P0308 on 2000-2002 Cadillac Eldorado: Cylinder 8 Misfire Causes and Fixes

What's Unique About the 2000-2002 Cadillac ELDORADO

The 2000-2002 Eldorado features the 4.6L Northstar V8 with a unique ignition system for these model years. Instead of individual coils for each spark plug, it uses two large 'ignition cassettes'—one for the front bank of cylinders (2, 4, 6, 8) and one for the rear bank (1, 3, 5, 7). 🎬 Watch: Northstar cylinder numbering and firing order explained. This design was used on FWD Northstar engines from 2000-2005. If the internal coil for cylinder 8 fails, the entire front ignition cassette assembly must be replaced. The front and rear cassettes are interchangeable, which is a critical detail for diagnosis.

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 vibrating idle
• Hesitation or stumbling during acceleration
• Noticeable loss of engine power
• Check Engine Light is on and steady
• Check Engine Light is flashing (indicates a severe misfire)
• Reduced fuel economy
• Smell of unburned gasoline from the exhaust
• Hot "popcorn" smell from the catalytic converter

Most Likely Causes

1. Failed Front Ignition Coil Cassette 🔴 High Probability → Shop Ignition Coil The integrated coil cassette design means a single coil failure requires replacement of the entire unit. Heat and age cause the internal coil windings to fail. This is a very common failure on 2000-2005 Northstar engines.
   How to confirm: Swap the front and rear ignition cassettes, as they are interchangeable. Clear the codes and drive the vehicle. If the misfire code changes to a cylinder on the rear bank (e.g., P0301, P0303, P0305, or P0307), the coil cassette is definitively faulty. This is the most effective diagnostic step before spending money on parts.
   Typical fix: Replace the entire front ignition coil cassette. It is highly recommended to replace all four spark plugs on that bank at the same time. Applying dielectric grease to the spark plug boots is good practice.
   Est. part cost: $200-$450
2. Worn or Fouled Spark Plug 🟡 Medium Probability → Shop Spark Plug Spark plugs are a standard wear item with a 100,000-mile service life for the original platinum plugs. Short-trip driving can lead to carbon fouling. Northstar engines from 2000-2002 were also noted to have issues with carbon buildup on piston rings which can contribute to fouling.
   How to confirm: Visually inspect the spark plug from cylinder 8. Look for a worn electrode, heavy carbon deposits, oil fouling, or a cracked porcelain insulator. The correct spark plug gap is 0.050 inches. Swap the plug with another cylinder and see if the misfire code follows.
   Typical fix: Replace the spark plug. It is best practice to replace all 8 spark plugs at once if they are near the end of their service life, though accessing the rear bank is notoriously difficult.
   Est. part cost: $8-$20 per plug
3. Faulty Fuel Injector ⚪ Low Probability → Shop Fuel Injector Injectors can become clogged over time from fuel deposits, an issue GM has addressed in TSB #03-06-04-030F. GM recommends using Top Tier detergent gasoline to prevent this.
   How to confirm: After ruling out spark issues, listen for a clicking sound from the injector with a mechanic's stethoscope. You can also test the injector's resistance with a multimeter or perform a fuel injector balance test using a scan tool like a Tech 2 or an AFIT tester. Swapping the injector with another cylinder is also a definitive test, though more labor-intensive.
   Typical fix: Replace the faulty fuel injector for cylinder 8. Sometimes, a professional fuel system cleaning can resolve minor clogging.
   Est. part cost: $70-$150
4. Vacuum Leak ⚪ Low Probability Rubber and plastic components in the engine bay become brittle over time. A notable weak point on this platform is the intake plenum boot, which can crack on the underside and be difficult to see, causing lean misfires.
   How to confirm: Perform a smoke test to find the source of the leak. Listen for hissing sounds around the intake manifold. A vacuum leak will often trigger lean codes (P0171/P0174) along with the misfire code.
   Typical fix: Replace the leaking gasket or hose. The intake manifold must be removed to replace a leaking plenum boot.
   Est. part cost: $10-$100

Rare But Worth Checking

• Low Cylinder Compression: While rare for a single-cylinder misfire, the Northstar engine can have mechanical issues, including head gasket failure which typically presents as combustion gases in the coolant. A compression test will confirm the mechanical health of the cylinder. A healthy cylinder should have compression over 140 PSI. Another rare cause on 2000-2002 models is excessive carbon buildup on valves, which can cause them to stick.
• Water Intrusion in Spark Plug Well: → Shop Spark Plug One owner on CaddyInfo forums reported a P0308 misfire immediately after washing the engine bay. Although no moisture was visible in the plug well, the timing suggested the water may have shorted the coil cassette's internal connection to that cylinder.

