P0308 on 2014-2018 GMC Savana: Cylinder 8 Misfire Causes and Fixes

What's Unique About the 2014-2018 Gmc SAVANA

The V8 engines available in the 2014-2018 GMC Savana (specifically the 4.8L LY2, 5.3L LC9, and 6.0L L96/LC8) have a well-documented, critical weakness, particularly those with Active Fuel Management (AFM). The hydraulic valve lifters used in the AFM system are a known high-failure item. These lifters can become stuck, collapse, or fail to activate properly, which prevents the valve from opening and causes a misfire code like P0308. If ignored, a failed lifter can destroy the camshaft lobe, sending metallic debris through the engine. This known issue is addressed in GM TSB 19-NA-219, which guides technicians to investigate worn camshafts and collapsed lifters for P030x codes. Therefore, a P0308 on a Savana V8 must be treated with more suspicion than on other vehicles.

Diagnostic Flowchart

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

Symptoms You May Notice

• Flashing or solid Check Engine Light
• Rough or shaking idle
• Noticeable loss of power and hesitation during acceleration
• A rhythmic ticking or tapping noise from the engine, which is a strong indicator of a collapsed or failing lifter.
• Increased fuel consumption
• Smell of unburned fuel from the exhaust
• Engine may run fine for a few minutes after startup and then begin to misfire once the AFM system engages/disengages.

Most Likely Causes

1. Faulty Ignition Coil 🔴 High Probability → Shop Ignition Coil Heat and vibration in the engine bay cause ignition coils to fail over time. Cylinder 8 is located at the rear of the engine on the passenger side, where heat can be trapped and access is tight, potentially accelerating wear.
   How to confirm: The easiest way to confirm a bad coil is to swap the ignition coil from cylinder 8 with another cylinder (e.g., cylinder 6, which is also on the passenger side). Clear the codes and drive the vehicle. If the code changes to P0306, the coil is faulty.
   Typical fix: Replace the faulty ignition coil. It is often recommended to replace the corresponding spark plug at the same time.
   Est. part cost: $30-$80
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 heavy carbon buildup, oil fouling, or a worn/damaged electrode. A plug fouled with oil can also indicate other internal engine problems like bad valve seals or piston rings.
   Typical fix: Replace the spark plug. It is best practice to replace all spark plugs at the same time, especially on a high-mileage engine, to ensure even performance.
   Est. part cost: $5-$20 per plug
3. Failed Hydraulic Lifter / Worn Camshaft 🟡 Medium Probability → Shop Engine Camshaft This is the most notorious issue for these GM V8s. The Active Fuel Management (AFM) system uses special lifters on cylinders 1, 4, 6, and 7, but all lifters in these engines are prone to failure. A collapsed lifter will fail to open its corresponding valve, causing a dead misfire. TSB 19-NA-219 (which supersedes PIP4138R) directly addresses this, pointing to worn cam lobes and collapsed lifters as a primary cause for P030x codes.
   How to confirm: This requires professional diagnosis. A mechanic will listen for the characteristic top-end ticking noise. They may need to remove the valve cover to observe valvetrain movement. A definitive diagnosis involves measuring lifter bleed-down and inspecting the camshaft lobes for scoring or wear, which requires significant engine disassembly.
   Typical fix: This is a major and expensive repair. It involves replacing all 16 lifters, the valve lifter oil manifold (VLOM), and often the camshaft if it has been damaged. Many owners opt to install an 'AFM Delete Kit' at this time, which replaces the failure-prone AFM components with standard, more reliable lifters and requires a PCM tune to disable the system.
   Est. part cost: $500-$1500+
4. Faulty Fuel Injector ⚪ Low Probability → Shop Fuel Injector
   How to confirm: A professional can perform a fuel injector balance test to measure fuel flow. A DIY method is to use a mechanic's stethoscope to listen for a consistent clicking sound from the injector; silence indicates a potential failure. You can also swap the cylinder 8 injector with another cylinder and see if the misfire code follows.
   Typical fix: Replace the faulty fuel injector.
   Est. part cost: $50-$150

