P0308 on 2004-2007 Buick Rainier 5.3L: Cylinder 8 Misfire Causes and Fixes

What's Unique About the 2004-2007 Buick RAINIER

This code is particularly noteworthy on the 2005-2007 Rainier's 5.3L V8 (engine code LH6), which features Active Fuel Management (AFM). This system deactivates four cylinders (1, 7, 4, and 6) under light loads to save fuel, but the special hydraulic lifters it uses are a known failure point. A collapsed or stuck AFM lifter is a common cause for a P0300 code on these specific model years, a problem less prevalent on the 2004 model's non-AFM 5.3L V8 (engine code LM4). The failure often stems from issues with the Valve Lifter Oil Manifold (VLOM), which controls oil flow to the lifters, or from oil pressure/quality issues that cause the lifter's internal locking pins to fail.

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 shaking idle.
• Hesitation or stumbling during acceleration.
• Noticeable loss of engine power.
• Solid or flashing Check Engine Light.
• Decreased fuel economy.
• A rhythmic ticking or tapping noise from the engine that increases with RPM, especially if a lifter has failed.
• Possible fuel smell from the exhaust.

Most Likely Causes

1. Failed Ignition Coil 🔴 High Probability → Shop Ignition Coil Coil-on-plug ignition systems are common, and individual coils can fail with age and heat cycles. It's a very frequent cause for single-cylinder misfires on these engines and often the first part to check.
   How to confirm: Swap the ignition coil from cylinder 8 with another cylinder (e.g., cylinder 6, which is easier to access). Clear the codes and drive. If the code changes to P0306, the coil is faulty.
   Typical fix: Replace the faulty ignition coil. It's recommended to use a quality OEM or aftermarket brand like ACDelco or Delphi.
   Est. part cost: $40-$75
2. Worn or Fouled Spark Plug 🔴 High Probability → Shop Spark Plug Spark plugs are a routine maintenance item with a service life of around 100,000 miles. If they are old, the electrode can be worn, or they can become fouled with oil or carbon deposits, preventing a proper spark. Cylinder 8 is notoriously difficult to access, so it may have been skipped during previous tune-ups.
   How to confirm: Remove the spark plug from cylinder 8 and inspect it. Look for a worn electrode, cracks in the porcelain, or heavy oil/carbon deposits. Compare it to a plug from a working cylinder.
   Typical fix: Replace the spark plug. If all plugs are near the end of their service life, it's best to replace all eight at the same time. Special tools like a stubby extension, a flex-head ratchet, and spark plug boot pliers can make the job on cylinder 8 easier. 🎬 Watch: Step-by-step guide for replacing spark plugs on this engine.
   Est. part cost: $8-$20 per plug
3. Failed AFM Lifter (2005-2007 models) 🟡 Medium Probability → Shop Engine Valve Lifter The LH6 engine's Active Fuel Management system uses special lifters that can get stuck or collapse, especially due to oil pressure issues, oil aeration, or internal wear of the locking pins. This prevents the valves for that cylinder from opening and closing correctly, causing a misfire. While cylinder 8 is not an AFM cylinder, problems in the oiling system related to AFM (like a faulty VLOM) can lead to failure of standard lifters as well.
   How to confirm: This is more difficult to diagnose. It often presents with a persistent ticking noise that sounds like a sewing machine and gets faster with engine RPM. A compression test will show low compression on cylinder 8. Advanced diagnosis involves removing the valve cover to observe valve train movement or using a stethoscope to pinpoint the noise. 🎬 See how to diagnose and replace a collapsed AFM lifter.
   Typical fix: This is a major repair that involves removing the cylinder head to replace the lifters. It is highly recommended to replace all 16 lifters (8 AFM and 8 standard), the lifter guides, and the Valve Lifter Oil Manifold (VLOM) at the same time. Many owners opt for an 'AFM delete kit' which replaces the AFM components with standard, more robust parts and requires an ECU tune to disable the system.
   Est. part cost: $350-$800 for a complete lifter and gasket kit or an AFM delete kit.
4. Clogged or Faulty Fuel Injector ⚪ Low Probability → Shop Fuel Injector
   How to confirm: Listen to the injector with a mechanic's stethoscope; it should make a consistent clicking sound. You can also test its resistance with a multimeter or swap it with another cylinder's injector to see if the misfire code moves.
   Typical fix: Replace the faulty fuel injector. It's often recommended to replace them as a set on higher mileage vehicles for balanced performance.
   Est. part cost: $50-$90

