Q: Rough idle, p0300 code, running rich

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Recently rebuilt engine, installed a larger cam shaft. Dyno tuned ran great, 600 miles on engine and now barely runs. Started when it stalled on me after I let off the accelerator going 30mph. Low on power but runs, stalls when accelerator is released and has rough idle. Replaced MAF sensor, long term fuel trims are -5% to -20%. Misfire count really high on cylinder #6. Checked plug, not fouled. Switched coil with no change. Pulled fuel rails and no injector is leaking with key in run. O2 sensor appears to be working. Evap purge solenoid not leaking. Fuel trims respond when vacuum leak is introduced to system, so it's running rich. Any ideas? Failing MAP sensor? High fuel pressure from faulty pressure regulator? EGR system is deleted, catalytic converter is deleted, AFM deleted mechanically and electrically. Any idea what the problem might be?

My car has 155000 miles.
My car has an automatic transmission.

Hi there. Let me refer you to this article explaining what causes the P-0300 OBD-II error code and possible solutions to resolve the issue. I believe reviewing this article will be very beneficial to you. Thanks.

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