Misfires can be a frustrating issue for any car owner, and when you’ve modified your 2018 WRX with a TGV (Tumble Generator Valve) delete, diagnosing the problem becomes even more crucial. While TGV deletes are a popular modification aimed at improving airflow and performance, they can sometimes introduce new variables that contribute to engine misfires. This guide is designed to help you systematically troubleshoot misfires on your 2018 WRX after a TGV delete, including when and how an OBD2 reset might be beneficial.
If you’re experiencing misfires, it’s essential to address them promptly to prevent potential engine damage. Before diving into complex diagnostics, let’s start with the basics and explore common causes, drawing upon general misfire knowledge and applying it to the specific context of a 2018 WRX with a TGV delete.
Here are common culprits behind misfires, especially relevant after a TGV delete:
-
Spark Plugs: Incorrectly Gapped, Worn, or Wrong Type. Spark plugs are fundamental to proper combustion. Ensure you are using the correct spark plugs for your 2018 WRX, and that they are gapped correctly (typically between .028″-.030″ for NGK plugs, which are often recommended). Aftermarket modifications, including TGV deletes and potential tuning changes, can sometimes necessitate adjustments to spark plug specifications or gap. Worn spark plugs are a common cause of misfires over time, regardless of modifications.
-
Fuel Injector Issues: Clogged, Dirty, or Faulty. Fuel delivery is critical for engine performance. Clogged or dirty fuel injectors can disrupt the fuel spray pattern and lead to misfires. Consider using a quality fuel injector cleaner or having them professionally cleaned. In more severe cases, injectors might fail. You can try swapping injectors between cylinders to see if the misfire follows, indicating a potential injector problem. Ensure you are using the correct octane fuel as recommended for your WRX, especially after any performance modifications.
-
Valve Clearances and Compression Problems. While less directly related to a TGV delete, valve clearances and compression issues are fundamental engine health factors that can cause misfires. Performing a compression and leak-down test can help diagnose these problems. Valve clearance adjustments are part of regular maintenance, although often overlooked.
-
MAF Sensor Problems: Dirty or Faulty Readings. The Mass Air Flow (MAF) sensor measures the air entering the engine, a crucial input for the engine control unit (ECU) to calculate the correct fuel mixture. A dirty MAF sensor can provide inaccurate readings, leading to misfires. Clean the MAF sensor with a specialized MAF sensor cleaner, being careful not to damage the delicate sensor wires. Ensure you are cleaning the MAF sensor itself and not confusing it with the Intake Air Temperature (IAT) sensor, which is often located nearby.
Properly functioning MAF sensor is crucial for accurate air flow readings in 2018 WRX engines.
-
Vacuum Leaks: Post-TGV Delete Installation Issues. Vacuum leaks are a very common issue after a TGV delete installation. The process involves disconnecting and reconnecting various vacuum lines and intake components. Carefully inspect all vacuum lines, especially around the intake manifold and where the TGV deletes were installed. Pay close attention to the Fuel Pressure Regulator (FPR) hose connection. Vacuum leaks can lean out the air-fuel mixture, leading to misfires and potentially triggering codes like P0171 (System Too Lean) or P0457 (Evaporative Emission Control System Leak Detected). Check the turbo inlet pipe for cracks or loose connections as well, as this is another potential source of pre-turbo leaks.
-
Grounding Issues: Poor or Loose Connections. Proper electrical grounding is essential for sensor accuracy and overall system stability. Check and clean the battery ground cable and any ground connections to the intake manifold and engine block. Ensure all sensor grounds are clean and securely fastened.
-
Cam Timing and Sensor Problems. Incorrect cam timing can cause significant engine performance issues, including misfires. Verify cam timing according to the service manual. Clean the faces of the Camshaft Position Sensor and Crankshaft Position Sensor, as debris can interfere with their readings.
-
Excessive PCV Blow-by. Excessive blow-by from the Positive Crankcase Ventilation (PCV) system can introduce oil and combustion gases back into the intake manifold. This can effectively lower the octane of the fuel-air mixture, leading to misfires and engine knock, especially in turbocharged engines like the WRX. Consider installing a catch can or air-oil separator to mitigate this.
-
Carbon Build-up in Cylinders. Carbon deposits can accumulate in the combustion chambers and on valves over time, especially in direct-injected engines like the FA20DIT in the 2018 WRX. These deposits can disrupt airflow and combustion, causing misfires. Consider using a carbon cleaning product like Seafoam or a top-end engine cleaner to help remove these deposits.
-
Lightweight Flywheel or Crank Pulley (Phantom Misfires). While less likely to be directly caused by a TGV delete, lightweight flywheels or crank pulleys can sometimes cause false misfire readings. The ECU’s misfire detection system relies on crankshaft speed fluctuations. Lightweight components can exaggerate these fluctuations, leading to “phantom” misfires. If you have these modifications, open-source tuning solutions might offer options to adjust misfire detection sensitivity.
