4Runner OBD2 P0304 Code: Diagnosing a Cylinder 4 Misfire

Encountering a check engine light in your Toyota 4Runner can be concerning, especially when accompanied by a P0304 OBD2 code. This code specifically points to a cylinder 4 misfire, indicating that the engine’s fourth cylinder isn’t firing correctly. Understanding the causes, symptoms, and fixes for a P0304 code is crucial for any 4Runner owner to ensure their vehicle runs smoothly and efficiently.

Understanding the P0304 Code in Your 4Runner

A P0304 code means the engine control unit (ECU) has detected a misfire in cylinder number 4. In simpler terms, one of your engine’s cylinders isn’t contributing to the combustion process as it should. This can lead to a variety of performance issues and potentially damage your engine if left unaddressed. Misfires can stem from various underlying problems, ranging from simple fixes to more complex mechanical issues.

Common Symptoms of a P0304 Misfire

Besides the check engine light illuminating, you might notice several symptoms indicating a cylinder 4 misfire in your 4Runner:

Rough idling: The engine may vibrate or shake noticeably when the vehicle is stationary.
Reduced engine power: The 4Runner might feel sluggish or less responsive than usual, especially during acceleration.
Poor fuel economy: Misfires can lead to inefficient combustion, resulting in decreased gas mileage.
Hesitation or stalling: The engine might hesitate or stumble during acceleration, or in severe cases, stall completely.
Unusual noises from the engine: You might hear popping or sputtering sounds.
VSC and Trac Off lights: As seen in the original post, misfires can sometimes trigger the Vehicle Stability Control (VSC) and Traction Control (Trac Off) lights in addition to the check engine light.

It’s important to note that these symptoms can vary in severity depending on the cause and frequency of the misfire.

Potential Causes of a P0304 Code in a 4Runner

Pinpointing the exact cause of a P0304 code requires a systematic approach to diagnosis. Here are some of the most common culprits behind cylinder 4 misfires in Toyota 4Runners:

1. Ignition System Issues

The ignition system is responsible for igniting the air-fuel mixture in the cylinder. Problems here are frequent causes of misfires:

Faulty Spark Plugs: Worn, fouled, or damaged spark plugs in cylinder 4 can fail to produce a strong enough spark for proper combustion. Spark plugs have a limited lifespan and should be replaced according to the manufacturer’s recommended intervals.
Failing Ignition Coil: Each cylinder in modern engines typically has its own ignition coil. A failing coil for cylinder 4 can result in a weak or nonexistent spark, leading to a misfire. Coils can degrade over time due to heat and electrical stress.
Spark Plug Wires (if applicable): While newer 4Runners often use coil-on-plug ignition, older models might have spark plug wires. Damaged or deteriorated wires can prevent sufficient spark energy from reaching the spark plug.

2. Fuel System Problems

Proper fuel delivery is essential for combustion. Issues in the fuel system can disrupt this process:

Clogged or Failing Fuel Injector: A fuel injector for cylinder 4 might be clogged, preventing it from delivering the correct amount of fuel. Injectors can become clogged with fuel deposits over time. A failing injector might also not spray fuel correctly.
Low Fuel Pressure: Insufficient fuel pressure can affect all cylinders, but cylinder 4 might be more sensitive, leading to a misfire. This could be due to a failing fuel pump, fuel filter, or fuel pressure regulator.

3. Compression Issues

Adequate cylinder compression is vital for proper combustion. Leaks or mechanical problems can reduce compression:

Leaking Valves: Damaged or improperly seated valves in cylinder 4 can allow compression to escape, resulting in a misfire. Valve issues can arise from wear and tear, carbon buildup, or valve train problems.
Worn Piston Rings: Piston rings seal the combustion chamber. Worn rings in cylinder 4 can lead to “blow-by,” where combustion gases leak past the pistons, reducing compression.
Cylinder Head Gasket Leak: Although less common for a single cylinder misfire, a head gasket leak near cylinder 4 could potentially cause compression loss or coolant intrusion, leading to a misfire.

4. Vacuum Leaks

Vacuum leaks can disrupt the air-fuel mixture, leaning it out and causing misfires:

Intake Manifold Gaskets: Leaks in the intake manifold gaskets near cylinder 4 can allow unmetered air to enter the engine, causing a lean mixture and misfire.
Vacuum Hoses: Cracked or disconnected vacuum hoses connected to the intake manifold can also create leaks.

