P0304 on Alfa Romeo: How to Diagnose & Fix
Cylinder 4 Misfire Detected
1 Alfa Romeo model · 1 documented issue · $500-$1,200 repair
P0304 on Alfa Romeo by model: symptoms, diagnosis and fix
The 2.0L GME turbo four is direct injected, so no fuel washes over the back of the intake valves. Crankcase vapour routed through the PCV system bakes onto the valve heads and forms hard carbon deposits that disrupt airflow and seal quality. What makes the Stelvio case notable is how early it appears: Stelvio and Giulia forum members report being sent for walnut blasting at Alfa's direction at roughly 6,000-6,500 miles because of cold-start misfires, an order of magnitude sooner than the 75,000-125,000 mile window typical of other direct-injection engines. Symptoms are worst on a cold start and often clear once the engine warms, which is why the condition is frequently dismissed on a test drive. This is a 2.0L issue and should not be merged with the 2.9L V6 Quadrifoglio's separate intake-side story.
Common Symptoms
- Rough idle and misfire on cold start that smooths as the engine warms
- Check engine light with cylinder misfire codes
- Hesitation or stumble under light throttle
- Reduced fuel economy
- Loss of low-end power
How to Fix
The real fix is mechanical cleaning - walnut shell blasting of the intake ports and valve faces with the intake manifold removed. Chemical induction/fuel-system services sold at dealers do not remove hardened deposits from the valve backs on a direct-injection engine and are not a substitute. Expect roughly $500-$1,200 at an independent specialist, more at a dealer, and budget for new intake manifold gaskets. Where misfires appear under warranty, dealers have performed the cleaning under warranty. Mitigation between services: use the specified oil, change it on a short interval, and consider a catch can to reduce vapour reaching the valves.
Sources for this issue (3)
P0304 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 P0304 mean on a Alfa Romeo?
P0304 stands for "Cylinder 4 Misfire Detected." This code means the engine computer detected a misfire in cylinder number 4 — that cylinder isn't igniting its air-fuel mixture fully or at all. The system detects this by monitoring small changes in crankshaft rotation speed as each cylinder fires. You may feel a rough idle, shaking, hesitation or reduced power, and a persistent misfire can dump unburned fuel into the exhaust and overheat or damage the catalytic converter. Since it's pinpointed to one cylinder, the problem typically lies in that cylinder's spark, fuel, or compression. On Alfa Romeo it is documented across 1 model: Stelvio.
What are the most common causes of P0304?
The most common causes of P0304 are: Worn or fouled spark plug in cylinder 4, Failed or weak ignition coil on cylinder 4, Faulty spark plug wire or boot (where applicable), Clogged, stuck or leaking fuel injector for cylinder 4, Vacuum or intake air leak near that cylinder, Low compression (worn rings, burnt/leaking valve, head gasket), Damaged wiring or connector at the coil or injector. The specific cause varies by vehicle.
How much does it cost to fix P0304 on a Alfa Romeo?
Published repair estimates on Alfa Romeo range from $500 to $1,200 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 P0304 on a different make?
View P0304 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.