P0308 on 2019-2022 GMC Savana: Cylinder 8 Misfire Causes and Fixes

What's Unique About the 2019-2022 Gmc SAVANA

The 2019-2022 GMC Savana with a V8 engine uses either the 6.0L L96 (2019-2020) or the 6.6L L8T (2021-2022). Crucially, neither of these heavy-duty truck engines are equipped with the Active Fuel Management (AFM) or Dynamic Fuel Management (DFM) systems found in many other GM V8s. This is significant because AFM/DFM lifter failure is a very common cause of misfires on other GM trucks and SUVs, but it is not a concern for this specific vehicle and engine combination. This simplifies diagnosis, making standard ignition or fuel system faults the most likely culprits for a P0308 code.

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
• Hesitation or stumbling during acceleration
• Noticeable loss of engine power
• Flashing or solid Check Engine Light
• Reduced fuel economy
• Smell of unburned fuel from the exhaust
• Engine may be difficult to start

Most Likely Causes

1. Faulty Ignition Coil 🔴 High Probability → Shop Ignition Coil Coil-on-plug ignition systems are common failure points due to constant exposure to high heat and vibration. It's the most frequent cause for a single-cylinder misfire.
   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 definitive DIY diagnostic step.
   Typical fix: Replace the faulty ignition coil. It's often recommended to replace the spark plug at the same time. Use of dielectric grease in the boot is recommended to prevent moisture intrusion and ensure a good connection.
   Est. part cost: $30-$70
2. Worn or Fouled Spark Plug 🟡 Medium Probability → Shop Spark Plug Spark plugs are a regular maintenance item with a typical 100,000-mile service life. An old plug, or one fouled by oil, carbon, or coolant, can't produce a strong enough spark. Cylinder 8 can sometimes be susceptible to fouling from minor coolant leaks dripping from heater core connections.
   How to confirm: Remove the spark plug from cylinder 8 and inspect it. Look for a worn electrode, carbon tracking, oil fouling, or a cracked insulator. Compare it to a new plug. Iridium plugs should not be re-gapped, if the gap is incorrect, the plug should be replaced.
   Typical fix: Replace the spark plug. It is best practice to replace all 8 spark plugs at the same time for even performance. The recommended gap is 0.040 inches.
   Est. part cost: $8-$25 per plug
3. Defective Spark Plug Wire ⚪ Low Probability → Shop Spark Plug The 6.0L L96 and 6.6L L8T engines use spark plug wires connecting the coil packs (mounted on the valve covers) to the spark plugs. These wires can degrade over time from extreme heat, causing the spark to arc to the engine block instead of reaching the plug.
   How to confirm: Visually inspect the wire for cracks, burns, or corrosion on the terminals. With the engine running in the dark, you may see the spark arcing from the wire to a metal surface. A resistance test with a multimeter can also identify a bad wire.
   Typical fix: Replace the spark plug wire for cylinder 8, or replace the entire set for preventative maintenance. It is highly recommended to replace wires when replacing plugs.
   Est. part cost: $50-$100 for a full set
4. Clogged or Failed Fuel Injector ⚪ Low Probability → Shop Fuel Injector Fuel injectors can become clogged with deposits over time or fail electrically, preventing the correct amount of fuel from reaching cylinder 8. In some GM V8s, the rearmost injector on the fuel rail (#8) can be more prone to collecting debris.
   How to confirm: After ruling out ignition issues, swap the fuel injector from cylinder 8 with another cylinder. If the misfire code follows the injector, the injector is the problem. This is an involved diagnostic step requiring removal of the fuel rail.
   Typical fix: Replace the faulty fuel injector. It is often recommended to replace all injectors on that bank or all 8 at once to ensure balanced performance.
   Est. part cost: $70-$150

Rare But Worth Checking

• Low Compression / Mechanical Issue: Although rare on these robust non-AFM engines, a mechanical issue like a burnt valve, worn piston rings, a worn camshaft lobe, or a broken valve spring could cause a loss of compression. This would lead to a persistent misfire that isn't fixed by ignition or fuel parts. A compression test and cylinder leak-down test are required for diagnosis.
• Intake Manifold Gasket Leak: → Shop Engine Intake Manifold A vacuum leak from a failed intake manifold gasket isolated to the cylinder 8 runner can lean out the air/fuel mixture enough to cause a misfire. This may be more noticeable at idle.
• Cylinder Head Porosity: → Shop Engine Cylinder Head GM TSB PIP5095H mentions a rare condition where porosity in the cylinder head casting can allow oil or coolant to leak into the spark plug tube, fouling the plug and causing a misfire. The fix is a cylinder head replacement.

