P0308 on 2015-2020 Chevrolet Suburban: Cylinder 8 Misfire Causes and Fixes

What's Unique About the 2015-2020 Chevrolet Suburban

The 2015-2020 Suburban is equipped with the Gen V EcoTec3 V8 engines (5.3L L83 or 6.2L L86) which feature both Direct Injection and Active Fuel Management (AFM), with later models using Dynamic Fuel Management (DFM). While a P0308 code can be a simple ignition issue, these engines have a well-documented history of AFM/DFM lifter failures. A collapsed or stuck lifter prevents the valve from opening, causing a persistent misfire that won't be fixed by replacing ignition or fuel parts. Furthermore, the belt-driven brake booster vacuum pump is located at the back of the engine, and its failure often involves an oil leak that drips directly onto the cylinder 8 ignition coil and spark plug, providing a unique and common cause for a P0308 on this platform.

Symptoms You May Notice

• Rough or vibrating idle.
• Hesitation or stumbling during acceleration.
• Noticeable loss of engine power.
• Flashing or steady Check Engine Light.
• A distinct ticking or chirping noise from the engine, especially on a cold start (often indicates a lifter issue).
• Smell of unburnt gasoline from the exhaust.
• Hard brake pedal, if the cause is a failing brake booster vacuum pump.

Most Likely Causes

1. Worn or Fouled Spark Plug 🔴 High Probability → Shop Spark Plug Spark plugs are regular maintenance items that wear out over time. Cylinder 8 is especially prone to being fouled by oil from a leaking vacuum pump or coolant from a leaking heater core hose connection.
   How to confirm: Remove the spark plug from cylinder 8 and inspect it for wear, carbon buildup, or oil/fuel fouling. A plug drenched in oil is a strong indicator of a leaking vacuum pump above it.
   Typical fix: Replace the spark plug for cylinder 8. 🎬 See this easy walkthrough for replacing the cylinder 8 spark plug. It is best practice to replace all 8 plugs if they are near the end of their service life (typically around 100,000 miles). If the plug is fouled, the source of the leak must be fixed.
   Est. part cost: $15-$25 per plug
2. Faulty Ignition Coil 🔴 High Probability → Shop Ignition Coil Ignition coils are subjected to high heat and vibration, and can fail over time. The cylinder 8 coil is at the rear of the engine bay, where heat can be more concentrated, and it is also susceptible to contamination from a leaking vacuum pump.
   How to confirm: Swap the ignition coil from cylinder 8 with another cylinder (e.g., cylinder 6, the next one forward). Clear the codes and drive. If the code changes to P0306, the ignition coil is bad.
   Typical fix: Replace the faulty ignition coil.
   Est. part cost: $40-$80
3. Failed AFM/DFM Lifter 🟡 Medium Probability → Shop Engine Valve Lifter The Active/Dynamic Fuel Management system in the L83 and L86 engines uses special lifters that are a widespread, well-documented failure point. They can become stuck, collapse, or break internally, causing a permanent misfire. TSB 19-NA-219 specifically covers misfires caused by collapsed lifters or worn camshaft lobes. Manufacturer bulletin #PIP4138R also notes that if standard diagnostics do not isolate the cause of a P0300-P0308 code, technicians should check for a worn camshaft lobe or lifter.
   How to confirm: This is an advanced diagnosis. A persistent ticking noise is a strong indicator. A mechanic would confirm by removing the valve cover to inspect the valvetrain. A compression test may also show low or zero compression. Many forum users report going through this after ignition parts don't solve the problem.
   Typical fix: This is a major repair that involves removing the cylinder head to replace the faulty lifter and lifter guide. Often, the camshaft is also damaged and requires replacement. Many owners opt to perform an 'AFM delete' at this time, replacing all AFM components with standard parts. 🎬 Watch: The permanent fix for GM 5.3L lifter failure explained.
   Est. part cost: $500-$1800+
4. Failed Brake Booster Vacuum Pump 🟡 Medium Probability The engine-driven vacuum pump is located at the rear passenger side of the engine block, directly above cylinder #8. It is a known failure item that leaks engine oil when its seals fail, dripping directly onto the #8 ignition coil, spark plug wire, and spark plug well, causing a misfire.
   How to confirm: Visually inspect the area around the cylinder 8 coil pack and the side of the engine block below the vacuum pump for signs of fresh oil. The pump itself may be wet with oil. A hard brake pedal is another symptom.
   Typical fix: Replace the brake booster vacuum pump and clean or replace the oil-fouled ignition components.
   Est. part cost: $150-$300 for the pump
5. Clogged or Faulty Fuel Injector ⚪ Low Probability → Shop Fuel Injector These engines use direct injection, where the injector is in the combustion chamber. They can become clogged with carbon over time. One owner on TahoeYukonForum reported a stuck-open injector flooding the cylinder.
   How to confirm: Listen to the injector with a mechanic's stethoscope; it should have a consistent clicking sound. A professional can perform a fuel injector balance test. Some owners have had success clearing a minor clog with a high-quality fuel system cleaner like Techron.
   Typical fix: Replace the fuel injector for cylinder 8.
   Est. part cost: $70-$150

