P0308 on 2004-2008 GMC Savana: Cylinder 8 Misfire Causes and Fixes

What's Unique About the 2004-2008 Gmc SAVANA

The V8 engines in this generation of GMC Savana are generally robust, but they are not immune to specific failures that can cause a P0308 code. Beyond the universal causes like spark plugs and coils, these Vortec engines have a documented history of potential valvetrain issues. Specifically, as noted in manufacturer bulletins, a persistent misfire that isn't resolved by ignition or fuel system repairs can point to a worn camshaft lobe or a collapsed lifter. This is a more complex and costly mechanical repair that is a known weakness in this engine family, often accompanied by a distinct ticking or chirping noise. 🎬 Watch: How to diagnose lifter tick versus other engine noises. Some later GM engines had issues with Active Fuel Management (AFM) lifters, but while the valvetrain wear is a concern on these 2004-2008 models, widespread AFM lifter collapse is more associated with later generations.

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 shaking idle.
• Hesitation or stumbling during acceleration.
• Noticeable loss of engine power.
• Flashing or solid Check Engine Light.
• Reduced fuel economy.
• A smell of unburnt fuel from the exhaust.
• Engine backfiring or popping sounds from the exhaust.
• A rhythmic ticking or chirping noise from the engine, especially if the cause is mechanical (lifter/cam).

Most Likely Causes

1. Worn or Fouled Spark Plug 🔴 High Probability → Shop Spark Plug Spark plugs are regular maintenance items. Cylinder 8 is the most difficult to access, located at the rear of the engine bay on the passenger side, which can lead to it being neglected during tune-ups.
   How to confirm: Remove the spark plug from cylinder 8 and inspect it for wear (worn electrode), carbon or oil fouling, or physical damage. Compare it to a plug from an easier-to-access cylinder.
   Typical fix: Replace the spark plug. It is highly recommended to replace all 8 spark plugs at the same time using OEM-spec plugs (e.g., ACDelco Iridium or Platinum) for even performance.
   Est. part cost: $5-$20 per plug
2. Faulty Ignition Coil 🔴 High Probability → Shop Ignition Coil
   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 coil is faulty. These 'coil-on-plug' units can fail from heat and age.
   Typical fix: Replace the faulty ignition coil. While you can replace just one, if the coils are original and high-mileage, consider replacing the set.
   Est. part cost: $30-$95
3. Bad Spark Plug Wire 🟡 Medium Probability → Shop Spark Plug
   How to confirm: Visually inspect the wire for cylinder 8 for any signs of cracking, melting, arcing (white marks), or corrosion on the contacts. You can also test its resistance with a multimeter; a good wire should typically have resistance between 250 and 1000 ohms per inch. A wire with infinite resistance is bad.
   Typical fix: Replace the spark plug wire. It's best practice to replace the entire set of wires with a quality brand.
   Est. part cost: $40-$100 for a set
4. Clogged or Failing Fuel Injector 🟡 Medium Probability → Shop Fuel Injector
   How to confirm: Listen to the injector with a mechanic's stethoscope to ensure it's clicking. Use a 'noid light' to verify it's receiving an electrical pulse from the PCM. You can also swap the injector with another cylinder to see if the misfire follows, though this is more labor-intensive. Some owners report success clearing a clogged injector by running a concentrated fuel system cleaner like Techron through the tank.
   Typical fix: Replace the fuel injector for cylinder 8.
   Est. part cost: $45-$100
5. Worn Camshaft Lobe / Failed Lifter ⚪ Low Probability → Shop Engine Camshaft As cited in GM Technical Service Bulletin #PIP4138R (later replaced by #19-NA-219), worn camshaft lobes and lifters are a known cause for persistent, isolated misfires on this engine family if standard diagnostics fail. This can happen due to oiling issues or simple mechanical wear over time.
   How to confirm: This is an advanced mechanical diagnosis. A key symptom is a persistent ticking or chirping noise that follows camshaft speed (half of engine RPM). A technician may remove the valve cover to observe rocker arm movement and measure valve lift. If lift is insufficient on cylinder 8's valves, the camshaft and lifter are the likely culprits.
   Typical fix: Replace the camshaft and all lifters (especially the lifters on the affected bank). This is a significant and labor-intensive repair, often costing over $2,000.
   Est. part cost: $400-$1000+

