P0308 on 2017-2022 Genesis G90 5.0L: Cylinder 8 Misfire Causes and Fixes

What's Unique About the 2017-2022 Genesis G90

The Genesis G90's 5.0L Tau V8 is a modern, direct-injection (GDI) engine known for its smooth power delivery. For this platform, a P0308 code typically points to the usual suspects of ignition system wear-and-tear. However, because it is a GDI engine, the intake valves are not cleaned by fuel spray, making them susceptible to carbon buildup over time, which can restrict airflow and cause misfires. Additionally, some owner experiences with the Tau V8 engine family suggest it can be sensitive to the quality of aftermarket ignition coils, with some owners resolving persistent misfires by using OEM or high-quality OEM-supplier brands like Denso. While extremely rare, a faulty ECU has also been reported as a root cause after all ignition components were replaced.

Symptoms You May Notice

• Rough or shaky engine idle
• Hesitation or stumbling during acceleration
• Noticeable loss of engine power
🎬 Watch: A quick breakdown of what causes a P0308 misfire.
• Check Engine Light is on and may be flashing
• A smell of unburned fuel from the exhaust
• Reduced fuel economy

Most Likely Causes

1. Faulty Ignition Coil 🔴 High Probability Coil-on-plug systems are highly effective but individual coils are a common failure point on most modern engines as they age from heat cycles. Cylinder 8's location at the rear of the driver's side bank may subject it to slightly more heat soak.
   How to confirm: Swap the ignition coil from cylinder 8 with another cylinder (e.g., cylinder 6 on the same bank). Clear the codes and drive the vehicle. If the code changes to P0306, the ignition coil is the faulty part.
   Typical fix: Replace the failed ignition coil. Many technicians and experienced owners recommend replacing them in a full set if they are original and the vehicle has high mileage, or at least using a reputable brand like Denso to avoid compatibility issues.
   Est. part cost: $50 - $120
2. Worn or Fouled Spark Plug 🔴 High Probability Spark plugs are a regular maintenance item with a finite lifespan. The OEM plugs are typically Iridium-tipped, but a worn electrode, carbon fouling, or incorrect gap can prevent a strong spark.
   How to confirm: After confirming the coil is good, remove the spark plug from cylinder 8. Inspect it for wear (rounded electrode), carbon buildup, or oil/fuel fouling. Compare it to a new plug. You can also swap it with another cylinder's spark plug to see if the misfire follows.
   Typical fix: Replace the spark plug. It is best practice to replace all eight spark plugs at the same time to ensure even performance and avoid future misfires.
   Est. part cost: $15 - $40 per plug
3. Clogged or Faulty Fuel Injector 🟡 Medium Probability The Tau V8 uses Gasoline Direct Injection (GDI), where injectors are exposed to high pressures and in-cylinder temperatures. This makes them more susceptible to clogging from fuel deposits or internal failure compared to port injectors.
   How to confirm: This is more complex. A professional can perform a fuel injector balance test. A DIY method is to listen for the injector's clicking sound with a mechanic's stethoscope. If cylinder 8's injector is silent or sounds different from the others, it may have failed.
   Typical fix: Replace the faulty fuel injector. This is a more involved and costly job on a GDI engine and may require professional service due to the high-pressure fuel system.
   Est. part cost: $105 - $220

Rare But Worth Checking

• Intake Valve Carbon Buildup: As a GDI engine, fuel does not spray over the intake valves to clean them. Over time, oil vapors from the PCV system can bake onto the valves, restricting airflow into the cylinder and causing misfires, especially on cold starts. This requires physical cleaning (e.g., walnut blasting).
• Low Cylinder Compression: If ignition and fuel components are confirmed good, a mechanical issue like a worn piston ring, a bad valve, or a leaking head gasket could be the cause. This requires a compression test to diagnose.
• Faulty Engine Control Unit (ECU): In very rare cases, the driver circuit within the ECU that controls the ignition coil for cylinder 8 can fail. This should only be considered after all other possibilities have been exhaustively ruled out. One owner of a tuned 5.0L Tau V8 reported needing an ECU replacement to solve a persistent misfire.