Rare But Worth Checking

• Leaking Heater Core Hose: The heater core quick-connect fittings are on the firewall, directly above cylinder 8. A coolant leak from these fittings can drip onto the spark plug wire and coil, causing a misfire. This has been reported by owners on forums.
• Damaged Spark Plug Wire/Boot: → Shop Spark Plug The short wire between the coil and plug can become damaged or corroded, especially from heat or fluid leaks (oil from vacuum pump, coolant from heater hose). TSB Bulletin #PIP5159C advises technicians to inspect for loose spark plug wires and check for a missing "C" clip on the wire end if a P0300-P0308 code is set.
• 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.
• Low Compression: A mechanical issue like a bent valve, worn piston rings, or a damaged cylinder head gasket can cause a loss of compression and a misfire.

Diagnosis Steps

1. Confirm the code with an OBD-II scanner. 🎬 Watch: A complete guide to diagnosing P0308 causes and fixes. Note if the Check Engine Light is steady or flashing. Manufacturer Bulletin #PIP5628G notes that specific information may need to be provided to technical assistance to aid in diagnosing misfires and DTC codes P0300-P0308 or P050D.
2. Locate cylinder 8, the rearmost cylinder on the passenger side, near the firewall.
3. Visually inspect the area around cylinder 8. Look for oil leaking from the vacuum pump above it. Also check the firewall for signs of a coolant leak from the heater core hoses dripping down.
4. Perform an 'ignition coil swap test'. Swap the ignition coil from cylinder 8 with cylinder 6 (the one in front of it). Clear the codes and run the engine. If the code changes to P0306, the coil is faulty.
5. If the code remains P0308, remove and inspect the spark plug. Look for signs of excessive wear, fouling (oil or fuel), or damage. Replace if necessary. An oil-soaked plug strongly suggests a vacuum pump leak.
6. Inspect the spark plug wire/boot for any cracks, corrosion, or signs of arcing.
7. If ignition parts are good, listen carefully to the top of the engine for a persistent ticking or tapping sound. A distinct tick strongly points towards a valvetrain issue, likely a failed lifter.
8. Listen to the fuel injector on cylinder 8 with a mechanic's stethoscope. It should make a regular clicking noise. Compare its sound to the injector on cylinder 6.
9. If the cause is still not found, a professional diagnosis is recommended. This will include a compression test and cylinder leak-down test to check for mechanical engine health, and potentially a fuel injector balance test.

