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)

Natrix

Related Links
Accel pedal position, throttle valve position, and idle air volume relearning walkthrough: https://www.youtube.com/watch?v=HIiaF0VaTlQ (also found on page EC-14 in FSM)

OBD II PID line-by-line walkthrough (with typical values/ranges) for VQ37VHR: https://www.youtube.com/watch?v=CXDX-89_ArU&t=706s - thanks to SonicVQ over on the MyG37 forum (I assume they are the same user here)

Natrix

Another helpful post about P0300 diagnosis from our friends over on the My350Z page: https://my350z.com/forum/maintenance...o300-code.html

Natrix

This thread in our forum documents two cases where a faulty ECM/ECU was the cause of misfiring:

P0302 Cylinder Misfire

I find it interesting because my car previously spent its life in the southern United States, notably the Houston, TX area. I wonder if the humidity could have led to undue corrosion issues with any of the electronics.

Averying

I have a P0300 code that started as a once-in-while temporary CEL, then started misfiring at idle but going away quickly after, to now misfiring noticeable above 2500rpm.

This post scares me… specifically all the things you replaced that did NOT fix the issue.

I sometimes also get a 02 sensor circuit code so I’m hoping that a new 02 sensor solves my issues. Maybe add some new 02 sensors to your to-do list?


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Natrix

Thanks for the input!

I did check resistance across t3 and t4 on both upstream O2 sensors, and they were in spec. They also output the ”correct” voltage range at idle, at least according to one example I found. I also checked continuity from each one to the ECU and checked for short circuits. I think all is well with my O2 sensors.

I would definitely recommend checking yours based on your issue, though. If you’re seeing a code pop up directly related to a component, it seems likely to be causing the issue.

On the 350Z, I have seen a case where the O2 sensor wire was not routed correctly (it was not pushed into the factory clips), and had gotten too close to the exhaust and melted. That caused a MIL and an O2 sensor code.

I hope you solve your P0300 issue without too much difficulty.

Averying

Do you have a light flywheel and solid drivetrain mounts installed? I think I’ve seen a couple posts where the added vibrations actually confuse the ecu jnto thinking there is a misfire. This seems like a very uncommon thing to happen though


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2009 Sport Touring | Platinum Graphite | GTM Stage 1 Supercharger | BC BR Coilovers | Hotchkis Swaybar | Z1 FUCAs | AME TM-02 Wheels | Z1 2-piece Front Rotors | Z1 Clutch & Flywheel | Invidia Gemini Catback | GTM Resonated Test Pipes

Natrix

The flywheel and engine mounts are stock. That is good thinking, though.

Natrix

Update
Last night and today, I pulled off the upper intake manifold and checked power and ground to each fuel injector connector, then confirmed that each injector resistance was within spec. All the harness branches, connectors, and terminals looked good as well. I picked up a stethoscope-style probe, and will try listening to the injectors once everything is put back together.

Finally, following the "rough idle" column in the EC-596 warranty symptom table, I took a look at the charging system (FSM CHG-21 and -22). I opened up the IPDM and the other engine room fuse box and checked each fuse. All fusible links on the battery positive coupler are intact, and both terminals are tightly connected. I checked continuity between IPDM connector E7 t76 and alternator connector t4. Again, all the wiring, connectors, and terminals look fine. There is no sign of corrosion or other damage anywhere, as far as I can tell.

Natrix

Link to another thread related to P0300 on cold starts:

P0300 cold mornings?

One person there reported a failed bearing related to oil system failure...scary!

Others having the same problem suspect cold air intakes (someone with the issue had Stillen Gen 3 intakes). I have doubts about this being the cause in my case, as the issue existed before installing the cold air intake system.

Natrix

Further down the rabbit hole...another thread linked in one I already posted. This one involves Nissan buying back someone's Z because they couldn't figure it out, Nissan engineers adding ground straps, and people replacing all sorts of sensors and the usual parts (coils, plugs).

Ignition Coils Misfiring anyone?

Coincidentally (?), at least two people there had 40th Anniversary models like mine.

2011 Nismo#91

Z1 Motorsports secondary fuel pump (anti-starvation) kit

I'd check anything related to the installation of that kit. Could be interfering with the fuel flow.

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Natrix

Thanks for the suggestion. I will take a look once the fuel level is lower (I just filled up recently, and it's at about 90% full).

Natrix

This person replaced the flywheel, clutch, slave cylinder, and CKP (crankshaft position sensor), and claims the issue was resolved for them:

Misfire Code p0300

Given that my car is almost certainly on its original clutch and flywheel, and the previous owner and I experienced the problem when the car was stock (they replaced all 6 ignition coils, too), this made me curious.

Zooming out for a moment, all of this seems generally limited to 2009-2012 Zs, and it makes me wonder if Nissan built them with a substandard part that was later improved through QC. It could also be something that happened occasionally during shipping or assembly before Nissan implemented process improvements or better packaging.

Natrix

Update
I decided none of the other items in the rough idle symptom table were likely causes, so I put the UIM back on with a new gasket. Before installing, I carefully cleaned the LIM surface with a razor blade to ensure a good seal. I followed the bolt tightening order prescribed by the FSM. I don't have a torque wrench that goes down to 8 ft.-lbs., so I went by feel...

...Put the ECM connectors back in, glove box back in, plugged in IPDM and alternator connectors, IAT connector, PCV hoses, throttle bodies, and bolted hard vacuum line brackets back onto the UIM. Checked everything a couple more times and plugged in the OBD scanner before firing it up.

Everything was fine until it tried to drop idle speed below 800 rpm...threw a P2101 (bank1 throttle control actuator). I was monitoring OBD II live data up until then, and all parameters were normal (IAT, timing, MAP, etc. all good).

I remembered seeing something in the FSM about running throttle position and idle learning after disconnecting TB connectors, and forgot that unplugging the ECU probably counts as doing that. ECT was above 158F, so I turned off the car and performed the throttle/APP/idle relearn sequence. However, it still had the P2101 code after doing that. I'll have to look into it some more.

Question: should the Z throttle bodies make a rattling/springy sound when gently tapped? One of mine has done that since I got the car, but no codes ever appeared until today. The one that rattles is coincidentally on bank 1(