Diagnosis Steps

1. Read the trouble codes with an OBD-II scanner to confirm P0308 and check for any other codes like P0300, P0171/P0174, or P0358.
2. Identify cylinder 8. On the transverse Northstar, it is the cylinder on the bank near the radiator (front bank), at the far driver's side (passenger side in some documentation, but physically on the driver's side of the transverse engine).
3. Perform the Cassette Swap Test: This is the most crucial step. The front and rear ignition cassettes are interchangeable. Unbolt and swap them. Clear the codes, start the engine, and see if the misfire code moves to a cylinder on the rear bank (e.g., P0301, P0303, P0305, P0307). If it does, the cassette you moved to the rear is bad. Purchase a new cassette.
4. If the code remains P0308 after the cassette swap, the problem lies with the cylinder itself. Remove and inspect the spark plug from cylinder 8. Check for wear, fouling, and correct gap (0.050"). You can swap it with another plug on the same bank (e.g., cylinder 6) to see if the code changes to P0306.
5. If the spark plug and cassette are confirmed good, investigate the fuel injector for cylinder 8. Listen for a steady clicking with a stethoscope or use a 'noid' light to confirm it's receiving a signal.
6. If ignition and fuel are ruled out, check for a vacuum leak near cylinder 8 using a smoke machine.
7. As a final step, perform a cylinder compression test to rule out mechanical engine problems like worn rings, valves, or a head gasket issue. A chemical block test should also be used to check for combustion gases in the coolant if a head gasket is suspected.

Parts You'll Likely Need

• Front Ignition Coil Cassette (OEM #1104075) — This is the most common failure point for a P0308 code on this engine, as the internal coil for cylinder 8 fails and the entire cassette must be replaced.
  Trusted brands: ACDelco, Delphi, Standard Motor Products
  OEM price range: $270-$400
  Aftermarket price range: $150-$250
• Double Platinum Spark Plugs (OEM #41-987) — A worn or fouled spark plug is a common cause of misfires and should always be replaced when installing a new coil cassette. The OEM plug is an ACDelco Double Platinum.
  Trusted brands: ACDelco, NGK, Denso, Autolite
  OEM price range: $10-$15 per plug
  Aftermarket price range: $8-$12 per plug

Related Codes That Often Appear With This One

• P0300 — If the misfire becomes more widespread or is intermittent between cylinders, a P0300 (Random/Multiple Cylinder Misfire) code may appear alongside P0308.
• P0171 / P0174 — If the misfire is caused by a large vacuum leak, such as a cracked intake plenum boot, lean condition codes for Bank 1 and Bank 2 may also be triggered.
• P0358 — This code indicates a specific fault in the ignition coil primary/secondary circuit for cylinder 8. If seen with P0308, it almost guarantees the problem is within the ignition cassette or its wiring harness.

Technical Service Bulletins (TSBs) & Recalls

• PIP5628G: A General Motors TSB that provides diagnostic aid for misfire codes P0300 through P0308, indicating this is a recognized issue requiring specific diagnostic procedures.
• 03-06-04-030F: Addresses various driveability issues, including misfires, caused by clogged or restricted fuel injectors due to fuel quality.
• 16-NA-383: Discusses rough idle and misfire due to excessive carbon buildup on intake and/or exhaust valves, which can cause a loss of compression when cold.

Platform-Specific Known Issues

• Difficult Rear Spark Plug Access: → Shop Spark Plug While cylinder 8 is on the easily accessible front bank, owners should be aware that replacing all 8 plugs at once is standard practice. The rear four plugs are notoriously difficult to access against the firewall, often requiring removal of engine brackets.
• Starter Motor Location: Unrelated to this code, but important for owners to know: the starter motor on the Northstar V8 is located under the intake manifold. This makes replacement a very labor-intensive job requiring intake removal and new g

Mechanic-Grade Diagnostic Values

• Ignition Coil Cassette Primary Resistance — expected: 0.5 - 0.9 Ohms. Failure: A reading outside this range, or an open circuit (infinite resistance). Note that a coil can still be faulty even if it tests within this range.
• Ignition Coil Cassette Secondary Resistance — expected: 5,000 - 10,000 Ohms (5kΩ - 10kΩ). Failure: A reading significantly outside this range. A known bad coil was measured at 8.26k Ohms, so this test is not always conclusive.
• Fuel Injector Resistance — expected: 10 - 16 Ohms. Failure: A reading outside this range, or an open/shorted circuit. The most important factor is that all injectors have very similar resistance values.
• Fuel Pressure (Key On, Engine Off) — expected: 40.5 - 47 PSI. Failure: Pressure below this range may indicate a weak fuel pump or leaking regulator.
• Fuel Pressure at Idle — expected: 30.5 - 44 PSI. Failure: Pressure should decrease from the KOEO reading due to engine vacuum acting on the regulator. If it doesn't drop, the regulator's vacuum line may be disconnected or the regulator is faulty.
• Cylinder Compression — expected: 120 - 150 PSI, with minimal variation between cylinders.. Failure: A reading significantly lower than other cylinders indicates a mechanical engine problem like worn piston rings or a valve sealing issue.