Parts You'll Likely Need

• Ignition Coil (OEM #12611424) — A very common failure point for isolated cylinder misfires. Easy to diagnose by swapping and easy to replace.
  Trusted brands: ACDelco, Delphi, NGK
  OEM price range: $70-$110
  Aftermarket price range: $40-$80
• Iridium Spark Plug (OEM #41-162) — Spark plugs are a routine maintenance item and a primary cause of misfires as they wear out. The OEM plug is an iridium design.
  Trusted brands: ACDelco, NGK, Bosch
  OEM price range: $15-$25
  Aftermarket price range: $10-$20
• AFM/DFM Lifter Kit (OEM #12645725) — A collapsed or stuck lifter is a very common cause of persistent misfires on these engines. Replacement often involves replacing all lifters on the affected bank or deleting the AFM system entirely.
  Trusted brands: ACDelco, Michigan Motorsports, Elgin
  OEM price range: $30-$50 per lifter
  Aftermarket price range: $200-$400 for a kit
• Brake Booster Vacuum Pump (OEM #12687986) — This pump is a known failure point, and its location directly above cylinder 8 means an oil leak frequently causes a P0308 code by fouling ignition components.
  Trusted brands: ACDelco, Dorman
  OEM price range: $200-$300
  Aftermarket price range: $150-$250

Related Codes That Often Appear With This One

• P0300 — If the misfire condition is severe or intermittent across multiple cylinders, a P0300 (Random/Multiple Cylinder Misfire) may be stored alongside P0308. This is common when a lifter failure begins.
• P050D — This code for 'Cold Start Rough Idle' is often seen with lifter-related misfires, as the engine struggles to maintain a stable idle before the lifter is fully pumped with oil. Manufacturer Bulletin #PIP5628G links this code with P0300-P0308 diagnostics.

Technical Service Bulletins (TSBs) & Recalls

• 23-NA-043: Addresses engine misfires (P0300-P0308) caused by an internal lifter concern.
• PIP5861: Earlier TSB related to misfires and lifter issues.
• 19-NA-219: Details diagnostic procedures for engine noises and misfires (P0300-P0308), specifically mentioning worn camshaft lobes and collapsed AFM lifters as potential causes.
• 18-NA-355: Not for P0308, but a critical TSB for the platform regarding transmission shudder, recommending a specific fluid flush.
• Bulletin #PIP4138R: Advises that technicians should check for a worn camshaft lobe and/or lifter if standard diagnostics do not isolate the cause of misfires (P0300-P0308).
• Bulletin #PIP5159C: Informs technicians to inspect for loose spark plug wires and missing "C" clips on the wire ends when P0300-P0308 codes are present.
• Bulletin #PIP5628G: Communicates information required for diagnosing vehicles with misfires and DTC codes P0300-P0308 and/or P050D.

Platform-Specific Known Issues

• Active Fuel Management (AFM) / Dynamic Fuel Management (DFM) lifter failure is the most significant known issue for this platform. A collapsed or stuck lifter is a very common cause for a single-cylinder misfire code like P0308. GM has issued multiple Technical Service Bulletins (TSBs) related to this problem, such as 23-NA-043, PIP5861, and 19-NA-219.
• The engine-driven brake booster vacuum pump is a major weak point. Located at the rear of the engine, its seals fail and leak oil directly onto the cylinder 8 spark plug and ignition coil, causing a targeted misfire.

Mechanic-Grade Diagnostic Values

• Low-Pressure Fuel System (In-tank pump) — expected: 40 to 90 PSI, averaging 46-55 PSI at idle. Can be commanded via a PWM signal.. Failure: Consistently low pressure can indicate a weak in-tank pump, while erratic pressure could be the Fuel Pump Control Module (FPCM).
• High-Pressure Fuel System (Engine-driven pump) — expected: 425 PSI to 2,200 PSI, depending on engine load.. Failure: Pressure that does not increase significantly under load can indicate a failing high-pressure fuel pump.
• Ignition Coil Secondary Resistance — expected: Approximately 8.22k Ohms for some aftermarket coils.. Failure: A reading significantly higher or lower than spec, or an open circuit (infinite resistance), indicates a failed coil.
• Ignition Coil Primary Resistance — expected: Approximately 0.45 Ohms for some aftermarket coils.. Failure: A reading significantly outside the specification points to a fault in the coil's primary winding.

