P0308 on 2002-2006 Cadillac Escalade EXT: Cylinder 8 Misfire Causes and Fixes

What's Unique About the 2002-2006 Cadillac ESCALADE EXT 1500

The 6.0L LQ9 V8 engine in the Escalade EXT is a robust and powerful motor, part of GM's legendary LS family. While generally reliable, a P0308 misfire isn't always a simple ignition issue. These engines are known to sometimes suffer from internal mechanical problems, specifically worn camshaft lobes and collapsed lifters, which can cause persistent misfires that don't resolve with a typical tune-up. This is documented in official GM Technical Service Bulletins and corroborated by numerous owner experiences in online forums.

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 Check Engine Light (especially under heavy acceleration)
• Reduced fuel economy
• Smell of gasoline from the exhaust
• Hissing or whistling noises from the engine bay, potentially indicating a vacuum leak

Most Likely Causes

1. Worn or Fouled Spark Plug 🔴 High Probability → Shop Spark Plug Cylinder 8 is the most difficult to access, located at the rear passenger side of the engine bay, and may be neglected during tune-ups.
   How to confirm: Remove the spark plug from cylinder 8 and inspect it. Look for heavy carbon buildup, oil fouling, coolant contamination (white residue), or a worn-down electrode. Compare it to a plug from a working cylinder.
   Typical fix: Replace the spark plug. It is highly recommended to replace all 8 spark plugs at the same time 🎬 See this step-by-step guide for changing your plugs and wires. for even performance using the specified ACDelco plugs.
   Est. part cost: $5-$20 per plug
2. Failed Ignition Coil Pack 🔴 High Probability → Shop Ignition Coil Heat from the engine can cause the coil's electronics to fail over time. Cylinder 8 is at the back of the engine bay, which can see higher temperatures and less airflow.
   How to confirm: Swap the ignition coil from cylinder 8 with another cylinder (e.g., cylinder 6, next to it). Clear the codes and drive the vehicle. If the code changes to P0306, the coil is bad.
   Typical fix: Replace the faulty ignition coil. This is a common DIY repair.
   Est. part cost: $30-$70
3. Defective Spark Plug Wire 🟡 Medium Probability → Shop Spark Plug The wire for cylinder 8 is long and routed in a hot, tight space, making it susceptible to heat degradation and physical damage over time.
   How to confirm: Inspect the wire for any visible damage, cracks, or burns near the exhaust manifold. You can test its resistance with a multimeter; a reading outside of the manufacturer's specification indicates a bad wire. Swapping with another cylinder's wire is also a valid test.
   Typical fix: Replace the spark plug wire. It's best practice to replace the entire set of 8 wires.
   Est. part cost: $50-$100 for a set
4. Clogged or Faulty Fuel Injector 🟡 Medium Probability → Shop Fuel Injector
   How to confirm: Listen to the injector with a mechanic's stethoscope; it should have a consistent clicking sound. You can also swap the injector with another cylinder to see if the misfire code follows, though this is more labor-intensive. A 'noid light' can be used to confirm the injector is receiving an electrical signal. In one forum case, a faulty aftermarket injector was the culprit.
   Typical fix: Replace the fuel injector for cylinder 8. Some owners report success using a quality fuel system cleaner like Techron as a first step.
   Est. part cost: $60-$120
5. Worn Camshaft Lobe / Failed Lifter ⚪ Low Probability → Shop Engine Camshaft As noted in GM TSB #PIP4138P, these engines can experience wear on the camshaft and lifters, leading to misfires that are not resolved by ignition or fuel system repairs. This is a well-documented failure mode in owner forums.
   How to confirm: This requires in-depth mechanical diagnosis. 🎬 Watch: A professional walkthrough for diagnosing and repairing 6.0L LS misfires. A distinct ticking or knocking noise from the engine is a strong indicator. A technician will remove the valve cover to inspect valvetrain movement. Low compression on the cylinder after ruling out other causes points to this issue.
   Typical fix: This is a major repair involving replacing the camshaft and all lifters. It should be performed by a professional mechanic. Owners have been quoted from $5,100 to over $9,000 for this repair at a dealership.
   Est. part cost: $400-$800+ for parts alone

