P0308 on 1999-2003 GMC Sierra: Cylinder 8 Misfire Causes and Fixes

What's Unique About the 1999-2003 Gmc SIERRA

The V8 engines in the GMT800 platform (1999-2006) are known for their durability, but high mileage can expose certain weaknesses. While a P0308 code is often a simple ignition issue, these engines have a documented history of valvetrain problems and specific leak-related failures. A GM Technical Service Bulletin (TSB #PIP4138L) points to worn camshaft lobes and lifters as a potential cause for misfires that aren't resolved by typical diagnostics. Furthermore, the plastic heater core quick-connect fittings on the firewall are known to leak coolant directly onto the cylinder 8 ignition coil, causing a short and a persistent 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

• Check Engine Light is on or flashing
🎬 Watch: A quick overview of P0308 causes and fixes.
• Rough idle or engine shaking
• Hesitation or stumbling during acceleration
• Noticeable loss of power
• Reduced fuel economy
• Strong fuel smell from the exhaust
• Ticking, chirping, or squealing noise from the engine (if related to valvetrain wear)

Most Likely Causes

1. Worn Spark Plug or Faulty Spark Plug Wire 🔴 High Probability Cylinder 8 is located at the rear of the engine on the passenger side, where it is exposed to significant heat and is difficult to access, sometimes causing it to be skipped during tune-ups. Wires can degrade over time from heat and vibration, and the connection at the coil or plug can become loose.
   How to confirm: Visually inspect the spark plug for wear, fouling, or damage. Inspect the spark plug wire for cracks, burns, or a loose connection. Test the wire's resistance with a multimeter; it should be within spec (typically under 1,000 ohms per foot).
   Typical fix: Replace the spark plug and spark plug wire for cylinder 8. It is best practice to replace all spark plugs and wires at the same time if they are near their 100,000-mile service interval.
   Est. part cost: $15-$50
2. Failed Ignition Coil 🔴 High Probability The coil-on-plug ignition system used on these engines means each cylinder has its own coil. These coils are subjected to constant heat and vibration and can fail individually over time. The coil for cylinder 8 is also susceptible to damage from coolant leaks.
   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 misfire code changes to P0306, the coil is faulty.
   Typical fix: Replace the failed ignition coil. The OEM coils are a round style.
   Est. part cost: $30-$80
3. Clogged or Faulty Fuel Injector 🟡 Medium Probability Over time, fuel injectors can become clogged with deposits or fail electronically. Some owners note that cylinder 8, being the last on the fuel rail, can be more susceptible to collecting debris.
   How to confirm: Listen to the injector with a mechanic's stethoscope to confirm it is clicking. Test the injector's electrical circuit with a 'noid' light. Swapping the injector with another cylinder is a definitive but more labor-intensive test.
   Typical fix: Clean or replace the fuel injector for cylinder 8.
   Est. part cost: $60-$150
4. Intake Manifold Gasket Leak 🟡 Medium Probability The original plastic-carrier intake manifold gaskets on these GM V8 engines are known to degrade and fail over time, causing a vacuum leak. A leak near the cylinder 8 runner will lean out the air/fuel mixture and cause a misfire, which can be worse at idle or on a cold engine.
   How to confirm: A mechanic can perform a smoke test to check for vacuum leaks. Alternatively, you can carefully spray short bursts of brake cleaner around the intake manifold runner for cylinder 8; a change in engine RPM indicates a leak.
   Typical fix: Replace the intake manifold gaskets. The upgraded Fel-Pro (MS 9801 T) gaskets use a steel carrier and improved rubber for better longevity.
   Est. part cost: $20-$60

Rare But Worth Checking

• Worn Camshaft Lobe and/or Lifter: Per GM TSB #PIP4138L, if standard diagnostics for codes P0300-P0308 do not isolate the cause, a worn camshaft lobe or lifter should be suspected. This is a significant mechanical failure that often produces a consistent ticking, chirping, or squealing noise that does not go away when disabling the cylinder's fuel injector.
• Coolant Leak onto Ignition Components: A well-documented issue on GMT800 trucks is a coolant leak from the plastic heater core quick-connect fittings at the firewall. The dripping coolant falls directly onto the cylinder 8 ignition coil and spark plug wire, causing corrosion and a short that leads to a P0308 misfire.
• Low Cylinder Compression: Internal engine damage such as a burnt valve, broken valve spring, or worn piston rings can cause a loss of compression in cylinder 8, leading to a persistent misfire. A compression test followed by a cylinder leak-down test is required to diagnose this.