Scan Tool Commands That Help

• GDS2 (GM Dealer Software): Cylinder Power Balance Test — Used to confirm a misfire on cylinder 8 and compare its contribution to other cylinders. The tool deactivates one cylinder at a time and measures the RPM drop. A small or no RPM drop for cylinder 8 confirms it is not contributing power.
• GDS2 (GM Dealer Software): Fuel Injector Balance Test — This test can help identify a clogged or faulty injector by measuring pressure drops as each injector is fired. It's a key step before physically replacing an injector.
• GDS2 (GM Dealer Software): Cylinder Deactivation (AFM/DFM) System Test — Allows a technician to command the AFM/DFM solenoids on and off to check for proper mechanical and electrical operation of the system, which can help isolate a stuck lifter or faulty solenoid.

Wiring & Ground Locations

• Engine Harness Grounds — There are two main ground wires for the engine harness, typically with ring terminals. They should be grounded to opposite cylinder heads at the back of the engine.. A poor ground connection on the passenger side cylinder head can cause erratic behavior or failure of the ignition coil and fuel injector for cylinder 8.
• Engine Harness Chafe Point 1 — Driver's side, where the harness can rub against the ECM/TCM bracket.. Although on the opposite side, chafing here can short various circuits in the harness, potentially causing widespread electrical issues that could manifest as a misfire code. GM has issued TSBs (#21-NA-149) regarding this issue.
• Engine Harness Chafe Point 2 — Driver's side, where the harness may contact the upper control arm or the shock tower bolt.. Engine roll and vibration can cause the harness to rub through on these components, leading to short circuits. This is a known issue addressed in GM service bulletins.

Real Owner Repair Stories

• GM-Trucks.com forum user (2017 GMC Sierra 5.3L) — Flashing check engine light, traction control light on, truck died on the highway. Later, a 'fluttering' sound under load between 3500-5000 rpm and an intermittent ticking sound.
  ❌ Tried (didn't work) Initial dealer diagnosis focused on a fouled spark plug in the misfiring cylinder.
  ✅ What actually fixed it The root cause was a broken exhaust valve spring on cylinder #3. The repair was complicated because the broken spring also bent a pushrod, which was discovered during a second visit to the dealer.
• Tahoe Yukon Forum (2007 Chevrolet Suburban) — P0308 code, later joined by P0300 (random misfire) and a 'Service Stabilitrak' message. A key symptom was a 'clear ticking sound that gets even more obvious as RPMs increase.'
  ❌ Tried (didn't work) Not specified, but the user's description implies that simple ignition fixes were not the solution.
  ✅ What actually fixed it The combination of codes and the distinct ticking noise was a textbook presentation for a collapsed AFM lifter, which required major engine work to resolve.

"I Checked Everything" — The Actual Cause

• A broken valve spring can cause a misfire without creating a vacuum leak that a smoke test would find. According to GM TSB 19-NA-219, a broken spring might only cause an intermittent misfire at specific conditions like high RPM, and may not even be found with a compression or leak-down test if the broken spring pieces remain stacked together.

OEM Part Supersession History

• 12571595, 12619820, 12639516 (AFM Lifters) → 12680871, 12645725 (newer AFM/DFM compatible lifters) — Design improvements to address high failure rates of the original AFM lifters. Newer DFM lifters are backward compatible with older AFM engines.
  Heads up: Original AFM-only lifters (e.g., Elgin HL-7011) will NOT work in newer DFM engines. However, the newer DFM lifters (e.g., Elgin HL-7025) ARE backward compatible and can be used in older AFM engines.

