P0304 on 2016 Acura MDX: Cylinder 4 Misfire Causes and Fixes

What's Unique About the 2016-2016 Acura MDX

The 2016 Acura MDX is powered by a 3.5L V6 J-series engine (J35Y5) featuring direct injection. While known for reliability, this engine is not immune to common misfire issues. A notable issue on some modern Honda/Acura direct-injection V6 engines relates to faulty fuel injectors, which have been known to cause persistent misfires and are addressed in service bulletins for related models. Additionally, because it is a direct-injection engine, carbon buildup on intake valves can be a cause for misfires over time, a less common issue on older port-injected engines. Finally, a valve adjustment is a known maintenance item on J-series engines that can resolve misfires if the valve lash is out of spec.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this vehicle.

Symptoms You May Notice

• Rough idle or engine vibration
• Hesitation or stumbling during acceleration
• Reduced engine power
• Flashing or solid Check Engine Light
• Decrease in fuel economy
• Unusual smell from the exhaust

Most Likely Causes

1. Faulty Ignition Coil 🔴 High Probability → Shop Ignition Coil Ignition coils are a common wear-and-tear item on most modern engines, including the Acura J35 V6. Heat and vibration eventually cause them to fail.
   How to confirm: Swap the ignition coil from cylinder 4 with an adjacent cylinder (e.g., cylinder 5). Clear the codes and drive. If the code changes to P0305, the ignition coil is faulty.
   Typical fix: Replace the faulty ignition coil. It is often recommended to replace all six coils and spark plugs at the same time for preventative maintenance, especially on higher mileage vehicles.
   Est. part cost: $40-$85 per coil
2. Worn or Fouled Spark Plug 🔴 High Probability → Shop Spark Plug
   How to confirm: Remove the spark plug from cylinder 4 and inspect it for wear, damage, or carbon/oil fouling. A healthy plug should have a light tan color. On related Honda V6 engines with VCM, plugs in certain cylinders are prone to fouling from oil consumption, which can be a sign of deeper issues.
   Typical fix: Replace the spark plug. It is best practice to replace all six spark plugs at the same time with OEM-spec (NGK or Denso) iridium plugs to ensure consistent performance.
   Est. part cost: $15-$30 per plug
3. Faulty Fuel Injector 🟡 Medium Probability → Shop Fuel Injector The 2016 MDX uses a direct injection system. Some modern Acura/Honda models have experienced premature fuel injector failures that can cause persistent misfires. There are numerous TSBs for fuel injector issues on related models, indicating a known area of concern for the manufacturer.
   How to confirm: Listen for a consistent clicking sound from the injector using a mechanic's stethoscope. A more definitive test involves swapping the injector with another cylinder, but this is a labor-intensive job requiring removal of the intake manifold. A professional can also perform a fuel injector balance test.
   Typical fix: Replace the faulty fuel injector. Due to the labor involved, some owners and shops recommend replacing all six injectors as a set. Acura offers an injector set for this purpose (part number 16010-R9P-305).
   Est. part cost: $50-$150 per injector

Rare But Worth Checking

• Valves Out of Adjustment: Honda J-series V6 engines require periodic valve clearance (lash) adjustments. Over time, exhaust valves can tighten and intake valves can loosen, leading to poor compression and misfires, especially when the engine is cold. If the vehicle has over 100,000 miles and has never had this service, it's a strong possibility.
🎬 Watch: How to perform a J-series valve lash adjustment.
• Low Engine Compression: This indicates a more serious mechanical problem, such as a burnt valve, worn piston rings, or a head gasket leak. A compression test followed by a leak-down test is needed to confirm and pinpoint the issue.
• Clogged EGR or Intake Passages: On some Honda/Acura V6 engines, carbon buildup in the EGR (Exhaust Gas Recirculation) system or intake manifold can cause misfires, particularly on specific cylinders. Some owners have reported success using an intake cleaning solution (like Seafoam) introduced through a specific vacuum hose to clear blockages.
• Internal Engine Failure: A safety recall (NHTSA Campaign Number: 23V751000) was issued for 2016-2020 MDX models for potential connecting rod bearing failure due to a manufacturing defect. This can lead to engine failure and may present as a misfire in its early stages.

