P0308 on 2000-2004 Cadillac Seville: Cylinder 8 Misfire Causes and Fixes

What's Unique About the 2000-2004 Cadillac SEVILLE

The 2000-2004 Cadillac Seville with the 4.6L Northstar V8 uses a unique ignition system. Instead of individual ignition coils for each spark plug, it has two large 'coil cassettes'. One cassette houses the coils for the rear bank of cylinders (1, 3, 5, 7) and the other houses the coils for the front bank (2, 4, 6, 8). This means if the coil for cylinder 8 fails, the entire front coil cassette must be replaced, which is different from most other vehicles where you can swap a single coil. This design was used on Northstar engines from 2000-2005.

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 raw fuel from the exhaust.
• Engine knocking or pinging noise

Most Likely Causes

1. Failed Front Ignition Coil Cassette 🔴 High Probability → Shop Ignition Coil The 2000-2004 models use a coil cassette design where a single internal coil failure requires replacing the whole unit. Heat and age cause the internal electronics and windings to break down. Owners report repeated failures of aftermarket cassettes, making OEM brands like ACDelco a safer bet.
   How to confirm: Since you cannot swap an individual coil, diagnosis involves first ruling out the spark plug. If the plug is good, the cassette is the next logical suspect. A diagnostic scanner with live data may show ignition command signals but no corresponding spark. The diagnostic process of swapping components between cylinders is not possible with this cassette design.
   Typical fix: Replace the front ignition coil cassette (servicing cylinders 2, 4, 6, and 8). It is highly recommended to replace all four spark plugs on that bank at the same time, as worn plugs can strain the new cassette.
   Est. part cost: $100-$275
2. Worn or Fouled Spark Plug 🟡 Medium Probability → Shop Spark Plug Spark plugs are a standard wear item. The manufacturer specifies platinum-tipped plugs with a long service interval, but oil leaks from the valve cover gaskets can foul the plugs, a common issue on higher-mileage Northstar engines.
   How to confirm: Remove the spark plug from cylinder 8 (front bank, driver's side). Inspect it for wear (worn electrode), carbon fouling, oil, or coolant contamination. A plug drenched in fuel can indicate a lack of spark or a stuck injector.
   Typical fix: Replace the spark plug. If one is worn, it's best practice to replace all 8. Use the specified high-quality Iridium or Platinum plugs (e.g., ACDelco 41-987).
   Est. part cost: $8-$20 per plug
3. Cracked Intake Plenum Boot 🟡 Medium Probability → Shop Engine Intake Manifold The rubber boot connecting the intake plenum to the throttle body is a well-known failure point on 2000-2004 Northstar engines. The rubber becomes brittle with age and heat, cracking on the underside and causing a significant unmetered vacuum leak. This often triggers random misfire code P0300 but can also cause single-cylinder misfires.
   How to confirm: Perform a smoke test to find the source of the leak. A common DIY method is to carefully spray brake cleaner or carburetor cleaner around the intake boot with the engine running; a change in engine idle indicates a leak is present. The crack is often on the bottom and not visible from above.
   Typical fix: Replace the cracked boot and the associated intake manifold gaskets. This requires removing the entire intake plenum. 🎬 See how to replace a cracked plenum boot and gaskets.
   Est. part cost: $40-$100 for the boot and gasket kit (e.g., Dorman 911-010).
4. Clogged or Faulty Fuel Injector ⚪ Low Probability → Shop Fuel Injector Injectors can become clogged with fuel deposits over time, or the internal coil can fail, preventing it from opening. This can lead to a lean misfire.
   How to confirm: Swap the fuel injector from cylinder 8 with another cylinder on the same bank (e.g., cylinder 6). Clear the codes and drive. If the code changes to P0306, the injector is faulty. This requires removing the fuel rail.
   Typical fix: Replace the faulty fuel injector. It's good practice to replace the O-rings on the other injectors while the fuel rail is off.
   Est. part cost: $50-$120
5. Vacuum Leak (Other Hoses) ⚪ Low Probability In addition to the main plenum boot, various smaller PCV hoses and vacuum lines on the Northstar engine are known to crack or become soft and collapse with age, causing unmetered air to enter the engine.
   How to confirm: Perform a smoke test to find the source of the leak. Visually inspect all rubber hoses connected to the intake manifold for cracks, especially at the elbows and connection points.
   Typical fix: Replace the cracked hose(s).
   Est. part cost: $20-$80

Rare But Worth Checking

• Low Cylinder Compression: While less common for a single-cylinder misfire, Northstar engines are infamous for head gasket issues due to cylinder head bolt thread failure ('pulling threads'). This can cause a loss of compression and a persistent misfire that does not respond to ignition or fuel system repairs. A compression test is required to confirm this serious issue.

