P0308 on 2007-2011 GMC Sierra Denali: Cylinder 8 Misfire Causes and Fixes

What's Unique About the 2007-2011 Gmc SIERRA DENALI

The 2007-2011 GMC Sierra Denali is equipped with V8 engines (like the 6.2L L94) that feature Active Fuel Management (AFM). This system is designed to improve fuel economy by deactivating cylinders 1, 4, 6, and 7 🎬 Watch: How AFM lifters work and why they fail. under light loads. The specialized hydraulic lifters used for this system are a well-documented failure point and can become stuck, collapsed, or fail to 'pump up' with oil, leading to misfires. While cylinder 8 is not an AFM cylinder, the entire valvetrain system, including the standard lifters, is prone to issues that can manifest as a P0308 code. TSB #PIP4138R specifically directs technicians to investigate for worn camshaft lobes and collapsed lifters when P030x codes are present after basic diagnostics fail.

Diagnostic Flowchart

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

Symptoms You May Notice

• Check Engine Light is on or flashing
• Rough or shaky engine idle
• Hesitation or stumbling during acceleration
• Noticeable loss of engine power
• Reduced fuel economy
• Ticking, tapping, or squeaking noise from the engine, which may indicate a lifter or camshaft issue
🎬 See how to identify engine ticking and misfire codes.
• "Service StabiliTrak" or "Service Traction Control" message may appear, as the engine instability can be misinterpreted by the chassis control systems

Most Likely Causes

1. Faulty Ignition Coil 🔴 High Probability → Shop Ignition Coil Coil-on-plug ignition systems are common, and individual coils can fail from heat and age. Cylinder 8 is the rearmost cylinder on the passenger side, a location that can be susceptible to higher temperatures and heat soak, potentially shortening coil life.
   How to confirm: Swap the ignition coil from cylinder 8 with another cylinder (e.g., cylinder 6, which is easier to access on the same bank). Clear the codes and drive the vehicle. If the code changes to P0306, the coil is bad.
   Typical fix: Replace the faulty ignition coil. It's often recommended to replace the corresponding spark plug at the same time.
   Est. part cost: $40-$115
2. Worn or Fouled Spark Plug 🔴 High Probability → Shop Spark Plug
   How to confirm: Remove the spark plug from cylinder 8 and inspect it. Look for a worn electrode, heavy carbon deposits, oil fouling, or coolant contamination. Compare it to a plug from a working cylinder. A plug that is wet with fuel can indicate a lack of spark, while heavy oil deposits can point to mechanical issues.
   Typical fix: Replace the spark plug. It is recommended to replace all 8 spark plugs at the same time for even performance, especially if they are near the end of their service life (around 100,000 miles for OEM iridium plugs).
   Est. part cost: $8-$25 per plug
3. Failed Active Fuel Management (AFM) Lifter or Related Valvetrain Component 🟡 Medium Probability → Shop Engine Valve Lifter The AFM lifters in these GM V8 engines are a well-documented failure point. They can collapse or become stuck, preventing the valve from opening correctly, which causes a misfire. This can be caused by oil pressure issues or the design of the lifter itself. Even though cylinder 8 is not an AFM cylinder, the standard lifters can also fail, and the camshaft can be damaged by a failed lifter on another cylinder, affecting overall engine timing and performance. TSB #PIP4138R directly points to this as a cause for P030x codes.
   How to confirm: After ruling out ignition and fuel issues, listen for a distinct ticking or tapping noise. A compression test is the next step. Low compression on cylinder 8 points to a mechanical problem. A physical inspection of the valvetrain by a professional is required to confirm a failed lifter, which often involves removing the valve cover and cylinder head.
   Typical fix: Requires replacing the faulty lifter(s). Often, the camshaft is also damaged ('wiped lobe') and requires replacement. Many owners opt to replace all lifters (both AFM and standard) or install an AFM-delete kit to prevent future failures. This is a labor-intensive job.
   Est. part cost: $400-$1200 for lifters, camshaft, and gaskets
4. Clogged or Faulty Fuel Injector 🟡 Medium Probability → Shop Fuel Injector
   How to confirm: Swap the fuel injector from cylinder 8 with another cylinder. If the misfire code follows the injector, it is the cause. You can also use a 'noid light' to verify the injector is receiving an electrical pulse from the ECM. A professional can perform a fuel injector balance test using a scan tool.
   Typical fix: Replace the faulty fuel injector.
   Est. part cost: $50-$120
5. Worn Camshaft Lobe ⚪ Low Probability → Shop Engine Camshaft This is almost always a direct result of a failed lifter. The roller on the lifter seizes and grinds against the camshaft lobe, wearing it down and causing the valve to not open fully. TSB #PIP4138R explicitly mentions checking for a worn camshaft lobe when diagnosing persistent misfires.
   How to confirm: Requires removal of the valve cover and potentially the cylinder head to visually inspect the camshaft lobe for wear after a failed lifter is suspected or confirmed. This is an advanced, labor-intensive diagnostic step.
   Typical fix: Replace the camshaft, all 16 lifters, and associated gaskets and seals. It is highly recommended to install an AFM delete kit at this time.
   Est. part cost: $250-$600 for the camshaft alone