Diagnosis Steps

1. Read the code with an OBD-II scanner to confirm P0308 is the primary code present.
2. Note if the Check Engine Light is solid or flashing. A flashing light indicates a severe misfire that can damage the catalytic converter.
3. Locate cylinder 8. On the Genesis 5.0L Tau V8, the cylinders are numbered 1-3-5-7 on the passenger side (Bank 1) and 2-4-6-8 on the driver's side (Bank 2), from front to back. Cylinder 8 is the rearmost cylinder on the driver's side, closest to the firewall.
4. Swap the ignition coil from cylinder 8 with the coil from cylinder 6 (the next one forward on the same bank).
5. Clear the fault codes using the scanner, then start the engine and let it run or take a short drive.
6. Re-scan for codes. If the code is now P0306, the ignition coil you moved is faulty and needs to be replaced.
7. If the code remains P0308, the coil is likely not the issue. Move the coil back to its original position.
8. Repeat the swap process with the spark plug from cylinder 8 and cylinder 6. If the code moves to P0306, the spark plug is the culprit.
9. If the code still remains P0308 after swapping both the coil and plug, the issue is likely with the fuel injector, wiring, or a mechanical problem in that cylinder.
10. At this point, testing the fuel injector (listening for clicks, checking wiring) or performing a compression test is recommended. For GDI injector issues or mechanical tests, professional assistance may be required.

Parts You'll Likely Need

• Spark Plug (OEM #18870-11071) — This is a standard maintenance item and a frequent cause of single-cylinder misfires due to wear or fouling.
  Trusted brands: Hyundai (OEM), NGK (Often the OEM supplier, e.g., SILKR8A-S), Denso
  OEM price range: $25-$40
  Aftermarket price range: $15-$25
• Ignition Coil (OEM #27300-3F100) — The coil-on-plug for cylinder 8 may fail due to heat and age, resulting in a weak or no-spark condition. This part number is shared across several Hyundai/Genesis V8 models.
  Trusted brands: Hyundai (OEM), Denso (Recommended by owners), Delphi, NGK
  OEM price range: $80-$150
  Aftermarket price range: $50-$100

Related Codes That Often Appear With This One

• P0300 — If the misfire becomes more widespread due to a systemic issue (like a vacuum leak, fuel delivery problem, or severe carbon buildup), a P0300 'Random/Multiple Cylinder Misfire' code may appear alongside P0308.

Technical Service Bulletins (TSBs) & Recalls

• TSB 22-EM-010H: While not for P0308 specifically, this bulletin provides guidelines for inspecting and cleaning the engine intake system (addressing carbon buildup) during major engine repairs, which is relevant to a potential cause of misfires on GDI engines.

Platform-Specific Known Issues

• Owners on forums for the Tau V8 engine family have noted that using cheap, unbranded aftermarket ignition coils can sometimes cause persistent or intermittent misfires, with issues being resolved upon switching to OEM or a known OEM-supplier brand like Denso.

Mechanic-Grade Diagnostic Values

• Ignition Coil Primary Resistance — expected: Approximately 0.45 - 0.55 Ω. Failure: A reading significantly outside this range, or an open circuit (infinite resistance), points to a failed coil.
• High-Pressure Fuel System (GDI) Pressure — expected: Idle: ~500-700 PSI. Under Load: 2,000 to 2,800 PSI (138 to 193 bar).. Failure: Pressure that is significantly below the specified range, especially under load, can cause lean misfires across multiple cylinders, but could be isolated if an injector is leaking.

Hidden / Shadow Codes Worth Checking

• Mode $06, Test ID $A2-$AF: This is not a trouble code, but a diagnostic monitor that tracks misfire counts per cylinder. A professional scan tool can display this data. For example, Test ID $A9 might correspond to Cylinder 8. Even if no P0308 code is set, a high count (e.g., several hundred over a drive cycle) indicates an intermittent or developing misfire that hasn't yet crossed the threshold to trigger the Check Engine Light. (see via A professional-grade scan tool with Mode $06 capability. The exact Test ID (TID) and Component ID (CID) for cylinder 8 may vary, but will be labeled in the scan tool's menu.)

Scan Tool Commands That Help

• Genesis GDS (Global Diagnostic System): Cylinder Power Balance Test — This function should be used after basic coil and plug swaps fail to identify the cause. The tool electronically disables one cylinder at a time and measures the corresponding drop in engine RPM. If disabling cylinder 8 results in little or no RPM drop compared to other cylinders, it confirms that cylinder 8 is not contributing power, pointing the diagnosis toward a fuel, spark, or compression issue specific to that cylinder.