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 can sometimes be detected by spraying brake cleaner near the gasket with the engine running and listening for a change in idle speed. 🎬 Watch: How to identify a misfire caused by a vacuum leak. Other symptoms include a hissing sound, coolant loss, or engine overheating.
• Low Cylinder Compression: Internal engine damage such as a bad valve, worn piston rings, or a failed head gasket can cause a loss of compression, leading to a misfire. A compression test is needed to confirm this.
• Broken Flexplate: While rare, a cracked flexplate (the plate connecting the engine to the torque converter) can cause erratic crankshaft position sensor readings, which the PCM may interpret as a misfire. This is often accompanied by a knocking or ticking sound from the transmission bellhousing area.
• Bad Engine Ground Strap: GM trucks of this era are known for issues with the main engine ground strap, which connects the engine block to the firewall. A corroded or broken ground can cause a host of electrical issues, including misfires on an entire bank of cylinders (2, 4, 6, 8). Symptoms can include flickering lights or other strange electrical behavior.

Diagnosis Steps

1. Read the code with an OBD-II scanner to confirm P0308 and check for any other codes like P0300.
2. Visually inspect the area around cylinder 8 (rear passenger side). Look for obvious damage to wiring, loose connectors, or signs of leaks.
3. Swap the ignition coil from cylinder 8 with cylinder 6. Clear the codes and drive. If the code changes to P0306, the coil is faulty.
4. If the misfire remains on cylinder 8, swap the spark plug from cylinder 8 with cylinder 4. If the code changes to P0304, the spark plug is faulty.
5. If the misfire still remains on cylinder 8, inspect the fuel injector and its wiring. Use a noid light to confirm the injector is getting a signal. Listen for a steady clicking sound from the injector.
6. Check for vacuum leaks around the intake manifold near cylinder 8 using the brake cleaner method. Listen for a hissing sound.
7. Inspect engine ground straps, particularly the one from the back of the engine to the firewall. Ensure it is clean, tight, and not corroded.
8. If all ignition and fuel components test good, perform a compression test on cylinder 8 to check for mechanical issues.
9. If compression is low, investigate for a bad valve, head gasket, or piston rings. If compression is good but a ticking noise is present, consult TSB #PIP4138P and investigate for a potential worn camshaft lobe or failed lifter.

Parts You'll Likely Need

• Ignition Coil (OEM #19418993) — Ignition coils are a common failure item that directly causes a misfire on a single cylinder. Cylinder 8's location makes it prone to heat failure.
  Trusted brands: ACDelco, Delphi, DENSO
  OEM price range: $50-$80
  Aftermarket price range: $30-$60
• Spark Plug (OEM #41-962 (May be superseded by 41-110 or 41-162 Iridium)) — Spark plugs are a routine maintenance item and the most frequent cause of a misfire. The original 41-962 is a double platinum plug. Many owners upgrade to the Iridium equivalent for longer life.
  Trusted brands: ACDelco, NGK
  OEM price range: $10-$18
  Aftermarket price range: $5-$12
• Spark Plug Wire Set (OEM #19351593 (ACDelco Professional)) — The wire carries the electrical charge from the coil to the plug. A cracked or high-resistance wire will prevent a good spark, and the wire for cylinder 8 is in a high-heat area.
  Trusted brands: ACDelco, NGK
  OEM price range: $80-$120
  Aftermarket price range: $50-$100

Related Codes That Often Appear With This One

• P0300 — If the misfire is intermittent or affects multiple cylinders, a P0300 (Random/Multiple Cylinder Misfire Detected) code will often accompany the specific cylinder code.
• P0171 or P0174 — If the misfire is caused by a vacuum leak, a lean condition code for one or both engine banks may also be present.

Technical Service Bulletins (TSBs) & Recalls

• PIP4138P: Warns that for P0300-P0308 codes where standard diagnostics don't find a cause, technicians should check for a worn camshaft lobe and/or lifter. This is a critical TSB for this platform.
• PIP5628C: Communicates information needed for diagnosing misfire codes P0300-P0308, indicating a specific diagnostic path may be required by GM's technical assistance center (TAC).