Diagnosis Steps

1. Read the code with an OBD-II scanner to confirm P0308 and check for any other codes like P0300, P0171, or P0174.
2. Locate cylinder 8. On the transverse Northstar, the even-numbered cylinders are in the front (near the radiator). Cylinder 8 is on the driver's side.
3. Inspect for vacuum leaks first, as this is a common platform issue. With the engine running, carefully spray short bursts of brake cleaner around the large intake plenum boot and other vacuum hoses. If the engine idle changes, you have found a leak. 🎬 Watch: How to find and fix common Northstar vacuum leaks. A smoke test is a more definitive method.
4. If no vacuum leaks are found, proceed to ignition. Remove and inspect the spark plug from cylinder 8. Look for excessive wear, oil, or fuel fouling. If the plug is old or fouled, replace it, clear the code, and test drive.
5. If the misfire persists with a good spark plug, the front ignition coil cassette is the most likely culprit. Replace the front cassette. It is strongly recommended to install new spark plugs in all four front cylinders (2, 4, 6, 8) at this time.
6. If replacing the cassette and plugs does not solve the issue, investigate the fuel system. Swap the fuel injector from cylinder 8 with another cylinder on the same bank (e.g., cylinder 6). If the misfire code moves to the other cylinder (P0306), you have a bad injector.
7. As a final step for persistent misfires, perform a cylinder compression test on cylinder 8 to rule out mechanical engine problems like worn piston rings, valves, or a head gasket failure. Compression should be between 120-150 PSI on a healthy engine.

Parts You'll Likely Need

• Front Ignition Coil Cassette (OEM #ACDelco D586 (replaces 1104075)) — This is the most common failure point for a P0308 code on this specific engine, as the internal coil for cylinder 8 cannot be replaced individually. Aftermarket units have a reputation for premature failure.
  Trusted brands: ACDelco (OEM), Delphi
  OEM price range: $200-$300
  Aftermarket price range: $100-$180
• Iridium or Platinum Spark Plug (OEM #ACDelco 41-987) — Spark plugs are a primary ignition component and a common cause of misfires. Worn plugs also strain the ignition coils, so they should be replaced with the cassette.
  Trusted brands: ACDelco, NGK, Denso
  OEM price range: $12-$20
  Aftermarket price range: $8-$15
• Intake Plenum Boot Kit (OEM #Dorman 911-010 (Kit)) — This rubber boot is a very common failure item on the 2000-2004 Northstar, causing vacuum leaks that lead to various misfire codes. It often needs to be replaced as preventative maintenance or to solve a persistent misfire.
  Trusted brands: Dorman, ACDelco
  OEM price range: $60-$100
  Aftermarket price range: $40-$70

Related Codes That Often Appear With This One

• P0300 — If the underlying issue, like a large vacuum leak from the intake boot or low fuel pressure, becomes more severe, it can cause misfires on multiple random cylinders, triggering a P0300 code.
• P0302, P0304, P0306 — If the entire front ignition coil cassette is failing electronically, you may see misfire codes for other cylinders on that bank (2, 4, 6) as well.
• P0171, P0174 — These codes indicate a lean condition on Bank 1 and Bank 2, respectively. A significant vacuum leak, such as from the intake plenum boot, will often cause these codes to appear alongside a P0300 or specific cylinder misfire codes.
• P0358 — This code indicates a specific fault in the ignition coil control circuit for cylinder 8. If you see this along with P0308, it strongly suggests an electrical problem with the coil cassette or its wiring, rather than a fuel or mechanical issue.

Technical Service Bulletins (TSBs) & Recalls

• PIP5628G: A general bulletin for technicians communicating with GM's Technical Assistance Center (TAC) regarding misfire codes P0300-P0308, indicating it's a common diagnostic area.
• 03-06-04-030H: A GM bulletin detailing the procedure for cleaning fuel injectors if updated PCM calibrations do not resolve lean codes (like P0171) that can accompany misfires.
• 19-NA-219: A diagnostic tip bulletin for various GM V6/V8 engines regarding misfire codes (P0300-P0308) that could be caused by worn cam lobes, lifters, or sticking valves.

Platform-Specific Known Issues

• The use of two 4-coil ignition cassettes is specific to the 2000-2005 Northstar engines. A failure of one coil requires replacing the entire cassette assembly.
• The intake plenum boot is known to crack on the underside, causing a hard-to-see vacuum leak that frequently results in P0300, P0171/P0174, and single-cylinder misfire codes.
• A cold-only misfire on this platform can be an early sign of a failing coil cassette or a carboned-up valve, which may not be apparent once the engine warms up.