Rare But Worth Checking

• 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 and cause a misfire. This is less common for a single-cylinder misfire but worth checking with a smoke test or by carefully spraying starter fluid around the intake port.
• Low Compression (Valves, Piston Rings): Beyond lifter failure, a bent valve, broken valve spring, worn valve seat, or failing piston rings can cause low compression and a persistent misfire. A compression test followed by a cylinder leak-down test will identify this.
• Coolant Leak onto Plug/Wire: In some instances, a coolant leak from a heater hose or fitting located above the passenger side of the engine can drip onto the cylinder 8 spark plug wire and boot, causing the spark to ground out and induce a misfire.

Diagnosis Steps

1. Read the code with an OBD-II scanner and confirm P0308 is present. Note any other codes and check freeze-frame data to see when the misfire occurs (e.g., idle, under load).
2. Visually inspect the area around cylinder 8 (rear passenger side). Look for obvious issues like a disconnected spark plug wire, damaged wiring, or coolant/oil leaks.
3. Swap the ignition coil from cylinder 8 with cylinder 6. Clear the codes and test drive. If the code changes to P0306, the coil is bad.
4. If the misfire remains on cylinder 8, swap the spark plug from cylinder 8 with cylinder 6. If the code moves to P0306, the plug was the issue. If not, inspect the old plug for signs of oil, fuel, or coolant fouling.
5. If the ignition system is good, test the fuel injector. You can swap it with another cylinder or use a noid light to check for a signal.
6. If ignition and fuel are confirmed to be working, perform a compression test on cylinder 8. Compare the reading to a healthy cylinder.
🎬 Watch: Diagnosing an intermittent misfire with a compression test.
7. If compression is low, the issue is mechanical. This strongly suggests a failed lifter and/or worn camshaft lobe, as documented in TSB #PIP4138R. At this point, professional diagnosis involving engine disassembly is recommended to confirm the failure and assess for collateral damage like a scored camshaft.

Parts You'll Likely Need

• Ignition Coil (OEM #12699382 (supersedes 12611424, 12570616)) — A common failure item causing a lack of spark to the cylinder.
  Trusted brands: ACDelco (GM Original Equipment), Delphi, Denso
  OEM price range: $80-$125
  Aftermarket price range: $40-$70
• Iridium Spark Plug (OEM #41-162) — Spark plugs are a routine maintenance item and can wear out or become fouled, causing a misfire.
  Trusted brands: ACDelco, NGK
  OEM price range: $15-$25
  Aftermarket price range: $8-$18

Related Codes That Often Appear With This One

• P0300 — If the condition causing the P0308 is severe or intermittent, or if other cylinders are also beginning to misfire (e.g., multiple failing lifters), a P0300 (Random/Multiple Cylinder Misfire) code may also be set.

Technical Service Bulletins (TSBs) & Recalls

• PIP4138R: Addresses engine misfire codes P0300-P0308, pointing towards mechanical issues like worn camshaft lobes, collapsed AFM lifters, and broken valve springs.
• PIP5628E: Provides a diagnostic template for technicians to follow when diagnosing misfires (P0300-P0308), emphasizing a structured approach starting with scan tool data and moving to compression and leak-down tests.

Platform-Specific Known Issues

• Active Fuel Management (AFM) lifter failure is a significant and widely documented problem on 2007-2011 GM V8 engines. A failure can lead to a misfire code, engine noise, and potentially catastrophic damage to the camshaft. TSB #PIP4138R directly addresses this, advising technicians to check for worn camshaft lobes and lifters when diagnosing P030x codes. The bulletin also recommends inspecting the Valve Lifter Oil Manifold (VLOM) filter screen underneath the oil pressure sensor, as debris here can indicate internal engine problems.

Mechanic-Grade Diagnostic Values

• Fuel Pressure (Key On, Engine Off) — expected: 50-60 PSI. Failure: Pressure below 50 PSI may indicate a weak fuel pump or leak.
• Fuel Pressure (Engine Idling) — expected: 35-45 PSI (will vary with vacuum). Failure: Pressure that drops significantly or is erratic can point to a failing fuel pressure regulator or pump.
• Compression Test — expected: 120-150 PSI, with less than 15% variation between cylinders. Failure: A reading significantly below 100 PSI or with high variance from other cylinders indicates a mechanical issue (rings, valves, head gasket, lifter).
• Injector Balance Rates (Scan Tool) — expected: Close to 0% for all cylinders.. Failure: A cylinder requiring significantly more fuel (a high positive percentage) to maintain crankshaft speed may have a mechanical or ignition issue, while one requiring less (a high negative percentage) may have a leaking injector.