Platform-Specific Known Issues

• Per TSB #PIP4138P, these vehicles can develop a misfire due to a worn camshaft lobe or a failed valve lifter. If standard ignition and fuel system checks do not

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. Pressure that drops quickly after the pump primes can indicate a leaking injector or check valve.
• Fuel Pressure (Engine Idling) — expected: Approximately 55 psi. Failure: Significantly lower pressure indicates a fuel delivery issue. Pressure may be slightly lower than KOEO reading.
• Ignition Coil Secondary Winding Resistance — expected: 9.5 - 11.1 kΩ (kilo-ohms). Failure: A reading outside this range indicates a faulty coil.
• Cylinder Compression — expected: 120 - 150 psi, with minimal variation between cylinders. Failure: A reading significantly below 100 psi or more than 15-20% lower than other cylinders points to a mechanical engine issue (rings, valves, head gasket).

Hidden / Shadow Codes Worth Checking

• Mode 6 Misfire Counts: The PCM tracks misfires per cylinder even before they are frequent enough to set a P0308 code. A technician can view 'Mode 6' data with a capable scan tool to see the raw misfire count for each cylinder during the current and previous 10 driving cycles. This can confirm a problem with cylinder 8 before the Check Engine Light is constant. (see via A professional scan tool with Mode 6 capability. The data is often presented under IDs like 'A2' through 'A7' for misfire data, with values up to 65535.)

Scan Tool Commands That Help

• Tech 2 / GDS2: Cylinder Power Balance Test — This function allows the technician to disable one fuel injector at a time and observe the corresponding drop in engine RPM. If disabling cylinder 8 causes little or no RPM drop compared to other cylinders, it confirms that cylinder 8 is not contributing power, isolating the fault without swapping parts.
• Tech 2 / GDS2: Fuel Injector Balance Test — This test pulses each injector for a set duration while monitoring fuel pressure drop with a mechanical gauge. It's used to identify a clogged or restricted injector. If the pressure drop for cylinder 8 is significantly less than the others, the injector is likely faulty.
• Tech 2 / GDS2: Crankshaft Position Variation Learn — After replacing a crankshaft position sensor, flexplate, or performing major engine work, this procedure must be performed. It calibrates the PCM to the minor variations in the crankshaft reluctor wheel, ensuring accurate misfire detection. Failure to do this can cause false misfire codes.

Wiring & Ground Locations

• G103 — On the cowl, in the left rear of the engine compartment, often near or above the brake booster.. This is a critical ground point for the Body Control Module (BCM) and Instrument Panel Cluster (IPC). While not directly for the ignition system, a poor connection here can cause a wide range of bizarre electrical issues that could be misdiagnosed.
• G104 — On the left rear of the engine block.. This ground is often used for various engine sensors. A corroded or loose connection here can lead to erratic sensor readings, potentially impacting ignition and fuel control.
• Cylinder 8 Coil Connector — On the ignition coil for cylinder 8, located at the rear of the passenger-side valve cover.. The connector itself or the wiring leading to it can become brittle from heat cycles, causing an intermittent connection. The PCM sends the signal to fire the coil through this connector. Check for a clean, secure connection and test for 12v power and the trigger signal at the pigtail.

Real Owner Repair Stories

• YouTube video from 'Sean's DIY & Reviews' (2006 Dodge Ram 1500 (similar V8 pushrod architecture and failure mode)) — P0308 code and an audible engine tick.
  ❌ Tried (didn't work) Replacing the spark plug on cylinder 8., Swapping ignition coils between cylinders.
  ✅ What actually fixed it The root cause was a failed lifter, which caused the rocker arm for one of the valves on cylinder 8 to completely fall off. This meant the valve could not open, leading to the misfire. The fix required removing the valve cover to identify the dislodged rocker arm and then replacing the failed lifters.

OEM Part Supersession History

• ACDelco 41-962 (Double Platinum) → ACDelco 41-110 (Iridium) — Iridium plugs were introduced on newer models for longer service life and improved performance. For a time, GM recommended the 41-110 as the premium replacement for older vehicles that came with the 41-962.
  Heads up: While both plugs work, GM later issued a bulletin suggesting that pre-2007 vehicles originally equipped with platinum plugs should be serviced with the platinum 41-962 plug, as the ignition system was designed for it. Using the 41-110 is generally considered an upgrade, but sticking with the OE-spec 41-962 is a safe, cost-effective choice.

Model Year Variations Within This Range

• 2003-2006: Around 2003, GM transitioned from the P01/P41 series Powertrain Control Module (PCM) to the more powerful P59 controller. While both manage the engine similarly, diagnostic strategies and software capabilities can differ slightly. The P59 has a different physical connector and pinout. This is important to know if performing advanced diagnostics or considering a PCM replacement.