Rare But Worth Checking

• Intake Manifold Gasket Leak: → Shop Engine Intake Manifold A vacuum leak near the cylinder 8 intake runner can lean out the air/fuel mixture, causing a misfire. This may also be accompanied by a P0171 or P0174 lean code. The leak can often be found by spraying brake cleaner or propane around the intake manifold sealing area for cylinder 8 and listening for a change in engine idle.
• Low Compression: A mechanical issue like a burnt valve, worn piston rings, or a leaking head gasket can cause low compression in cylinder 8, leading to a misfire. A compression test and a cylinder leak-down test are needed to confirm and pinpoint the cause.
• Coolant Leak onto Ignition Components: On platform mates like the Silverado, there are documented cases of the heater core quick-connect fittings, located on the firewall near cylinder 8, developing a leak. Dripping coolant can foul the spark plug wire and coil, causing a persistent P0308 until the leak is fixed and the components are cleaned or replaced.

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 to see when the misfire occurs (e.g., idle, load, cold/hot).
2. Note if the Check Engine Light is flashing. If so, avoid driving to prevent catalytic converter damage.
3. Locate Cylinder 8: It is the rearmost cylinder on the passenger side of the engine.
4. Inspect the spark plug wire, and ignition coil for cylinder 8. Look for obvious damage, corrosion, loose connections, or signs of coolant contamination from above.
5. Perform a 'swap test' with the ignition coil. Move the cylinder 8 coil to another cylinder (like 6, the next one forward on the same bank), clear the codes, and see if the misfire code moves to P0306. If it does, the coil is bad.
6. If the coil is good, inspect the spark plug. If it's old, fouled, or damaged, replace it. Consider replacing all 8 if they are due.
7. If the ignition components appear good, check the fuel injector. Listen for a steady clicking sound with a stethoscope. Use a noid light to confirm it's getting a signal from the PCM.
8. Check for vacuum leaks around the intake manifold near cylinder 8 using propane or a smoke machine.
9. If ignition and fuel delivery seem okay, perform a compression test on cylinder 8 to rule out mechanical engine problems like bad rings or valves. A healthy cylinder should have compression that is within 15% of the others.
10. If compression is good but the misfire persists, listen carefully for a ticking or chirping noise from the passenger side valve cover area. This could indicate a valvetrain problem.
11. If a valvetrain issue is suspected, the final diagnostic step is to remove the valve cover to inspect valve movement and measure lift, which can confirm a worn camshaft lobe or collapsed lifter as described in TSB #19-NA-219 (which supersedes PIP4138R).

Parts You'll Likely Need

• Ignition Coil (OEM #12570616 (ACDelco D510C) or 12558693 (ACDelco D585))

Related Codes That Often Appear With This One

• P0300 — If the misfire is intermittent or starting to affect other cylinders, a P0300 (Random/Multiple Cylinder Misfire) code may appear alongside P0308.
• P0171 or P0174 — If the misfire is caused by a vacuum leak near cylinder 8 (Bank 2), the engine may run lean, triggering a 'System Too Lean' code for that cylinder bank (P0174 for Bank 2).
• P050D — This code for 'Cold Start Rough Idle' can sometimes accompany misfire codes on GM vehicles, as noted in TSB #PIP5628G, indicating a potential related issue during startup.

Technical Service Bulletins (TSBs) & Recalls

• PIP4138R / 19-NA-219: Advises technicians to check for a worn camshaft lobe and/or lifter if a P0300-P0308 misfire code cannot be isolated by other diagnostics. Symptoms can include a chirp, squeak, or tick noise.
• PIP5628G: An internal diagnostic aid for technicians contacting GM's Technical Assistance Center (TAC) for persistent misfire codes (P0300-P0308). It provides a template of information to collect, including freeze-frame data, fuel trim values, and results of compression/leak-down tests, to aid in complex diagnoses.

Platform-Specific Known Issues

• Persistent P0308 codes that are not resolved by replacing ignition components or fuel injectors should lead to an inspection of the valvetrain. TSB #19-NA-219 (superseding PIP4138R) specifically calls out checking for a worn camshaft lobe or collapsed lifter as a potential cause for misfires on these engines.
• Cylinder 8's location at the rear passenger side makes it the hardest to work on and susceptible to heat soak, potentially shortening the life of the coil and wire.
• Owners of platform-mate trucks have reported coolant leaks from heater hose connections on the firewall dripping directly onto the cylinder 8 coil and wire, causing the misfire.

