P0300, P0174, specific cylinder misfires - help please!
Hi Mafia, I've read about this issue for several days and can't really figure out where to proceed. I don't do a ton of my own work, so forgive the naïveté.
The other day, with no prior warning, my car started making these awful vibrations and lawnmower/helicopter-like noises when I hit the gas (no problem when foot was off the gas). I looked up the symptoms, figured it was a misfire, ordered and received an ODB2 scanner, and sure enough:
P0300
P0305
P0306
P0174
I understand the 5/6 are specific cylinder misfires, but the P0300 random misfire has me stumped. I figured it was probably my coils, since my plugs were changed about 30k miles ago, and that seems fast for them to wear out. A few days and a bit of driving later, today I swapped coils around, and it SEEMED as though the misfires were following the coils, but I scanned it again, and the P0174 is gone, but now FOUR cylinders are misfiring, and the P0300 is still there.
However, When I disconnect each coil, I can hear a change in the engine noise, which to me suggests that all the coils and plugs are actually fine (6 might be genuinely bad, hard to tell). At idle, or in gear with the clutch pedal depressed, there is a very noticeable, rhythmic knocking/tapping noise that gets louder the more coils are disconnected. When I get back in the car, all coils connected, and hit the gas while in neutral, the noise stops, though the car still feels like it's misfiring, with loud general engine noise and vibrations.
Does this sound like bad coils or plugs? Does the phantom P0174 suggest a vacuum leak somewhere? I bought the car 30k miles ago knowing it had a slightly cracked lower intake boot, which I never did anything about, but it seemed to be fine for 3 years. Could it be the DISA? MAF? Could it be a compression issue in the cylinders? What would cause more and more cylinders to misfire?
