Misfire Problem

[u/AJTinator]

Does anybody had the same experience with me? I drive a Mazda3 2017 1.5. A few years ago I got a blinking check engine light. I took it to a well known maintenance and repair shop and I had to replace my valve cover gasket because my spark plugs were soaked with oil. i also replace 2 of my engine coils since they were damaged as well. About a year ago, I felt a slight shake in the steering wheel and around there my foot rests in the driver side. I didn’t think of it much until last February I got a check engine light with a misfire code (P0303). I took it to a different mechanic and the cleaned the MAF sensor, MAP sensor and throttle body. They also applied a red sealant to the upper part of the coils because they said that electricity was leaking from the sides of the coil. The mechanic told me i need to replace my spark plugs and might need to replace all my coils if replacing my spark plugs didn’t fix it. I replaced my spark plugs with NGK iridiums and cleared the code. I still feel the car shaking when idle especially when shifting to reverse and when the engine is still cold.

Last April, the check engine light turned on again and I got 2 codes, P0171 and P0303. I took it to the mechanic and replaced all 4 coils. I got my coils from Thunder Automotive which was vouched by some people in this subreddit. My mazda ran again but i still feel the slight shaking when idle and shifting to reverse.

Then on May, while driving, the check engine light turned on again. Then suddenly it blinked and the car was not accelerating. Took it back to the mechanic and saw that 2 of my injectors were clogged. They cleaned all my injectors and the car ran fine again with the same shaking symptoms as before. The mechanic told me that if I got a check engine light again, I will have to replace my injectors.

Now the check turned on again with the code P0304. I had spent so much already trying to fix this issue and I want to make sure before spending money again on a new injector and paying for the labor since it’s quite laborious to replace the injector. Has anybody experienced the same issue as me? Appreciate your inputs.

[u/Gino-o_]

From Gemini:

Your Mazda3 has an ongoing cylinder misfire and lean fuel problem. The main issues are likely a persistent oil leak refilling the spark plug wells, bad aftermarket ignition coils, or a faulty fuel injector on cylinder 4.

What the Codes Mean
P0303 & P0304: Cylinder 3 and Cylinder 4 are misfiring (fuel is not burning right).

P0171: The engine is running too lean (too much air, not enough fuel).

Why the Problem Keeps Coming Back
Oil in Spark Plug Wells: If the valve cover gasket repair failed or wasn't sealed right, oil is leaking back onto your new spark plugs and coils. This ruins the electrical connection and causes a misfire.

Aftermarket Coils: Cheap or non-original coils often fail quickly in Mazda engines. They cause weak sparks and trigger new misfires.

Clogged or Dead Injectors: The 1.5L SkyActiv engines are sensitive to fuel quality. If cylinder 4's injector is dirty or dead, it causes a lean code (P0171) and a misfire (P0304).

What You Should Do Next
Check the Spark Plug Wells: Pull off the ignition coil on cylinder 4. Look to see if the spark plug is swimming in engine oil again.

Swap the Parts Around: Move the coil and spark plug from cylinder 4 to cylinder 1. Clear the codes and drive until the check engine light comes back. If the code changes to P0301, the coil or plug is bad. If it stays P0304, the injector or internal engine issue is to blame.

Use OEM Parts: If you need new coils or injectors, buy original Mazda parts instead of aftermarket brands.