Diagnosis Steps

1. Read the code with an OBD-II scanner to confirm P0304.
🎬 Watch: Step-by-step guide to diagnosing a cylinder four misfire.
2. Note if the Check Engine Light is solid or flashing. A flashing light indicates a severe misfire that requires immediate attention.
3. Locate cylinder 4 on the 3.5L V6 engine. The front bank (closest to the radiator) contains cylinders 4, 5, and 6 (from passenger side to driver side). The rear bank is 1, 2, and 3.
4. Swap the ignition coil from cylinder 4 with cylinder 5. Clear the codes, start the engine, and see if the misfire code moves to P0305. If it does, the coil is bad.
5. If the code remains P0304, inspect the spark plug from cylinder 4. Look for signs of wear, oil fouling, or damage. If the plug is suspect, replace it. It's best to replace all six.
6. If ignition components are good, inspect the wiring and connector for the cylinder 4 ignition coil and fuel injector for any damage or loose connections.
7. Listen to the fuel injector on cylinder 4 with a mechanic's stethoscope to ensure it is clicking. Compare the sound to a known good injector on another cylinder.
8. If the vehicle has high mileage (over 100k miles) and the above steps do not identify the issue, consider a valve adjustment, as tight valves are a known cause of misfires on J-series engines.
9. If a mechanical issue is suspected, perform a cylinder compression test, followed by a leak-down test to check for issues like burnt valves, bad piston rings, or a faulty head gasket.

Parts You'll Likely Need

• Ignition Coil (OEM #30520-5G0-A01) — This is the most frequent cause of a single-cylinder misfire and is a common wear item. This part number is correct for the 2016 MDX.
  Trusted brands: Acura (Genuine), Denso, NGK, Hitachi
  OEM price range: $60-$85
  Aftermarket price range: $40-$70
• Iridium Spark Plug (OEM #NGK DILKAR7G11GS) — Spark plugs are a routine maintenance item and a worn or fouled plug is a primary cause of misfires. The service interval is around 100,000 miles. 🎬 See this walkthrough for replacing your MDX spark plugs.
  Trusted brands: NGK, Denso
  OEM price range: $20-$30
  Aftermarket price range: $15-$25
• Fuel Injector Set (6) (OEM #16010-R9P-305) — If the misfire is persistent and not related to ignition, a faulty direct fuel injector is a likely cause on this engine. They are often replaced as a set.
  Trusted brands: Acura (Genuine)
  OEM price range: $800-$1200 (for the set)
  Aftermarket price range: $400-$700 (for a set)

Related Codes That Often Appear With This One

• P0300 — If multiple cylinders are misfiring randomly, a P0300 (Random Misfire) code may appear alongside the specific cylinder code. On Hondas, it's common for a single-cylinder misfire to trigger codes for other cylinders as well.
• P0301, P0302, P0303, P0305, P0306 — These codes indicate misfires in other cylinders. Seeing them together can point to a systemic issue like a bad fuel pump, clogged EGR system, or the need for a valve adjustment.
• P0171, P0174 — These codes indicate the engine is running lean on Bank 1 and Bank 2, respectively. A vacuum leak or a widespread fuel delivery issue could cause both lean conditions and misfires.

Technical Service Bulletins (TSBs) & Recalls

• BPAS04242017901: Mentions the potential for P0301 through P0306 misfire codes to be stored, confirming manufacturer awareness of misfire issues on this engine.
• TSB 19-011: Safety recall concerning the fuel pump and a software update.

Platform-Specific Known Issues

• Acura issued Technical Service Bulletin #BPAS04242017901 which acknowledges that misfire codes (P0301-P0306) can be stored in the vehicle's computer, indicating a known issue with the engine platform. [⭐ MANUFACTURER TSB — highest authority, 18]
• Some owners of newer Acura models with direct injection have reported fuel injector failures leading to misfire codes, which were resolved by replacing all injectors under warranty or as a set.
• Acura TSB 19-011 addresses a potential fuel pump failure that could lead to stalling, which may present as a misfire. The fix is a PGM-FI software update.
• A major safety recall (NHTSA 23V751000) for 2016-2020 MDX models involves improper crankshaft manufacturing that can lead to connecting rod bearing failure and catastrophic engine damage. A misfire could be an early symptom of this serious issue.