-
Coil Pack Issues: Faulty Coils or Connectors. Ignition coil problems are a common cause of misfires. Test each coil pack and try swapping them between cylinders to see if the misfire follows the coil. This would indicate a faulty coil pack. Inspect the coil pack connectors for damage or corrosion and ensure they are securely connected. Use dielectric grease on the connections to improve electrical contact and prevent corrosion. Also, check the coil pack boots for cracks or damage.
Inspecting and testing ignition coils is essential for diagnosing misfires in 2018 WRX.
-
Fuel Pressure Regulator or Fuel Pump Problems. Insufficient fuel pressure can lead to lean conditions and misfires. Check fuel pressure using a fuel pressure gauge. It should typically be around 35-38 psi at idle at sea level. Removing the vacuum line to the FPR at idle should increase the pressure to around 43 psi. The FPR is designed to increase fuel pressure by 1 psi for every 1 psi of manifold pressure.
-
Faulty Front O2 Sensor. The front (upstream) Oxygen sensor plays a crucial role in air-fuel ratio control. A faulty O2 sensor can provide incorrect feedback to the ECU, leading to misfires. While the OBD2 system should detect a completely failed O2 sensor, a sensor that is degrading or providing inaccurate readings might still cause issues. Replacement is usually recommended for a suspect O2 sensor.
-
Damaged Crank Timing Gear. This is a less common issue but should be considered if other possibilities have been ruled out. Damage to the crank timing gear (behind the crank pulley) can disrupt timing and cause misfires.
-
Misfires on All Cylinders: If you experience misfires across all cylinders simultaneously, consider these specific causes:
- Mixed-up Coil Pack Connectors: Ensure the coil pack connectors are correctly installed (typically black to the rear cylinders, white to the front).
- Lightweight Flywheel: As mentioned earlier, this can cause phantom misfires.
- Bad or Unplugged Crank or Cam Sensor: A faulty or disconnected crankshaft or camshaft position sensor will severely disrupt engine operation and cause widespread misfires.
- Incorrectly Gapped Spark Plugs or Wrong Spark Plugs: Incorrect spark plugs or gaps will affect all cylinders.
- Cam Timing Off: Significant cam timing issues will impact all cylinders.
OBD2 Reset and its Role
After addressing potential mechanical or electrical issues causing misfires, an OBD2 reset can be a useful step. The OBD2 system stores Diagnostic Trouble Codes (DTCs) and freeze frame data when misfires occur. Resetting the OBD2 system clears these codes and any learned fuel trims.
When to Perform an OBD2 Reset:
- After Repairing a Misfire Cause: Once you’ve addressed the suspected cause of the misfire (e.g., replaced spark plugs, fixed a vacuum leak), an OBD2 reset will clear the check engine light and allow the ECU to relearn fuel trims without the influence of the previous fault condition.
- Troubleshooting Step: If you’re unsure of the exact cause, a reset can sometimes help. If the misfire returns quickly after a reset, it indicates a persistent underlying issue. If it takes longer to reappear, it might have been a transient problem or related to something temporarily affecting sensor readings.
How to Perform an OBD2 Reset:
- Using an OBD2 Scanner: The most reliable way is to use an OBD2 scanner. Connect the scanner to your WRX’s OBD2 port (usually under the dashboard on the driver’s side). Follow the scanner’s instructions to read and clear DTCs. This will typically reset the system.
- Battery Disconnect (Less Recommended): Disconnecting the negative battery terminal for 15-30 minutes can also reset the ECU in many vehicles. However, this method is less precise and can sometimes cause other issues (e.g., radio presets lost, need to relearn idle). It’s generally preferable to use an OBD2 scanner.
Important Considerations After TGV Delete and OBD2 Reset:
- Tuning: After a TGV delete, especially on a 2018 WRX, proper engine tuning is highly recommended, if not essential. The TGV delete modifies airflow characteristics, and the factory ECU tune may not be optimized for these changes. A custom tune can ensure optimal performance, fuel efficiency, and prevent issues like misfires that might arise from an improper air-fuel ratio after the modification.
- Monitoring: After a TGV delete and OBD2 reset (and ideally a tune), closely monitor your WRX for any recurring misfires or other issues. Use an OBD2 scanner or monitoring tool to check for DTCs and engine parameters.
Conclusion
Troubleshooting misfires on a 2018 WRX after a TGV delete requires a systematic approach. By understanding the common causes of misfires, especially those related to modifications like TGV deletes, and knowing how to use an OBD2 reset effectively, you can diagnose and resolve these issues. Remember to prioritize safety and consult with a qualified mechanic or tuner if you are not comfortable performing these diagnostics or repairs yourself. Proper tuning after a TGV delete is crucial for long-term engine health and performance.