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 enough to cause a misfire. This is less common for a single-cylinder misfire but is possible.
• Low Engine Compression: Internal engine damage, such as a bad valve, bent pushrod (often from a failed lifter), or worn piston rings, can cause a loss of compression in cylinder 8. This is a serious issue and would require a compression test to confirm.
• Coolant Leak onto Spark Plug Wire: → Shop Spark Plug In a documented case on a similar GM truck platform, a coolant leak from the heater core quick-connect lines dripped directly onto the #8 spark plug wire, causing an intermittent short and a P0308 code.
• Excessive Carbon Buildup on Valves: As noted in TSB 16-NA-383, excessive carbon can cause valves to stick, leading to misfires. This typically occurs after higher mileage and can be exacerbated by the AFM system.

Diagnosis Steps

1. Read the code with an OBD-II scanner to confirm P0308 and check for any other codes, especially P0300 or P050D.
2. Listen for Engine Noise: Start the engine and listen carefully for a distinct ticking or tapping sound from the top of the engine, particularly the passenger side. A ticking noise strongly points towards a valvetrain/lifter issue.
3. Inspect the spark plug wire and the wiring harness going to the ignition coil and fuel injector for cylinder 8 (rear passenger side). Check for damage, loose connections, or evidence of fluid leaks from above.
4. Swap the ignition coil from cylinder 8 with cylinder 6 (front passenger side). Clear the codes and drive. If the misfire code changes to P0306, the coil is bad.
5. If the misfire does not move, swap the spark plug from cylinder 8 with another cylinder. If the code follows the plug, the plug is bad.
6. If ignition components are good, listen for a steady clicking from the fuel injector on cylinder 8 using a mechanic's stethoscope. An absence of clicking could indicate a bad injector or wiring issue.
7. At this point, if the cause is not found and especially if a ticking noise is present, a mechanical issue is highly likely. A professional diagnosis is strongly recommended.
8. A professional should perform a compression test and a cylinder leakage test to check the mechanical health of cylinder 8. Low compression could mean a bad valve, piston ring, or a valve that isn't opening due to a collapsed lifter.
9. If compression is low, the next step is removing the valve cover to inspect the valvetrain for a bent pushrod, broken spring, or rocker arm not moving, which would confirm a lifter/camshaft issue as described in TSB 19-NA-219.

Parts You'll Likely Need

• Ignition Coil (OEM #12611424) — Ignition coils are a common failure item that directly causes a single-cylinder misfire. The original part number in Pass 1 (12632479) is primarily for V6 engines.
  Trusted brands: ACDelco, Delphi, NGK
  OEM price range: $45-$70
  Aftermarket price range: $25-$50
• Spark Plug (OEM #41-110) — As a routine maintenance item, spark plugs wear out and can become fouled, leading to a misfire. This is an Iridium plug specified for many GM V8s.

Related Codes That Often Appear With This One

• P0300 — If the misfire is intermittent or starting to affect other cylinders due to a widespread issue (like a failing camshaft or low fuel pressure), a P0300 (Random/Multiple Cylinder Misfire) code may appear alongside P0308.
• P050D — This code for 'Cold Start Rough Idle' is often seen with P030x codes. TSB PIP5628G specifically groups these codes together for diagnosis, suggesting a potential common root cause that is more apparent during a cold start.

Technical Service Bulletins (TSBs) & Recalls

• TSB 19-NA-219: Supersedes PIP4138R. This is a key diagnostic guide for technicians encountering P0300-P0308 misfire codes with or without engine noise. It explicitly tells them to investigate for a worn camshaft lobe, collapsed AFM lifter, sticking valve, or broken valve spring if initial diagnostics don't find the cause.
• TSB PIP5628G: A diagnostic aid bulletin for technicians dealing with misfires (P0300-P0308) and/or rough idle on cold start (P050D). It provides a template of information to gather before contacting technical assistance, indicating the complexity of these issues.
• TSB 16-NA-383: Referenced in other bulletins, this TSB provides instructions for engine decarbonizing procedures, which may be necessary if excessive carbon buildup is causing valves to stick and misfire.