Diagnosis Steps

1. Read the code with an OBD-II scanner to confirm P0308 and check for any other codes. Note the freeze-frame data.
2. Check if the Check Engine Light is flashing. If so, avoid driving to prevent catalytic converter damage.
3. Locate cylinder 8. On GM V8s, it is the rearmost cylinder on the passenger side (front cylinders are 1 & 2).
4. Swap the ignition coil from cylinder 8 with the coil from cylinder 6 (the cylinder in front of it). Clear the codes, run the engine, and re-scan. If P0306 appears, the coil is faulty.
5. If the misfire remains on cylinder 8, inspect the spark plug wire for damage. Check for cracks, burn marks, or corrosion. If it looks good, swap the wire with another cylinder if possible.
6. If the misfire still remains, remove and inspect the spark plug from cylinder 8. Replace it if it appears worn, fouled, or damaged. Check the gap on the new plug before installation (should be 0.040").
7. If ignition components are ruled out, the next step is to check the fuel injector. This is more complex and may require a professional. A 'noid' light can be used to check for an electrical signal to the injector.
8. If all else fails, a mechanical compression test and a cylinder leak-down test on cylinder 8 are needed to check for internal engine problems like bad rings, valves, or a head gasket leak.

Parts You'll Likely Need

• Ignition Coil (OEM #12699383) — This is the most common failure point for a single-cylinder misfire on modern GM V8s.
  Trusted brands: ACDelco, Delphi, Standard Motor Products
  OEM price range: $40-$60
  Aftermarket price range: $25-$45
• Iridium Spark Plug (6.0L L96) (OEM #41-110) — Spark plugs are a routine wear item. The 6.0L L96 uses this plug. They come pre-gapped to 0.040" and should not be adjusted.
  Trusted brands: ACDelco, NGK
  OEM price range: $10-$15
  Aftermarket price range: $8-$12
• Iridium Spark Plug (6.6L L8T) (OEM #41-168) — The newer 6.6L L8T engine uses a different spark plug. It is also an Iridium plug with a 100k mile service interval.
  Trusted brands: ACDelco, NGK
  OEM price range: $12-$18
  Aftermarket price range: $9-$15
• Spark Plug Wire Set (OEM #9748HH (ACDelco set for L96)) — The wire connecting the coil to the plug can fail, preventing spark delivery. It's best practice to replace as a set.
  Trusted brands: ACDelco, NGK, Carquest Premium
  OEM price range: $70-$100
  Aftermarket price range: $40-$70
• Fuel Injector (6.6L L8T) (OEM #12698484) — If ignition parts don't solve the issue, a faulty injector is the next logical cause. This part is for the 2021-2022 models. Note that TSB 20-NA-098 advises to verify the part number directly from the old injector, as multiple parts may be used.
  Trusted brands: ACDelco
  OEM price range: $115-$175
  Aftermarket price range: $70-$120

Related Codes That Often Appear With This One

• P0300 — If the misfire on cylinder 8 is frequent or severe, it can sometimes be intermittently detected on other cylinders, leading to a P0300 (Random/Multiple Misfire) code being stored as well.
• P0174 — If the misfire is caused by a vacuum leak affecting the passenger side bank (Bank 2), a P0174 (System Too Lean Bank 2) code may accompany the P0308.

Technical Service Bulletins (TSBs) & Recalls

• PIP5628G: A bulletin for technicians on diagnosing misfire codes (P0300-P0308) and what information to provide to the Technical Assistance Center (TAC).
• 19-NA-219: Details potential mechanical causes for P0300-P0308 codes, including worn cam lobes, sticking valves, or broken valve springs, which are relevant after ruling out ignition/fuel issues.
• PIP5095H: Addresses a rare condition of cylinder head porosity leading to spark plug fouling and misfires.
• 20-NA-098: Advises technicians to verify fuel injector part numbers directly off the old component before ordering a replacement, as multiple parts may have been used.

Platform-Specific Known Issues

• The provided Technical Service Bulletin #PIP5628G mentions diagnostic procedures for codes P0300-P0308, indicating that GM has specific steps for technicians to follow when diagnosing misfires on these engines, which may sometimes be complex.
• Access to cylinder 8 is notoriously difficult due to the engine's position deep inside the engine bay ('doghouse'). This can make what would be a simple repair on a pickup truck a much more labor-intensive job on a Savana van.
• A less common but documented issue is TSB PIP5095H, which points to potential cylinder head porosity causing oil/coolant to foul the spark plug, requiring a cylinder head replacement to fix the misfire.