Rare But Worth Checking

• Worn Camshaft Lobe: → Shop Engine Camshaft As cited in GM Technical Service Bulletin #PIP4138R (superseded by 19-NA-219), a worn camshaft lobe can cause a persistent misfire and/or a ticking/chirping noise. This is often discovered during a lifter replacement job, as a failed lifter roller can quickly destroy the corresponding cam lobe.
• Intake Manifold Gasket Leak: → Shop Engine Intake Manifold A vacuum leak near the intake runner for cylinder 8 can lean out the air/fuel mixture enough to cause a misfire. This can sometimes be found by spraying brake cleaner near the gasket with the engine running and listening for a change in idle speed.
• Low Compression (other causes): Beyond a failed lifter, low compression can be caused by a damaged piston, worn piston rings, or a valve that isn't sealing correctly (bent valve, worn valve seat). A compression test and cylinder leak-down test will diagnose this. A broken valve spring is also a possible cause.
• Faulty Valve Lifter Oil Manifold (VLOM): On AFM engines (2005-2007), the VLOM under the intake manifold contains solenoids and oil passages that control the AFM system. A faulty VLOM can cause oil pressure issues leading to lifter failure. Sometimes the VLOM gasket leaks or the bolts come loose, causing similar issues.

Diagnosis Steps

1. Read the code with an OBD-II scanner to confirm P0308 and check for any other codes.
2. Note if the Check Engine Light is steady or flashing. A flashing light indicates a severe, catalyst-damaging misfire requiring immediate attention.
3. Identify Cylinder 8: It is the rearmost cylinder 🎬 Watch: A quick breakdown of common P0308 causes and fixes. on the passenger side of the engine.
4. Swap the ignition coil from cylinder 8 with an adjacent, easy-to-access cylinder (like 6). Clear the codes and run the engine. If the code changes to P0306, the ignition coil is bad.
5. If the code remains P0308, swap the spark plug from cylinder 8 with the one from cylinder 6. If the code moves to P0306, the spark plug was the issue. Be prepared for difficulty accessing the #8 plug.
6. Inspect the wiring and connector for the cylinder 8 ignition coil and fuel injector for any damage, corrosion, or looseness.
7. If ignition parts are good, listen to the fuel injector on cylinder 8 with a stethoscope. It should have a regular, audible clicking sound.
8. If the above steps don't identify the issue, perform a compression test on cylinder 8 to check for mechanical problems. Low compression often points to a valve train issue.
9. On 2005-2007 models, if compression is low and/or a rhythmic ticking noise is present, suspect a failed lifter. This may require removing the valve cover for visual inspection of valve train movement.

Parts You'll Likely Need

• Ignition Coil (OEM #12573190 (superseded by 12739916)) — This is one of the most common failure points for a single-cylinder misfire. Heat and vibration cause the internal windings to fail over time.
  Trusted brands: ACDelco, Delphi, Dragon Fire Performance
  OEM price range: $60-$80
  Aftermarket price range: $40-$60
• Spark Plug

Related Codes That Often Appear With This One

• P0300 — If the misfire is intermittent or other cylinders are also beginning to fail (common with AFM issues), a P0300 (Random/Multiple Cylinder Misfire) code may appear alongside P0308.
• P050D — TSB #PIP5628G notes that P050D (Rough Idle On Cold Start) can appear with P030x codes, indicating a potential need for deeper diagnosis with GM's technical assistance.

Technical Service Bulletins (TSBs) & Recalls

• PIP4138R / 19-NA-219: Diagnostic Tip for Misfire, Chirp, Squeak, Squeal or Tick Noise, Malfunction Indicator Lamp (MIL) Illuminated - DTC P0300-P0308 Set.
• PIP5628G: Information on Diagnosing Misfire DTCs P0300 - P0308 and/or P050D.

Platform-Specific Known Issues

• TSB #PIP4138R directly addresses engine misfires (P0300-P0308) on the 2006 Rainier, pointing technicians to check for a worn camshaft lobe if standard diagnostics don't find the cause.
• TSB #PIP5628G mentions that codes P0300-P0308 may require gathering specific information to send to GM's Technical Assistance Center (TAC) for diagnosis, indicating the complexity of some misfire issues on this platform.
• A clogged filter screen for the AFM system, located under the oil pressure sensor, can restrict oil flow and contribute to lifter failure.