Platform-Specific Known Issues

• Active Fuel Management (AFM) Lifter Failure: This is the most significant and costly issue. The specialized hydraulic lifters used in the AFM system are prone to failure, causing them to collapse or stick. This directly results in misfires and can lead to camshaft destruction. TSB 19-NA-219 specifically addresses this widespread problem across GM's V8 truck and van lineup. A P0308 code accompanied by a ticking noise is a classic symptom of this failure.
• Cylinder 8 Accessibility: Cylinder 8 is the rearmost cylinder on the passenger side. Access for replacing the spark plug, wire, or coil is notoriously tight, often requiring removal of the air intake housing or other components to reach.
• Oil Consumption: The AFM system has also been linked to increased oil consumption, which can lead to low oil levels that exacerbate wear on valvetrain components, including the lifters. Keeping the oil level full is critical on these engines.

Mechanic-Grade Diagnostic Values

• Fuel Pressure (Gasoline Engines) — expected: 55-65 PSI. Failure: Pressure below this range can cause lean misfires.
• Engine Compression Test — expected: 120-150 PSI per cylinder, with minimal variation between cylinders.. Failure: Significantly lower pressure in cylinder 8 indicates a mechanical issue like a bad valve, worn piston rings, or a damaged head gasket.

Scan Tool Commands That Help

• GDS2 (GM Global Diagnostic System 2): Cylinder Power Balance Test — This function allows a technician to electronically disable individual cylinders one by one to see the effect on engine RPM. If disabling cylinder 8 causes no change in how the engine runs, it confirms that cylinder is not contributing (misfiring).
• GDS2: Fuel Injector Balance Test / AFIT (Automated Fuel Injector Test) — This test measures the pressure drop when an injector is commanded to fire, helping to identify a clogged or electrically faulty injector without removing it from the engine. It's a key step after verifying the ignition system is working correctly.
• GDS2: Crankshaft Position Variation Learn — This procedure should be performed after any major engine work or if the crankshaft position sensor has been disturbed. It calibrates the PCM to recognize minor variations in the crankshaft reluctor wheel, ensuring accurate misfire detection. Failure to perform this can sometimes lead to false misfire codes.

Wiring & Ground Locations

• G102 — On the left (driver's side) rear of the engine block or cylinder head.. This is a primary ground point for all eight ignition coils. A loose or corroded connection at G102 can cause weak or intermittent spark on one or more cylinders, leading to misfire codes.
• G103 — On the right (passenger's side) rear of the engine block or cylinder head.. This is another critical engine ground. Given its proximity to cylinder 8 (rear passenger side), ensuring this ground is clean and tight is an important diagnostic step for any electrical issues on that bank.

Real Owner Repair Stories

• Reddit user on r/ChevyTrucks (2011 Suburban (similar GM truck platform)) — P0308 code.
  ❌ Tried (didn't work) Initial diagnosis pointed towards common ignition parts.
  ✅ What actually fixed it The heater core quick-connect lines were dripping coolant directly onto the #8 spark plug wire. Fixing the coolant leak and cleaning the wire resolved the P0308 code without replacing any ignition components.
• LS1TECH Forum user (2003 GMC Yukon 5.3L (similar V8 architecture)) — P0300 with misfires concentrated on the passenger side bank (cylinders 2, 4, 6, 8). No power over 2,500 RPM and flashing check engine light.
  ❌ Tried (didn't work) Replaced O2 sensors, spark plugs, plug wires, and fuel pressure regulator.
  ✅ What actually fixed it The passenger side catalytic converter was clogged. Cutting out the clogged converter resolved the misfires and power loss issue.

Model Year Variations Within This Range

• 2014-2018: The primary V8 engine in the Savana 2500/3500 is the 6.0L L96 (or the bi-fuel LC8 variant). Crucially, unlike the 5.3L and 6.2L engines in the Silverado/Sierra 1500s of the same era, the L96 in the Savana does NOT use Active Fuel Management (AFM). This means the notorious AFM lifter failures are not a direct cause for P0308 on this specific platform, a major correction from common assumptions about GM V8s of this period.