Model Year Variations Within This Range

• 2015-2018 (approx.): These model years primarily use the 5.3L L83 engine with Active Fuel Management (AFM), which deactivates a fixed bank of four cylinders (1, 7, 4, and 6).
• 2019-2020: For 2019, the 5.3L L84 engine was introduced, replacing AFM with Dynamic Fuel Management (DFM). DFM is more advanced, capable of deactivating any number of cylinders in 17 different patterns. This means all 8 cylinders have deactivation-style lifters, increasing the number of potential failure points compared to AFM. The brake vacuum pump was also removed on L84-equipped models.

Diagnostic Flowchart

Other Known Issues on This Vehicle

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

• 8-Speed (8L90/8L45) Transmission Shudder 🔴 High — Extremely common, typically appears between 40,000-80,000 miles. Feels like driving over rumble strips at light throttle. (Ref: TSB 18-NA-355 (recommends fluid flush with Mobil 1 Synthetic LV ATF HP).)
• AFM/DFM Lifter Failure 🔴 High → Shop Engine Valve Lifter — Widespread, well-documented issue across all model years. Can occur at low mileage (under 50k) or higher mileage. Leads to misfires, ticking noise, and potential camshaft damage. (Ref: Multiple TSBs exist, including 19-NA-219, 23-NA-043, PIP5861.)
• A/C Condenser Leak 🟠 Medium — Common on 2015-2017 models. The condenser can crack, leading to a loss of refrigerant and A/C failure. (Ref: Special Coverage Adjustment #N172096380 was issued for some vehicles, but many owners report paying for repairs out-of-pocket.)
• Brake Booster Vacuum Pump Failure 🔴 High — Common failure. The pump, located at the rear of the engine, can fail internally and leak oil directly onto the cylinder 8 spark plug and ignition coil, causing a P0308 misfire and potentially a hard brake pedal.
• Cracked Dashboards 🟡 Low — Many owners report cracks developing on the dashboard, typically around the passenger airbag area or instrument cluster, especially on earlier models of this generation (2015-2016).

Used vs. New Parts: Buying Guide for This Vehicle

When a used part is the smart pick: Used ignition coils are a reasonable choice, as they are easy to test (by swapping) and replace if they are faulty. A used brake booster vacuum pump can also be a cost-effective option if it can be visually inspected for leaks before purchase.

Donor-vehicle mileage cap: roughly under 80000 miles for the part to have meaningful remaining life.

What to inspect on the donor part:

• For an ignition coil, check for cracks in the housing and corrosion on the electrical connector pins.
• For a vacuum pump, ensure the housing is dry and free of any signs of oil seepage, especially around the seals and mounting surfaces.
• Check the vehicle history of the donor car if possible; avoid parts from vehicles with signs of poor maintenance or high mileage.

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

• AFM/DFM Lifters: Given the extremely high failure rate and the intensive labor required for replacement, using used lifters is strongly discouraged. Always use new, preferably updated-design, OEM (ACDelco) or high-quality aftermarket lifters and guides.

Aftermarket brands forum-validated for this vehicle:

• Ignition Coils: Delphi, NGK, Standard Motor Products (SMP).
• AFM/DFM Lifters & Kits: Michigan Motorsports, Elgin Industries.

Brands owners have reported issues with on this vehicle:

• Unbranded, no-name ignition coils and lifters from online marketplaces are frequently reported to have high failure rates shortly after installation.

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 — A random or multiple cylinder misfire code that often accompanies P0308 if the root cause (like AFM lifter failure or vacuum pump leaks) affects more than one cylinder.
• P0306 — Used during the 'ignition coil swap test' to confirm a faulty coil; if the coil from cylinder 8 is moved to cylinder 6 and the misfire follows, P0306 will trigger.
• P0301-P0307 — Other single-cylinder misfire codes mentioned in TSB 23-NA-043 and 19-NA-219 as being caused by the same internal lifter or camshaft concerns.

Frequently Asked Questions