The P0300 Saga

Natrix

I decided it was time to document this issue that my 370Z has had ever since I purchased it back in 2019. I hope others with the same problem find it helpful in some way. The issue has yet to be solved...

Background
I purchased the car in 2019 with 51k miles, completely stock. It had two previous owners and spent time in Tennessee and Texas. I received a good amount of service history (receipts and work orders) with the car, and it was fairly well-cared-for. However, every few months, it likes to throw a P0300 (multiple/random misfire) pending code. The previous owners hinted that this was happening to them, too. For me, it seems to always happen upon startup, particularly on cold mornings. In addition, it is stuck with a "permanent" P0300 code that never goes away. There is no other perceivable symptom; the car runs great.

Vehicle Information
2010 370Z 40th Anniversary Edition

Modifications
I bought the car to enjoy in the canyons and on the track, so as time went on and I found its weak points, I added a few things. Keep in mind that the frequency of occurrence of P0300 never seemed to change throughout this whole process. It has been a challenge every other year to pass CA smog inspection.

• Z1 Motorsports secondary fuel pump (anti-starvation) kit
• Z1 Motorsports 25-row oil cooler with thermostatic sandwich plate
• CSF triple-pass radiator & A/C condenser
• aFe Takeda Stage II cold air intake (CARB-compliant)
• HKS Hi-Power cat-back exhaust
• Z1 Motorsports silicone radiator, heater, and PCV hoses
• Many chassis/suspension mods (not likely relevant, but who knows...can post up later if needed)

Related Maintenance
See attached PDF for full list and results.

• Throttle body cleaning
• MAF sensor cleaning
• EC-14 Pedal position, throttle position, and idle air volume relearning
• 2 tanks of fuel with injector cleaner
• PCV valves replaced
• Checked/cleaned accessible grounds
• Exhaust manifold and catalytic converter gaskets replaced (in case of upstream leaks; none found) - forgot to add this one to the list
• Spark plugs replaced
• Ignition coils replaced

Next Steps

• Check fuel injector function and circuits...will have to pull off the UIM to access the connectors
• Check fuel pump circuit - Although the pumps prime every time the ignition switch is turned on and the car seems to run well (fuel trims and O2 voltages seem normal), I did modify this circuit to install the secondary fuel pump. Basically, the two pumps are running in parallel on the same circuit.
• Check for continuity and shorting on other circuits in the symptom table on EC-596
• Take to dealer for power balance test, MAF sensor test, and maybe fuel pressure check (I don't really want to mess around with gasoline in my garage)
• Compression test? Seems unlikely to be the culprit...
• Try another ECM/ECU?

Attached Files

230101 P0300 Documentation.pdf (61.7 KB, 8 views)