Mechanic-Grade Diagnostic Values

• Ignition Coil Primary Winding Resistance — expected: 0.49 - 0.57 Ohms. Failure: A reading outside this range or an open loop (OL) indicates a faulty coil.
• Ignition Coil Secondary Winding Resistance — expected: 9.5 - 11.1 kOhms. Failure: A reading significantly outside this range suggests internal winding failure.
• Fuel Injector Resistance — expected: 10 - 18 Ohms. Failure: A reading below 10 Ohms or an open loop (OL) reading indicates a failed injector coil.
• Low-Pressure Fuel Pump Pressure — expected: 57 - 64 PSI. Failure: Pressure below this range indicates a weak in-tank pump or clogged filter.
• High-Pressure Fuel Rail Pressure — expected: 4,352 to 21,755 PSI (30 to 150 MPa) depending on load. Failure: Significantly lower pressure than commanded by the PCM, especially under load. May trigger code P0087.

Hidden / Shadow Codes Worth Checking

• Mode $06, TID $A2-$A7: These Test IDs in Mode $06 track the misfire count for each cylinder (A2 for Cyl 1, A3 for Cyl 2, etc.). A technician can view these raw counts on an advanced scanner to see which cylinder is misfiring, even if the misfire count isn't high enough to set a specific P030x code yet. (see via Advanced OBD-II scanner with Mode $06 functionality.)

Scan Tool Commands That Help

• Acura HDS (Honda Diagnostic System) or compatible bidirectional scanner: Misfire Cylinder Test / Power Balance Test — Used to isolate a misfiring cylinder when only a P0300 (Random Misfire) is present. The tool deactivates spark or fuel to each cylinder one by one; the cylinder that causes the least change in RPM when deactivated is the culprit.
• Acura HDS (Honda Diagnostic System) or compatible bidirectional scanner: CKP Pattern Relearn — This procedure is necessary after certain repairs or if the PCM's learned crankshaft position sensor data becomes corrupted, which can cause false misfire codes. It allows the PCM to relearn the precise pattern of the crankshaft reluctor wheel.
• Acura HDS (Honda Diagnostic System) or compatible bidirectional scanner: Reset All Learned Values — After replacing major components like fuel injectors or performing a valve adjustment, this function clears long-term fuel trims and other learned adaptations, forcing the PCM to establish a new baseline with the new parts.

Real Owner Repair Stories

• YouTube channel 'JNB Tuning' (Acura MDX (3rd Gen, similar to 2016)) — P0304 code, rough idle, check engine light.
  ❌ Tried (didn't work) Replacing the spark plug on cylinder 4., Swapping the ignition coil from cylinder 4 to another cylinder (misfire did not move)., Performing a compression test (results were good)., Inspecting the cylinder with a borescope (no visible issues).
  ✅ What actually fixed it After all standard diagnostic steps showed no faults, the owner added a fuel system/injector cleaner to the gas tank and used a different brand of fuel. The misfire code did not return after a month of driving, suggesting the issue was a partially clogged or dirty fuel injector that was cleared by the treatment.
• YouTube channel 'honda acura' (Honda/Acura V6 (similar J-series engine)) — P0304 cylinder 4 misfire.
  ❌ Tried (didn't work) Using Seafoam through the brake booster hose., Using Seafoam through another general vacuum hose.
  ✅ What actually fixed it The owner fixed the issue by introducing Seafoam cleaner directly into the vacuum hose that leads to the EGR valve passages. This suggests the misfire was caused by a specific carbon blockage in the intake/EGR runners for cylinder 4, which was cleared by the targeted cleaning.

OEM Part Supersession History

• 16450-R70-A02, 16450-R70-A03 → 37240-R70-A04 — Supersession for the rocker arm oil pressure switch. Earlier versions may be prone to failure.
• N/A → 30520-5G0-A01 — This is the current, widely used ignition coil for many modern Honda/Acura V6 engines, including the 2016 MDX.
  Heads up: Acura also lists part number 30520-5G0-S01. While similar, it is crucial to verify the correct part number by VIN to ensure compatibility.