Hidden / Shadow Codes Worth Checking

• P124F: Injector 8 Positive Voltage Control Circuit Shorted to Control Circuit. This is a GM-specific code that can accompany a P0308 and points directly to a wiring or electrical fault with the cylinder 8 fuel injector circuit. (see via Professional-grade OBD-II scanner with manufacturer-specific code reading capabilities.)

Scan Tool Commands That Help

• Tech 2 / GDS2: Cylinder Power Balance — To identify which cylinder is contributing least to engine power. For a P0308, this test should show cylinder 8 with a significant power drop compared to others.
• Tech 2 / GDS2: Fuel Injector Balance Test — To command each injector to fire and measure the resulting pressure drop. A cylinder 8 injector that shows a significantly different pressure drop than the others is likely faulty or clogged.
• Tech 2 / GDS2: Misfire Data / Misfire Counters — To view current and historical misfire counts for each cylinder. This helps confirm the misfire is isolated to cylinder 8 and see under what conditions (RPM, load) it occurs most frequently.

Wiring & Ground Locations

• G102 — Located at the left rear of the engine.. This ground point is for all 8 ignition coils. A poor connection here can cause weak spark and misfires on any or all cylinders, including cylinder 8.
• G103 — Located at the left rear of the engine compartment on the cowl, above the brake booster.. This is a primary ground for the Engine Control Module (ECM), Body Control Module (BCM), and Instrument Panel Cluster (IPC). A faulty ground here can cause a wide range of erratic electrical issues, including incorrect sensor readings and false misfire detection.
• Engine Block to Chassis/Firewall Ground Strap — Typically runs from the rear of the passenger-side cylinder head to the firewall.. This main ground strap is critical for the entire engine's electrical system. It is notoriously prone to corrosion and breaking on GMT900 trucks. A degraded connection increases electrical resistance, which can cause weak spark, injector issues, and various seemingly unrelated electrical problems.
• Cylinder 8 Ignition Coil Connector — Rearmost connector on the passenger side valve cover.. The wiring and connector pins for the cylinder 8 coil should be inspected for corrosion, especially if there has been a history of coolant leaks from the heater hoses routed nearby.

Real Owner Repair Stories

• ScannerDanner Forum User (2016 GMC Sierra 5.3L (Note: Newer model, but same P0308 lifter/cam issue)) — Flashing P0308 check engine light.
  ❌ Tried (didn't work) A dealership performed a camshaft and lifter replacement for a collapsed lifter on cylinder 8, costing $6,500.
  ✅ What actually fixed it The P0308 code returned just two days after the major repair. The owner's father, using a scan tool, noted the bank 2 catalytic converter was 50 degrees colder than bank 1, suggesting the misfire was still present and the dealership may have missed a bent/sticking valve or other mechanical issue during the initial repair. The final resolution was not posted, but the story highlights that even a full cam/lifter job might not be the complete fix if underlying damage is missed.
• Reddit user on r/gmcsierra (2014 GMC Sierra (Note: Newer model, but relevant wiring issue)) — Blinking check engine light, weak and shaking engine, P0308 and P124F codes.
  ❌ Tried (didn't work) Swapping ignition coils between cylinders did not move the misfire code., Replacing spark plugs and wires a year prior.
  ✅ What actually fixed it The user suspected the ignition coil wiring harness for cylinder 8 was the culprit due to corrosion from a previous coolant leak at the firewall. Unplugging the coil connector on cylinder 8 made no change to the engine's running condition, confirming it wasn't firing. The combination of P0308 and the injector-specific P124F code strongly pointed to a wiring fault affecting both ignition and fuel circuits for that cylinder, rather than a mechanical engine problem.

Model Year Variations Within This Range

• 2007-2008: The primary 6.2L V8 was the L92. It features Variable Valve Timing (VVT) but does NOT have Active Fuel Management (AFM). While still susceptible to standard lifter and cam wear, it is not prone to the specific AFM lifter collapse that is common in later years.
• 2009: The 6.2L engine was updated to the L9H. The main change was the addition of Flex Fuel (E85) capability. Like the L92, it has VVT but does NOT have AFM.
• 2010-2011: The 6.2L engine was the L94. This version kept the VVT and Flex Fuel capability of the L9H but ADDED Active Fuel Management (AFM). This makes these specific model years highly susceptible to AFM-related lifter failures, which are a primary cause of misfire codes.