Wiring & Ground Locations

• Engine Bay Negative Jump Point — Under a small plastic cover on the driver's side of the engine bay, separate from the fuse box. It is a dedicated metal post for connecting a negative jumper cable.. This is a primary chassis ground point in the engine bay. A loose or corroded connection here can cause widespread electrical issues, including weak spark. Verifying this connection is clean and tight is a crucial step in diagnosing any electrical fault.
• Cylinder 8 Ignition Coil Connector — The electrical connector plugging into the ignition coil on the rearmost cylinder of the driver's side bank.. The wiring harness in this area is subjected to significant heat. Over time, the plastic connector can become brittle and the wires can chafe. The availability of aftermarket pigtail connectors suggests this is a known failure point. A visual inspection for cracked insulation or damaged pins is warranted if a new coil doesn't fix the misfire.

OEM Part Supersession History

• 27300-2G000 → 27300-3F100 — Likely an updated internal design for improved reliability or performance. This is a common practice for ignition coils.
  Heads up: While the older part may physically fit, using the latest superseded part number (27300-3F100) is always recommended to get the most current revision.

Model Year Variations Within This Range

• 2017-2022: The 5.0L Tau V8 engine and its core components related to the ignition and fuel system remained consistent throughout the first generation G90's production run. A significant facelift for the 2020 model year primarily introduced cosmetic and infotainment updates (like a new touchscreen with phone connectivity) and did not alter the engine or the common causes for a P0308 code.

Diagnostic Flowchart

Other Known Issues on This Vehicle

Issues unrelated to this code that are worth knowing about as an owner of this generation:

• Starter Solenoid Water Intrusion 🔴 High — Affects enough vehicles to warrant a recall (NHTSA ID: 22V696). Can occur at any mileage. (Ref: Recall 22V696: Water may enter the starter solenoid and cause an electrical short, which can result in an engine compartment fire while parked or driving.)
• Intake Valve Carbon Buildup 🟠 Medium — Common on GDI engines over 50,000-80,000 miles. Can lead to rough idle, hesitation, and eventually misfires if not addressed. (Ref: TSB 22-EM-010H provides guidelines for inspecting and cleaning the intake system during engine work, acknowledging the issue.)
• Oil Leak from Turbocharger Oil Feed Pipe (3.3T Models) 🟠 Medium — Specific to the 3.3T V6 engine, but a notable issue for the G90/G80 platform. A recall was issued. (Ref: Recall 22V477: The left turbocharger oil feed pipe may deteriorate and leak oil, posing a fire risk.)
• Coolant Leak at Rear Intake Manifold Connector 🟡 Low — Reported by some Tau V8 owners. A slow leak can occur at the rear of the engine near the firewall, often evaporating before it drips, making it hard to detect without a pressure test. (Ref: A TSB was reportedly issued for older models, recommending replacement of two O-rings in the connector.)

Used vs. New Parts: Buying Guide for This Vehicle

When a used part is the smart pick: For this repair, using a used part is generally not recommended for the primary failure items. However, if you need to replace a purely mechanical component like an engine cover or bracket during the repair, a used part from a reputable salvage yard is a cost-effective choice.

Donor-vehicle mileage cap: roughly under 60000 miles for the part to have meaningful remaining life.

What to inspect on the donor part:

• For an ignition coil, inspect the plastic housing for any cracks or signs of melting.
• Check the electrical connector pins on the used part for any corrosion or damage.
• Ask for the donor vehicle's VIN to verify its mileage and check for any reported accident history that could have damaged engine components.

OEM-only on this vehicle (don't cheap out):

• Ignition Coil: Owner reports strongly indicate that cheap aftermarket coils can be unreliable or fail prematurely on the Tau V8. Stick with genuine Hyundai/Genesis or a known OEM supplier like Denso.
• GDI Fuel Injector: These are high-precision components operating under extreme pressure. To ensure proper function and avoid leaks, using an OEM injector is the safest choice.

Aftermarket brands forum-validated for this vehicle:

• Denso (Ignition Coils)
• NGK (Spark Plugs - often the OEM supplier)

Brands owners have reported issues with on this vehicle:

• Unbranded, 'white-box' ignition coils sold on online marketplaces are frequently cited as a cause of persistent misfires.

Real Owner Stories

Aggregated from forums and TSBs cited above. Mileages and costs reflect what owners reported in those sources.

Related OBD-II Codes

• P0300 — Random or Multiple Cylinder Misfire Detected; often accompanies P0308 when the misfire is not isolated to a single cylinder.
• P0306 — Misfire detected in cylinder 6; used as a comparison cylinder during the diagnostic swap process for coils and spark plugs.
• P0301-P0307 — Misfire codes for other cylinders; the Equus and G80 with the same Tau V8 are documented to experience the full range of these codes.

Frequently Asked Questions