Diagnosis Steps

1. Use an OBD-II scanner to confirm the P0308 code and check for any other codes. Note if the Check Engine Light is steady or flashing.
2. Perform a visual inspection of the cylinder 8 area (rear passenger side). Look for obvious damage, loose connectors, or signs of coolant leaks (pink/orange crust) from the heater hoses on the firewall.
3. Swap the ignition coil from cylinder 8 with another cylinder (e.g., cylinder 6). Clear the codes and see if the misfire moves to cylinder 6 (P0306). If it does, the coil is bad.
4. If the misfire remains on cylinder 8, remove and inspect the spark plug. Replace it if it is worn, fouled, or damaged.
5. Inspect the spark plug wire for cylinder 8, ensuring it is secure and not damaged. Test its resistance with a multimeter.
6. If ignition components are good, test the fuel injector circuit with a 'noid' light and listen for its operation with a stethoscope.
7. Check for vacuum leaks around the intake manifold near cylinder 8 using a smoke machine or by carefully spraying brake cleaner and listening for RPM changes.
8. Perform a compression test on cylinder 8 to rule out mechanical engine problems like bad rings or valves.
9. If all other tests fail and a ticking/chirping noise is present, inspect the valvetrain for a worn camshaft lobe or lifter, as indicated by TSB #PIP4138L.

Parts You'll Likely Need

• Spark Plug (OEM #ACDelco 41-962) — Spark plugs are a common maintenance item and a primary cause of single-cylinder misfires. The original Iridium plugs (41-110) were superseded by these Platinum plugs per a GM TSB.
  Trusted brands: ACDelco, NGK
  OEM price range: $8-$15
  Aftermarket price range: $5-$10
• Ignition Coil (OEM #ACDelco D585 (GM 10457730)) — Individual ignition coils are a frequent failure point. This is the round-style coil used on these engines.
  Trusted brands: ACDelco, Delphi
  OEM price range: $50-$80
  Aftermarket price range: $30-$60
• Spark Plug Wire Set — The insulation on spark plug wires breaks down over time, causing weak or arcing spark, especially on the hard-to-reach cylinder 8.
  Trusted brands: ACDelco, NGK
  OEM price range: $40-$70
  Aftermarket price range: $25-$50
• Intake Manifold Gasket Set (OEM #Fel-Pro MS 9801 T) — These gaskets are a known failure point on GM V8s of this era. The Fel-Pro set is a popular, improved replacement with a metal carrier.
  Trusted brands: Fel-Pro, ACDelco
  OEM price range: $40-$60
  Aftermarket price range: $20-$40

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 Detected) code may appear alongside P0308.
• P0171 / P0174 — If the misfire is caused by a significant vacuum leak, such as a failed intake manifold gasket, the engine may run lean and trigger 'System Too Lean' codes for Bank 1 and/or Bank 2.

Technical Service Bulletins (TSBs) & Recalls

• PIP4138L: Notes that for engine misfire codes P0300-P0308, a worn camshaft lobe and/or lifter should be investigated if common diagnostics do not find the cause. Often accompanied by a chirp, squeak, or tick noise.

Platform-Specific Known Issues

• A worn camshaft lobe and/or lifter can cause a persistent P0308 code, often accompanied by a ticking or chirping noise, as documented in TSB #PIP4138L.
• Failing intake manifold gaskets are a common source of vacuum leaks that can cause misfires on one or more cylinders.
• Leaking plastic heater core quick-connect fittings on the firewall can drip coolant directly onto the cylinder 8 ignition

Mechanic-Grade Diagnostic Values

• Fuel Pressure (Key On, Engine Off) — expected: 55 to 62 PSI for standard gasoline engines (4.8L, 5.3L, 6.0L). 48 to 54 PSI for Flex Fuel (L59) engines.. Failure: Pressure below the specified range indicates a weak fuel pump, clogged filter, or faulty regulator.
• Fuel Injector Coil Resistance — expected: 10 to 15 Ohms (typically around 12-13.5 Ohms).. Failure: A reading significantly outside this range, or an open/shorted circuit, indicates a failed injector coil.
• Spark Plug Wire Resistance — expected: Under 1,000 ohms per foot of wire length.. Failure: Excessively high resistance indicates the wire is internally broken or worn out and cannot deliver sufficient spark energy.

Hidden / Shadow Codes Worth Checking

• Misfire Counter Data (Mode $06): A live data parameter that shows the real-time count of misfires for each individual cylinder, even if the count is not high enough to trigger a formal P030x trouble code. (see via Accessible via a professional scan tool like the GM Tech2 or GDS2, or some high-end consumer OBD-II scanners. Look for PIDs related to 'Misfire History' or 'Cylinder Misfire Data'.)