Mechanic-Grade Diagnostic Values

• Fuel Pressure (Key On, Engine Off) — expected: 55-62 PSI. Failure: Pressure below 55 PSI can indicate a weak fuel pump or faulty regulator, potentially causing a lean misfire.
• Ignition Coil Primary Resistance — expected: 0.4 to 2.0 ohms. Failure: A reading outside this range, or an open circuit (infinite resistance), indicates a faulty coil.
• Ignition Coil Secondary Resistance — expected: 6,000 to 10,000 ohms (6k-10k Ω). Failure: A reading significantly outside this range suggests the internal secondary windings are failing.
• Fuel Injector Resistance (at operating temperature) — expected: 12 to 16 ohms. Failure: A reading significantly higher or lower, or an open/shorted reading, points to a failed injector coil. All injectors should have similar resistance values.
• Compression Test (per cylinder) — expected: 120 to 150 PSI, with all cylinders within 15% of each other. Failure: A reading on cylinder 8 that is significantly lower than the others indicates a mechanical engine problem like worn rings, a bad valve, or a head gasket leak.

Hidden / Shadow Codes Worth Checking

• Mode 6, Test ID $A2-$A9 (Misfire Counts): While not a hidden code, Mode 6 data on a generic OBD-II scanner provides raw misfire counts for each cylinder. Test ID $A9 would correspond to Cylinder 8. This allows a technician to see the severity of the misfire, even if it's not yet bad enough to keep the check engine light on continuously. (see via Any advanced OBD-II scanner that can display Mode $06 data.)

Scan Tool Commands That Help

• GM Tech 2 / GDS2: Cylinder Power Balance Test — This bidirectional test allows the technician to disable the fuel injector for each cylinder one by one and observe 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, helping to isolate the issue without swapping parts.
• GM Tech 2 / GDS2: Fuel Injector Balance Test — This function can be used to check if an injector is clogged or flowing poorly. The tool commands the injector to open for a specific duration, and the resulting drop in fuel rail pressure is measured. A smaller pressure drop for cylinder 8 compared to others would indicate a restricted injector.

Wiring & Ground Locations

• G102 — On the left rear of the engine (driver's side).. This is a primary ground point for all 8 ignition coils. A loose or corroded connection here can cause weak spark or intermittent misfires on any cylinder, including cylinder 8.
• G103 (Gasoline Engine) — On the right rear of the engine (passenger's side).. This ground serves the Engine Control Module (ECM). A poor connection here can cause a wide range of erratic engine behavior and fault codes, including misfire detection problems.
• Cylinder 8 Injector Control Wire — Pink/Black wire at the cylinder 8 fuel injector connector and at the PCM connector.. This is the specific wire the PCM uses to pulse the injector for cylinder 8. A continuity or voltage drop test on this wire can rule out a wiring harness issue between the PCM and the injector.

Real Owner Repair Stories

• Chevrolet Forum user (2004 Chevy Express (sister van to Savana) with 6.0L V8) — P0308 (Cylinder 8 Misfire) and P0300 (Random Misfire).
  ❌ Tried (didn't work) Replaced spark plug, Replaced spark plug wire, Replaced ignition coil
  ✅ What actually fixed it The plastic heater core quick-connect fitting on the firewall above cylinder 8 had a small crack and was leaking coolant directly onto the #8 coil and plug wire, causing the misfire. Replacing the fitting and cleaning the ignition components resolved the codes.

OEM Part Supersession History

• 10457730 (GM) → 19005218 (stamped on bracket), ACDelco D585 — This is the well-known 'LS truck coil' with a heat sink, often considered a robust and high-performance factory coil.
  Heads up: The ACDelco D585 is a specific design. Be cautious of generic replacements labeled 'D585' from unknown brands, as quality can vary significantly. Genuine ACDelco or Delphi parts are recommended for reliability.

Model Year Variations Within This Range

• 2004-2007 vs 2007-2008: Around 2007, GM began transitioning from Gen III to Gen IV Vortec engines. While many 2007 Savanas still had Gen III engines (24x crank reluctor, rear cam sensor), some later models may have early Gen IV versions (58x crank reluctor, front cam sensor). This affects the ECU, sensors, and some internal components, which is critical to know when ordering parts. The L59 5.3L V8 was a Flex-Fuel (E85) capable version of the LM7.
• 2003-2005+: The fuel pressure regulator location changed. On earlier Vortec engines, it was often part of the central port injection system inside the intake. On the 2003+ V8 models covered here, it is a more accessible part located on the fuel rail.