5. Other Potential Causes

While less frequent, other factors can contribute to a P0304 code:

Engine Timing Issues: Problems with the timing chain or belt can affect valve timing and potentially cause misfires.
Sensor Malfunctions: While less direct, faulty sensors like the Mass Air Flow (MAF) sensor or Oxygen (O2) sensors can provide incorrect data to the ECU, potentially leading to misfires under certain conditions.
Catalytic Converter Issues: In rare cases, a severely restricted catalytic converter could cause back pressure and contribute to misfires.
ECU/PCM Problems: While extremely rare, a fault within the engine control unit (ECU) or powertrain control module (PCM) itself could theoretically cause a misfire code, although this is usually diagnosed after ruling out all other possibilities.

Diagnosing and Fixing a P0304 Code on Your 4Runner

Diagnosing a P0304 code typically involves a step-by-step process to systematically eliminate potential causes:

OBD2 Scanner Verification: Use an OBD2 scanner to confirm the P0304 code and check for any other related codes. Clear the code and see if it returns after driving. Note any freeze frame data if available.
Visual Inspection: Inspect the spark plug wires (if applicable), ignition coil, and fuel injector for cylinder 4 for any obvious damage, loose connections, or signs of wear. Check vacuum hoses for cracks or disconnections.
Spark Plug and Ignition Coil Swap: A common first step is to swap the spark plug and ignition coil from cylinder 4 with those from another cylinder (e.g., cylinder 3). Clear the code and see if the misfire moves to the swapped cylinder (e.g., P0303). If it does, you’ve likely identified the faulty component (spark plug or coil).
Fuel Injector Testing: If swapping ignition components doesn’t resolve the issue, the fuel injector for cylinder 4 should be tested. This can involve checking injector resistance, listening for injector clicking with a stethoscope, or performing a fuel injector balance test.
Compression Test: Perform a compression test on all cylinders, including cylinder 4. Low compression in cylinder 4 indicates a potential issue with valves, piston rings, or cylinder head gasket.
Vacuum Leak Test: Use a smoke machine or carburetor cleaner (sprayed carefully around potential leak areas while monitoring engine RPM) to check for vacuum leaks around the intake manifold and vacuum hoses.
Further Diagnostics: If the above steps don’t pinpoint the problem, further diagnostics might involve checking fuel pressure, inspecting engine timing, and potentially more advanced sensor testing or ECU diagnostics.

Addressing the Original Scenario

The original forum post describes a 2004 4Runner with a P0304 code that the seller claims was previously resolved with spark plug replacement. However, the code reappeared during a test drive after the vehicle sat for a couple of months.

Is it risky to buy this 4Runner? Yes, it carries a moderate level of risk. While the seller cleared the code, the fact that it returned quickly suggests the underlying issue might not be fully resolved. The smog shop’s broad suggestions (fluke, coil, plugs, valve train) highlight the uncertainty.

Recommendations for the potential buyer:

Don’t solely rely on the seller’s word. A cleared code doesn’t guarantee the problem is fixed.
Insist on a pre-purchase inspection by a trusted mechanic. A mechanic can perform a thorough diagnosis of the P0304 code and identify the root cause.
Compression test is crucial. Given the history and the smog shop’s mention of valve train issues, a compression test is essential to rule out mechanical problems.
Negotiate the price. Factor in the potential cost of repairs when negotiating the purchase price. If the issue is minor (e.g., spark plug or coil), it might be worth the risk at a reduced price. However, major engine work can be costly.
Walk away if uneasy. If the seller is unwilling to allow a pre-purchase inspection or if the diagnosis reveals significant issues, it’s wise to consider other 4Runner options.

Conclusion

A P0304 code in your Toyota 4Runner indicates a cylinder 4 misfire, which can stem from various ignition, fuel, compression, or vacuum system issues. Proper diagnosis is key to identifying the root cause and implementing the correct repair. For potential buyers encountering a 4Runner with a P0304 code, proceeding with caution and conducting a thorough pre-purchase inspection is highly recommended to avoid unexpected repair costs and ensure a reliable vehicle. Addressing misfires promptly is crucial for maintaining your 4Runner’s performance, fuel efficiency, and longevity.