Scan Tool Commands That Help

• GM Tech2 (or equivalent professional scanner): Cylinder Power Balance Test — When a misfire is felt but the specific cylinder is uncertain. This test disables one fuel injector at a time and graphs the RPM drop. If disabling cylinder 8 causes little or no change in RPM, it confirms that cylinder is not contributing power.
• GM Tech2 (or equivalent professional scanner): Fuel Injector Balance Test — To test for a clogged or weak fuel injector after confirming a power loss on cylinder 8. The tool pulses the injector and measures the corresponding drop in fuel rail pressure. An injector with a smaller pressure drop compared to others is likely restricted.

Wiring & Ground Locations

• Engine to Firewall Ground Strap — Typically runs from the rear of the passenger-side cylinder head to the firewall.. This is a primary ground for the engine block. If corroded, loose, or broken, it can cause a weak or intermittent spark on any cylinder, including cylinder 8. This can lead to a persistent, hard-to-diagnose misfire that appears to be a component failure but is actually a grounding issue.
• Cylinder 8 Ignition Coil Connector — On the ignition coil for the rearmost cylinder on the passenger side.. Due to its location, this connector is highly susceptible to corrosion from coolant leaking from the heater hose quick-connects on the firewall. A corroded connector can cause a P0308 even if the coil and wire are good.

Real Owner Repair Stories

• Reddit user in r/gmcsierra (2014 GMC Sierra) — Blinking check engine light, weak and shaking engine, codes P0308 and P124F (Injector 8 circuit short).
  ❌ Tried (didn't work) Swapping the ignition coil from cylinder 8 to cylinder 2; the misfire code did not move and remained P0308.
  ✅ What actually fixed it The root cause was corrosion on the ignition coil wiring harness connector for cylinder 8. This was caused by a previous, known issue of coolant leaking from the firewall hoses directly onto the harness. The fix was to address the faulty wiring connector, not the coil itself.
• ScannerDanner Forum user (2016 GMC Sierra 5.3L) — Flashing P0308 code returned two days after a major engine repair.
  ❌ Tried (didn't work) A dealership performed a $6,500 repair, replacing the camshaft and all lifters to fix a collapsed lifter on cylinder 8.
  ✅ What actually fixed it The original, persistent misfire had dumped enough raw fuel to damage the catalytic converter on Bank 2. A scan tool showed the Bank 2 cat was running 50 degrees cooler than Bank 1, indicating it was no longer functioning. The P0308 was likely being caused or exacerbated by the resulting exhaust blockage. The final fix required addressing the damaged catalytic converter.

"I Checked Everything" — The Actual Cause

• A common scenario is performing a smoke test to check for vacuum leaks and finding none, yet the P0308 persists. The actual cause is often a coolant leak from the plastic heater hose quick-connect fittings on the firewall. This leak drips directly onto the cylinder 8 ignition coil, spark plug wire, and harness connector, causing the spark to short to ground. This is an external fluid leak, not a vacuum leak, so a smoke test will not detect it.

When the Usual Fixes Don't Work

• While the TSB pointing to a worn camshaft is a critical piece of information, assuming it's the final fix can be a misdiagnosis. In a documented case, a P0308 returned just two days after a complete camshaft and lifter replacement. The root cause of the recurring code was a damaged catalytic converter on the corresponding bank, which was destroyed by excess fuel from the original misfire. This highlights that even after a major, TSB-indicated repair, consequential damage must be diagnosed and addressed.

OEM Part Supersession History

• GM Square-Style Ignition Coils (e.g., ACDelco D580) → GM Round-Style Ignition Coils (ACDelco D585 / GM 10457730) — The D585 coil, often called the 'truck coil', is known for its higher energy output and durability.
  Heads up: While physically adaptable with different brackets, using D585 coils on a vehicle originally equipped with and tuned for D580 coils may require adjusting the ignition dwell time in the PCM tune for optimal performance and to prevent premature coil failure.

Model Year Variations Within This Range

• 1999-2001 vs 2002-2003: Early models in this range may have come with a 'square' style ignition coil. This was phased out for the more common 'round' style coil (ACDelco D585). While interchangeable with the correct bracket, the electrical characteristics and required dwell times can differ.
• All years within range: The presence of a Flex Fuel 5.3L V8 (RPO code L59, 8th digit of VIN is 'Z') changes fuel system diagnostics. These engines have a lower fuel pressure specification (48-54 PSI) compared to standard gasoline engines (55-62 PSI) and may use a different fuel pressure regulator.