Mechanic-Grade Diagnostic Values

• Cylinder Compression — expected: 120-150 PSI. Failure: Readings significantly below 120 PSI or a variance of more than 15-20% between cylinders indicates a mechanical engine problem.
• Fuel Pressure (Key On, Engine Off) — expected: Approximately 40-46 PSI. Failure: Significantly lower pressure indicates a weak fuel pump or clogged filter. Note: The 2000-2004 Seville uses a 'returnless' system where the regulator is in the tank, so pressure at the rail may not drop with a bad regulator like in older systems.
• Ignition Coil Cassette Secondary Resistance (General) — expected: 6,000 - 15,000 ohms (6kΩ - 15kΩ). Failure: A reading outside this range (either open-loop/infinite or near zero) on the pins corresponding to cylinder 8 would indicate a failed internal coil. This test is difficult without a pinout diagram for the cassette connector.

Hidden / Shadow Codes Worth Checking

• Misfire Counters (Current/History): Most professional scan tools can access live data PIDs that show a running count of misfires for each individual cylinder. This data is far more sensitive than the CEL; it can show a cylinder is misfiring occasionally even if it's not frequent enough to set a P0308 code. (see via A GM Tech 2, or a high-quality aftermarket scanner (Autel, Snap-on, Foxwell) with GM-specific software. Look for PIDs like 'Misfire Current Cyl 8' or 'Misfire History Cyl 8'.)

Scan Tool Commands That Help

• GM Tech 2 (or equivalent): Misfire Graph / Misfire Counters — Use this function to watch for misfires in real-time while driving or power-braking the engine to replicate the fault conditions. It can confirm if the misfire is happening on cylinder 8 and under what specific load/RPM, which helps differentiate from random misfires.
• GM Tech 2 (or equivalent): Crankshaft Position Variation Learn (CASE Relearn) — This procedure should be performed after replacing a crankshaft position sensor, PCM, or performing major engine work. If the stored variation data is corrupt, the PCM can incorrectly interpret normal crankshaft speed fluctuations as misfires. Running this procedure can resolve false misfire codes.
• GM Tech 2 (or equivalent): Injector Balance Test / Cylinder Power Balance — This function allows the technician to disable one fuel injector at a time. When you disable the injector for cylinder 8, if there is no change in engine idle speed or quality, it confirms that cylinder was not contributing power. This helps verify the misfire is on cylinder 8 before tearing parts down.

Wiring & Ground Locations

• G106 / G110 — Located on the left rear of the engine assembly.. Wiring diagrams for the 2003 Seville STS indicate this ground is used for both Ignition Control Modules (Bank 1 and Bank 2). Since the coil cassettes have integrated ICMs, a poor connection at this ground point could cause weak or intermittent spark for the entire front bank, including cylinder 8.
• G108 — Located on the right rear of the engine.. Provides another critical engine ground path. Verifying all engine-to-chassis grounds are clean and tight is a crucial step in diagnosing any intermittent electronic issue, including misfires.
• Front Coil Cassette Connector — The main electrical connector plugging into the front ignition coil cassette.. The terminals and wiring at this connector can become corroded or damaged from heat and vibration, leading to a poor connection and misfires on one or more cylinders serviced by the cassette. The fault may be in the connector, not the cassette itself.

Real Owner Repair Stories

• Tahoe/Yukon Forum user '5150TAHO' (2002 Chevy Tahoe (Similar GM V8 architecture)) — Persistent cylinder 8 misfire.
  ❌ Tried (didn't work) Replacing the ignition coil.
  ✅ What actually fixed it The injector harness connector for cylinder 8 was not making a good connection. Replacing the harness pigtail with one from a salvage yard resolved the issue.
• YouTube channel 'Vince's Auto Fix' (2003 Cadillac DeVille (Same 4.6L Northstar engine and intake design)) — Rough idle that eventually set a P0300 random misfire code, with cylinders 1 and 8 showing the most misfires.
  ❌ Tried (didn't work) Initial diagnosis pointed towards ignition or injectors.
  ✅ What actually fixed it The root cause was a large crack in the rubber intake plenum boot, located under the main intake manifold. Replacing the boot and associated gaskets completely resolved the misfire.

"I Checked Everything" — The Actual Cause

• In some cases, a smoke test for vacuum leaks will come back clean, and swapping ignition or fuel components won't solve the problem. One owner found the issue was an intermittent connection at the fuel injector's wiring harness plug. The injector itself was good, but it wasn't receiving a consistent signal from the PCM. This type of electrical fault is often missed during standard diagnostic procedures.

Model Year Variations Within This Range

• 2000-2004: The 2000-2004 Seville uses a coil-on-plug system with two 4-cylinder ignition 'cassettes'. This is different from the 1993-1999 Northstar engines which used four individual remote coil packs, and later Northstar engines (in models like the SRX and XLR) which used eight individual coil-on-plug units. This makes the diagnosis unique, as you cannot swap a single coil from cylinder 8 to another cylinder.