P0308 on Ford: How to Diagnose & Fix
Cylinder 8 Misfire Detected
4 Ford models · 4 documented issues · 2 critical · $150-$1,500 repair
P0308 on Ford by model: symptoms, diagnosis and fix
The optional 4.6L 2-valve modular V8 (shared with the Mercury Mountaineer) has aluminum heads that engage only a few spark-plug threads (~0.25 in). Under thermal cycling the plug can strip the threads and eject from the head, sometimes blowing out the coil boot with it. Ford acknowledged the stripped/missing-thread condition in TSB 07-21-2 for 1997-2008 4.6/5.4/6.8L 2V engines. Ejection causes a loud pop, sudden misfire and power loss, and a fuel-vapor fire risk.
Common Symptoms
- Loud pop/bang followed by exhaust-like ticking
- Sudden misfire and power loss
- Ejected spark plug and/or coil boot in the engine bay
- Check engine light with cylinder misfire code
- Fuel/burning smell (fire risk)
How to Fix
Repair the stripped port with a thread insert designed for the job — Ford's authorized LOCK-N-STITCH aluminum insert kit, which TSB 07-21-2 names as the only Ford-authorized spark plug thread repair — rather than replacing the head where possible. Reinstall a new plug and coil boot to spec torque. Prevent recurrence by torquing plugs correctly and anti-seizing per updated practice; inspect all plugs when one lets go.
Sources for this issue (1)
The 4.6L 2-valve engine uses spark plugs that thread into aluminum heads with only a few threads of engagement. The plugs can eject from the head under load, stripping the threads. Ford addressed stripped or missing spark plug threads on 1997-2008 Crown Victoria 2V heads in TSB 07-21-2, authorizing a Lock-N-Stitch thread insert repair.
Common Symptoms
- Loud popping sound
- Misfire
- Loss of power
- Spark plug shoots out of engine
How to Fix
Sources for this issue (2)
A failed coil sets a cylinder-specific misfire (P0301-P0308) and often a matching coil-circuit code (P0351-P0358), causing a shake at idle, hesitation, and a flashing check-engine light.
Common Symptoms
- Cylinder-specific misfire, rough idle, engine shake
- Hesitation/bog on acceleration
- Flashing check-engine light
- Fuel smell from exhaust, slight economy drop
How to Fix
Confirm the bad coil by swapping it to an adjacent cylinder and seeing if the misfire follows. Replace the failed coil (and its spark plug), apply dielectric grease to the boot to seal out moisture. Consider a full coil/plug set at high mileage.
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
- Ignition CoilCross-ref0221504704
Bosch Engine Management Sensors 0221504704. Fitment confirmed against the ShowMeTheParts catalog for this vehicle's year and engine.
As an Amazon Associate, we earn from qualifying purchases; other part links may also earn au7o a commission. Confirm fitment by VIN before buying.
Sources for this issue (2)
The 4.6L and 5.4L Triton 2-valve V8s in 1997-2004 Expeditions are notorious for spark plugs ejecting from the aluminum cylinder heads. The 2-valve heads have only 4 threads holding each spark plug, which is insufficient for the clamping force needed. Over time, the threads strip and the spark plug launches out of the head under combustion pressure, taking the ignition coil with it. This typically happens when spark plugs are removed and reinstalled, or spontaneously at high mileage.
Common Symptoms
- Loud popping noise from engine
- Spark plug and coil found on top of engine
- Engine misfire on one cylinder
- Hissing noise from plug hole
- Loss of power and rough running
How to Fix
If a spark plug has blown out, a thread repair kit (Time-Sert or Heli-Coil) can permanently restore the threads without removing the cylinder head ($150-400 per plug at a shop). Prevention: Use anti-seize on spark plug threads, torque to 22-25 ft-lbs (do not overtighten), and avoid changing plugs when the engine is hot. Time-Sert inserts are considered the superior repair ($30-50 per kit DIY). Some shops specialize in this repair and can do it in 30-60 minutes per plug.
Sources for this issue (2)
P0308 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P0308 mean on a Ford?
P0308 stands for "Cylinder 8 Misfire Detected." P0308 means the engine computer detected a misfire on cylinder number 8 — that cylinder is failing to combust properly or at the correct time. The PCM identifies it by monitoring crankshaft speed variations as cylinder 8 fires. Misfires waste fuel, increase emissions, and can overheat and damage the catalytic converter, so a hard misfire may cause a flashing check engine light. Typical symptoms include rough idle, vibration, hesitation, and reduced power. On Ford it is documented across 4 models: Explorer, Crown Victoria, F-150, Expedition.
What are the most common causes of P0308?
The most common causes of P0308 are: Worn, fouled, or incorrectly gapped spark plug on cylinder 8, Failed ignition coil or coil-on-plug for cylinder 8, Faulty, clogged, or leaking fuel injector, Vacuum or intake air leak affecting that cylinder, Low compression (worn rings, burnt/leaking valve, head gasket), Damaged spark plug wire or boot (on applicable engines), Carbon buildup or sticking valve. The specific cause varies by vehicle.
How much does it cost to fix P0308 on a Ford?
Published repair estimates on Ford range from $150 to $1,500 depending on the model and root cause. If an issue mentions a recall or campaign, check your VIN with the manufacturer or NHTSA and confirm eligibility and coverage with a dealer.
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Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.