P0308 on Ford: How to Diagnose & Fix
Cylinder 8 Misfire Detected
5 Ford models · 6 documented issues · 3 critical · $150-$1,500 repair
P0308 on Ford by model: symptoms, diagnosis and fix
The 2-valve Triton engines (4.6L, 5.4L, 6.8L V10) used in 1997-2008 E-Series vans are notorious for spark plugs ejecting from the cylinder heads. Ford used only 4 threads in the aluminum heads to secure the spark plugs, which is insufficient to withstand combustion pressures. The plug strips the threads and blows out of the head, often taking the ignition coil with it. This can happen without warning and leaves the vehicle running on fewer cylinders. The problem is most common on plugs 4 and 8 (passenger side).
Common Symptoms
- Loud pop followed by engine misfire
- Spark plug found sitting on top of engine or on ground
- Check engine light with misfire codes P0301-P0310
- Loud exhaust noise from engine bay on one cylinder
- Ignition coil damaged or blown off
How to Fix
The standard repair uses a Time-Sert 5553 or Cal-Van 38900 thread repair insert kit to install a steel thread insert in the damaged cylinder head. The repair can be done with the engine in the vehicle. Preventive installation of inserts on all cylinders is recommended when one blows out. Anti-seize should be used on spark plugs but do NOT over-torque — spec is 11 lb-ft.
Sources for this issue (2)
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. Later models (2004+) use a 2-piece plug design that can break during removal.
Common Symptoms
- Loud popping sound
- Misfire
- Loss of power
- Spark plug shoots out of engine
How to Fix
Sources for this issue (1)
The 4.6L and 5.4L Triton V8s use one coil-on-plug per cylinder, and these coils are a very common failure item — subjected to heat, vibration, and moisture from cowl leaks — typically starting after 80k miles. 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. Repeat failures on the same cylinder usually mean oil is leaking from the valve-cover gasket into the plug well and shorting the coil.
Common Symptoms
- Cylinder-specific misfire, rough idle, engine shake
- Hesitation/bog on acceleration
- Flashing check-engine light
- Fuel smell from exhaust, slight economy drop
- Repeat coil failure on the same cylinder (oil in plug well)
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. If the same cylinder fails repeatedly, replace the valve-cover gasket and clean oil from the plug tube. 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 (1)
The 3.5L EcoBoost's high-pressure direct (GDI) injectors run at diesel-like pressures and are a common failure item, especially at higher mileage and after heat-soak. A stuck-open or electrically failing injector floods a cylinder and can wash down neighboring cylinders on the same bank, producing a cold-start misfire that often smooths out once warm. It commonly sets injector-circuit codes (P0201-P0206) alongside misfire codes (P0300-P0308) and occasionally a total-misfire/knock code. It is frequently misdiagnosed as coils or plugs before the injector is found.
Common Symptoms
- Cold-start misfire / stumble that smooths when warm
- Rough idle and hesitation on acceleration
- Check-engine light, often flashing on start
- Fuel smell from exhaust, poor fuel economy
- Single-cylinder or same-bank multi-cylinder misfire
How to Fix
Read freeze-frame and identify the specific cylinder; compare fuel-injector balance/contribution data. Swap or replace the failed GDI injector (intake manifold removal required for access) and inspect for fuel-washed plugs. Replace plugs/coils on the affected cylinder if fouled. Verify high-pressure fuel pump output and low-side pressure to rule out supply issues before replacing multiple injectors.
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
- High-pressure direct-injection (GDI) fuel injector — 2011-2016 3.5L EcoBoost (Gen 1)OEMBL3Z-9F593-BCross-refMotorcraft CM-5202 (superseded by BL3Z-9F593-B), Ford BL3Z-9F593-A (earlier revision, superseded), Ford engineering number BL3E-9F593-HB / BL3E-HB$95–$185
Sold individually; the engine takes 6 (Require Quantity: 6). Gen 1 (2011-2016) 3.5L EcoBoost is direct-injection ONLY, so this is the single injector type on the engine.
- High-pressure direct-injection (GDI) fuel injector — 2017-2020 3.5L EcoBoost (Gen 2)OEMHL3Z-9F593-BCross-refBosch 0261500307, Motorcraft/Ford service reference CM5254, Ford 82511115$66–$140
Sold individually; 6 per 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 (1)
The 5.4L Triton 2-valve V8 in 1997-2008 Expeditions is 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 exactly 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 (1)
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 heli-coil-style thread insert designed for the job — a Time-Sert or Ford's authorized LOCK-N-STITCH aluminum insert kit — 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)
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 5 models: E-Series, Crown Victoria, F-150, Expedition, Explorer.
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.
Looking for P0308 on a different make?
View P0308 across all makes →Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.