I’ve been dealing with an issue on my 2002 Toyota Corolla (120k miles) where it’s been misfiring without throwing any error codes. Finally, today it threw a P0300 code, indicating a random misfire. I could use some help troubleshooting where to focus next.
A few days ago, after starting fine in my garage, I noticed significant hesitation and even heard a bang sound while driving about 5 minutes later. This has happened a few times recently. When I parked and tried restarting after a short break, it was harder to start, especially on a hot day (around 80-85F). I’m not sure if the weather’s a factor, but issues seem more prominent on hot or humid days. Also, I noticed a burning or strange dust-like smell when I got home, which is new.
Since I first noticed the misfire weeks ago, I’ve replaced spark plugs (NGK), ignition coils (Denso), and the valve gasket cover. I also replaced the car battery and cleaned the MAF sensors with cleaner. I’ve done a vacuum leak test (all good), checked the serpentine belt, fluids, and air filter. Just recently, I used Chevron fuel injector cleaner when my gas tank was low.
I haven’t looked into the fuel filter or pump much because I’ve heard they’re usually not the issue with this car, but I could be mistaken.
Any ideas on what I should check next for this P0300 issue would be greatly appreciated!
Make sure your spark plugs are NGK or Denso. The ignition coils must be Denso.
The hesitation you’re experiencing is often caused by the mass air flow sensor. Try cleaning it, and while you’re at it, consider cleaning the idle air control sensor too. Both sensors need to be removed for cleaning, and be careful as the screws can strip easily.
Thanks for getting back to me.
Yes, I’m using NGK spark plugs and Denso ignition coils.
I forgot to mention earlier (and just updated my post) that I also cleaned the MAF sensor using cleaner. I haven’t tried cleaning the idle air control sensor yet, but I’ll give that a shot shortly.
With random misfires, potential issues could include clogged fuel injectors, incorrect spark plug gaps, or fuel-related issues. The bang sound you heard is likely unburnt fuel that didn’t ignite during combustion and exited through the exhaust.
Do you happen to know when the fuel injectors were last removed and cleaned? My suspicion is that they might be clogged and could benefit from a thorough cleaning and new injector O-rings.
If you have access to a scan tool, please share the following information:
MAF sensor data
Long term and short term fuel trims
Upper and lower oxygen sensor readings
Please capture live data or freeze frame data at both idle and 3000 RPM. Sharing this information here will help others assist you better. Without these details, diagnosing the issue accurately is challenging. It’s important to pinpoint the root cause rather than randomly replacing parts.
Also, consider performing a compression test to further diagnose the issue.
Regarding the question about when the fuel injectors were last cleaned, I haven’t addressed this yet. I had someone I know take a quick look, and based on their initial troubleshooting, they didn’t think it was a likely issue. However, I’m open to exploring this further.
As for the live data, I plan to gather that soon and will post it here in a few days. Unfortunately, my current scanner doesn’t have the capability to capture data, so I’m waiting to acquire a better one that can provide live data. I appreciate your assistance and will update this thread as soon as I’m able to gather more information.
Start by inspecting your air filter and replace it if it’s dirty. The MAF sensor measures unmetered air entering the intake box. During cold starts, the idle may be higher. As the engine warms up, the RPMs will decrease. Typically, normal idle ranges between 700-900 RPMs.
It’s possible that your MAF sensor is dirty, affecting its ability to accurately measure airflow. Initially, MAF readings may be higher on a cold engine, stabilizing around 2-3 grams/second as it warms up. Use MAF sensor cleaner specifically for cleaning the sensor.
When starting your car, does the engine crank slowly or quickly when it’s hard to start? Check your battery by measuring voltage across the positive and negative terminals before attempting to start. A healthy battery should measure around 12.6 volts. Additionally, have the starter tested, preferably through a bench test, for a more accurate assessment compared to testing it on the battery terminals.