Mechanic-Grade Diagnostic Values

• Ignition Coil Primary Resistance — expected: 0.820 ohms. Failure: A reading significantly higher or lower than the specification, or an open circuit (infinite resistance).
• Ignition Coil Secondary Resistance — expected: 7.10k ohms. Failure: A reading significantly outside the specification.
• Stock Fuel Injector Resistance (Impedance) — expected: 12.5 Ohms (High Impedance). Failure: A reading significantly different from the specification, indicating a potential issue with the injector's internal coil.
• Mode 6 Misfire Monitor Data — expected: A low count of misfires on cylinder 8 during the last drive cycle.. Failure: A high or incrementing value for cylinder 8 (Component ID $08) under Test ID $A2 (Misfire Cylinder 1 Data) through Test ID $A9 (Misfire Cylinder 8 Data) indicates current misfires, even if they aren't frequent enough to set a code immediately.

Scan Tool Commands That Help

• GM Tech2 / GDS2: Cylinder Power Balance — This test allows the technician to sequentially disable the fuel injector to each cylinder. When cylinder 8 is disabled, if there is no change in engine RPM or smoothness, it confirms that cylinder 8 was not contributing to begin with, pinpointing the misfire.
• GM Tech2 / GDS2: Misfire Graphic — Provides a real-time graph of misfire counts per cylinder, which is useful for seeing the effect of diagnostic steps (like wiggling a wire or spraying carb cleaner) immediately.
• GM Tech2 / GDS2: Fuel Injector Balance — This test fires each injector for a set amount of time and measures the corresponding drop in fuel pressure. A smaller pressure drop for cylinder 8 compared to others would indicate a clogged or restricted injector.

Wiring & Ground Locations

• G102 — Located at the left rear of the engine.. This ground is specifically for all 8 ignition coils. A loose or corroded connection here could cause weak spark or intermittent misfires on any cylinder, including cylinder 8.
• G103 — Located on the cowl, above the brake booster at the left rear of the engine compartment.. This is a major ground point for the Body Control Module (BCM), Instrument Panel Cluster (IPC), and the Data Link Connector (DLC). While not a direct cause, a poor ground here can cause erratic communication and diagnostic issues.
• G108 — On the lower left side of the engine block.. This is a primary ground for the Engine Control Module (ECM) on the 5.3L engine. A poor connection here can cause a wide range of engine performance issues, including misfires.
• Cylinder 8 Coil Connector — On the ignition coil for the rearmost cylinder on the passenger side bank.. The wiring and connector pins for the cylinder 8 coil should be inspected for corrosion, damage, or looseness. Given its location at the rear of the engine, it is susceptible to heat damage and moisture intrusion.

Real Owner Repair Stories

• LS1TECH Forum User (2001 Firebird LS1 (mechanically similar V8)) — Misfire codes on all passenger side cylinders (P0302, P0304, P0306, P0308).
  ❌ Tried (didn't work) Initial diagnosis focused on individual cylinder components.
  ✅ What actually fixed it The problem was a faulty Bank 2 (passenger side) oxygen sensor. A bad O2 sensor was sending incorrect data to the ECM, causing the entire bank to run with an improper fuel mixture, leading to misfires on all four cylinders on that side.
• GMTNation Forum User (GMT360 Platform with 5.3L LH6 engine) — Misfire on cylinder 1 (P0301) with low and inconsistent compression readings (80-90 psi).
  ❌ Tried (didn't work) Driving the vehicle normally.
  ✅ What actually fixed it The user was advised to try a top engine cleaner (ACDelco Top Engine Cleaner) sprayed directly into the cylinder and allowed to soak. The theory was that a carbon deposit on the valve seat was preventing the valve from sealing properly, causing the low compression. This procedure, if successful, dissolves the carbon and restores compression without major mechanical work.

OEM Part Supersession History

• 12571609 (example) → 12698755 — Updated design for the Valve Lifter Oil Manifold (VLOM) for AFM engines.
  Heads up: Older part numbers for the VLOM have been superseded. When replacing, it is critical to use the latest version and verify fitment by VIN, as there were several revisions.

Model Year Variations Within This Range

• 2005-2006 vs 2007: The 2005-2006 LH6 engines used a 24x crankshaft reluctor wheel, while the 2007 LH6 switched to a 58x reluctor wheel. This is a critical difference for the ECM and ignition system timing and means that engine control modules and crankshafts are not interchangeable between these year groups without a signal conversion module.