poorly performing fuel injector after ignition issues have been ruled out. The tool commands each injector to fire and, when used with a fuel pressure gauge, allows the technician to measure the pressure drop for each cylinder to find one that is weak or clogged.

Wiring & Ground Locations

• G100 — On the frame, in the area behind the right (passenger side) headlamp.. Cylinder 8 is on the front (radiator side) bank, close to the passenger side. This ground is a primary candidate for causing electrical issues for the cylinder 8 ignition coil and fuel injector due to its proximity.
• G115 — Located on a transmission stud, to the right of the starter motor.. This is a major powertrain ground. While less specific to a single cylinder, a poor connection here can cause a variety of erratic engine management issues, including misfires.
• ECM Location — Left front of the engine compartment.. Knowing the location of the Engine Control Module (ECM) is essential for tracing the control wiring for the cylinder 8 coil and injector back to their source and checking for continuity or damage.

OEM Part Supersession History

• GM 12571595 (and others) → GM 12645725, 12698946 — Design improvements to the Active Fuel Management (AFM) lifters to increase durability and reduce failure rates.
  Heads up: Newer Dynamic Fuel Management (DFM) lifters are generally backward-compatible with older AFM engines, but original AFM lifters will NOT work in newer DFM engines. When replacing, it is best to use the latest available part number for all AFM lifters.
• ACDelco D585 (Square 'Truck' Coil) → ACDelco D514A (Round Coil) — Change in ignition system design for later model years.
  Heads up: The coils are physically different shapes (square vs. round) and have different mounting brackets and potentially different wiring connectors. It is critical to visually inspect the existing coils before ordering replacements.

Model Year Variations Within This Range

• 2006: Vehicles in this year used the E40 Engine Control Module (ECM).
• 2007-2009: Vehicles in these years switched to the more advanced E67 Engine Control Module (ECM), which impacts diagnostics and tuning capabilities.
• 2008-2009 (approx.): May have switched from the square D585 style ignition coil to a round D514A style coil. Visual verification is required before purchasing replacements.