Hidden / Shadow Codes Worth Checking

• Live Misfire Data / Misfire History: The PCM stores historical and real-time misfire counters for each cylinder. This allows a technician to see not just the current misfire, but also intermittent misfires on other cylinders that haven't yet triggered a code. (see via GM Tech 2 or equivalent professional scan tool. Look for 'Misfire Data' or 'Cylinder Misfire Counts' in the engine data list.)
• P1381: Misfire Detected - No Communication with Brake Control Module. This manufacturer-specific code can appear with a P030x code and points towards a data communication or module issue that could be related to the misfire detection logic. (see via Most comprehensive OBD-II scanners, not just basic code readers.)

Scan Tool Commands That Help

• GM Tech 2: Misfire Graph — This function displays a live bar graph of misfire events for each cylinder, making it easy to visually confirm which cylinder is the primary offender and see if other cylinders are having intermittent issues.
• GM Tech 2: Fuel Injector Balance Test — This bidirectional test commands each injector to fire for a set duration and measures the corresponding drop in fuel pressure. A smaller pressure drop for cylinder 8 would indicate a clogged or failing injector.
• GM Tech 2: Cylinder Deactivation — Allows the technician to manually disable one cylinder at a time while monitoring engine RPM drop. If disabling cylinder 8 results in little or no change to engine speed, it confirms that cylinder was not contributing power.

Wiring & Ground Locations

• G104 — On the left front corner of the engine compartment.. This is a critical ground point that serves both the Bank 1 (rear) and Bank 2 (front) ignition control modules. Corrosion or a loose connection here can cause erratic ignition performance, including misfires.
• G101 — Located on the lower front of the engine block.. This is a primary ground for the Powertrain Control Module (PCM). A poor connection here can cause a wide range of engine management issues, including false misfire detection.
• G110 — Lower left front corner of the engine.. This is another engine-area ground point. Verifying all engine grounds are clean and tight is a crucial step when diagnosing any electrical fault, including persistent misfires.
• Ignition Cassette Connector — The large electrical connector on the front-facing side of the front ignition cassette.. The terminals and wiring at this connector can become corroded or damaged, leading to a poor connection for the cylinder 8 coil circuit. It should be inspected when replacing the cassette.

Real Owner Repair Stories

• YouTube channel 'WTBM123 Car repair' (Cadillac DTS with Northstar V8 (similar PCV/intake design)) — P0300 random misfire, but only occurring at idle with the throttle plate completely closed. Misfire counts were highest on one cylinder.
  ❌ Tried (didn't work) Initial diagnosis pointed towards a single cylinder, but the misfire disappeared with any throttle input, making a coil or injector less likely.
  ✅ What actually fixed it The mechanic found a small vacuum leak at the PCV hose elbow where it connects to the intake manifold. Holding the elbow tight stopped the misfire. The final fix was to remove the intake manifold to replace the leaking hose, and the intake boot and gaskets were replaced at the same time.
• YouTube channel 'The Auto Rules' (Cadillac with 2000-2004 Northstar V8) — Multiple 'ghost codes', including misfires (P0300) and MAF sensor codes (P0101).
  ❌ Tried (didn't work) The video notes that owners often mistakenly replace the MAF sensor, spark plugs, coil packs, and injectors trying to solve the problem.
  ✅ What actually fixed it The actual cause was a large crack in the rubber intake plenum boot (throttle body coupler) located between the throttle body and the intake manifold. This part becomes brittle with age and causes a massive, unmetered air leak that the PCM cannot compensate for.

"I Checked Everything" — The Actual Cause

• A cracked intake plenum boot (throttle body coupler) can cause a massive vacuum leak leading to misfires, but may not be found with a smoke test. The crack may only open under specific conditions of engine torque or temperature, remaining sealed during a static test in the garage. The part must be physically removed and inspected for cracks, especially on the underside.

Model Year Variations Within This Range

• 2000-2002: The Eldorado was available in two main trims with different engine tunes. The ESC (Eldorado Sport Coupe) used the LD8 version of the 4.6L Northstar with 275 hp. The ETC (Eldorado Touring Coupe) used the high-output L37 version with 300 hp and a more aggressive final drive ratio (3.71:1). While the cause of P0308 is identical for both, it's important to know which engine is being worked on.
• 2000-2002: Starting in the 2000 model year, the Northstar engine was significantly revised to improve reliability, including changes to the cylinder head bolts to reduce head gasket failures. It was also retuned to produce full power on regular unleaded gasoline, whereas earlier versions required premium.