Another Dreaded PO300 Thread
 

Hello everyone, it's been years since I posted on the forums but don't know where to go next with my car. I have a variety of other threads on this topic, so mods feel free to reprimand me if I am beating a dead horse.

2011 370z sport mt, factory engine apart from K&N filters, 105,xxx miles. Routine maintenance regularly and new spark plugs ~10,000mi ago.

In January 2020 when the weather got cold, after a 3 hour drive my car started throwing a PO300 code and the ECU would raise the idle slightly otherwise driveability was fine. I the code multiple times with my OBDII reader but it would come back on soon thereafter. At that time, I also installed a new clutch and flywheel. No other codes were being shown. After about a month I took it to the local dealership where they read then turned off the code and it stayed off until January 2021 when it shows the same symptoms.

I have since had the injectors bench tested/cleaned, cleaned MAFS, done a compression test, done a smoke test, sprayed the running engine with starter fluid to find leaks and the CEL continues to come back on. I have now had the car to Nissan four separate times and the codes they pull will show either an unspecified misfire; a misfire on bank 1 or a misfire on bank 2. They also performed a "fuel system service" which is I guess something they recommended. At the same time, once they reset the CEL, they say everything seems to be running well and within a few days of leaving the dealership the light comes back on.

Anyways, as of today my car won't pass state inspection and registration is coming due so it looks like I have a nice nissan paperweight. Looking for recommendations. Otherwise the next step I guess is to start throwing parts at the car (fuel pump, 4x O2 sensors, knock sensors, cam sensors etc) or pull the head. I can also repeat a leakdown test to see if anything has changed, but don't have air at my house.

oof. I was hoping my car would just last until the 400Z came out and I could rid myself of it.

Read

Since you have an OBD scanner, you could use OBDFusion and look at the mode $06 misfire counters.
It is shown as part of OBDFusion's wonderful diagnostic report.

This might give you an idea which cylinder has the highest count and that tells you which bank.

Also look at the fuel trims.
For each bank, add the short term and long term together to get the TOTAL fuel trim. This is how much the ECU is adding/removing fuel from the fueling calculations.
Negative is reducing fuel, positive is adding fuel.
Generally acceptable TOTAL fuel trims are -15 to +15%

The next time the CEL is turned on, run the diagnostic report and it will capture the freeze frame data, and that show the conditions when the misfire happened. This is very helpful when troubleshooting!

If you want to post the full report (just blank out your VIN) I will have a look at it and give my thoughts.