2007 350Z Enthusiast (bought new) and 125,000 miles of road racing miles. Only mods are Fujita F5 CAI and Stillen True Dual Catback, among several suspension and interior mods.

I had a persistent P2A03 code, so i changed out the Bank 2 sensor 1 AF O2 Sensor with an aftermarket one from Summit (NGK-25679) back in February 2019 and went on to pass an emissions test in May, then around October I had the code come back. After countless forum searches and hours of reading i decided i need to go through the ignition and vacuum systems to make sure the code wasn't due to a misfire or air leak between the MAF and A/F sensor (which it did end up being, keep reading). The plan was to replace the original spark plugs while i was in there.

So i tore it down, replaced the spark plugs (also i sealed off the valve cover spark plug tube seal leak on cylinders 1,3,5 with Permatex Ultra Black). I tested all the coils and they all checked out. I cleaned the MAF sensors and the throttle bodies and I put everything back together and fired her up.

P0300!!! so now i have a bigger problem, it's running like garbage, choppy idle and backfiring, acting like it wants to stall, revving like an angry horse every time i come to a stop. So after several pedal dances to reset the idle and having a horrible running condition i decided to replace the O2 sensor again, but this time with the OEM "Denso Exact Fit Oxygen Sensor 234-9072" from Autozone... thinking maybe it's just getting bad info from the aftermarket sensor or sensor mismatch to Bank 1.

... as a last ditch effort before replacing the O2 sensor again, and facing the reality of having to take this to the stealership to try to isolate the misfire, I decided to work on a running engine, removing coil wires one by one to see if i could isolate a specific cylinder and that's when the absolutely ABSURD happened.. I heard a faint whistling sound coming from the top of the engine - i'd never noticed it with the plastic cover on the engine and hadn't run it sans cover before, as i removed the cover as i started the tear-down. I suspected i missed something in my vacuum system check and that's when it struck me to put my finger over the two molded-in nuts on the intake plenum that hold the stupid Nissan cover onto an otherwise beautiful motor. Here's what happened.. i had to video it because it was such a benign manufacturing defect that caused all of this troubleshooting and part replacement.. basically the intake was sucking air through this insert boss on the plenum and it was throwing off the air/fuel mix to the point it was causing rich running and misfiring. I simply thread sealed the bolt that goes in the hole and voila! A perfectly running, perfectly idling 12 year old VQ35HR.

Hope this helps somebody else out there that might be at whits end contemplating an ECU or throttle body replacement. Truly a $0.05 fix for potentially VERY expensive repair.



P0300 FIX VQ35HR

P0300 FIX VQ35HR

 

Glad you got it sorted, I had the same issue and also siliconed the tops of all plenum screws. No big deal, you peel it off for access.
Word to the wise (anyone else) Idle surging is usually vacuum leak related. Gl!

 

I read your saga of all of the work that you had done to try to fix this and watched the video. So glad that you found out just what the problem was!

 

Regarding the "ABSURD" label.. I should have prefaced that I've made a career in industrial design as a mechanical design engineer and I used to run a product development and prototyping company specializing in plastic injection molding, before I realized it was easier and much less liability to sell engineered products than to engineer them. I've personally seen mold-flow issues in prototyping when dealing with insert molding, and seen hoop stress failure issues from press-in interference fasteners and these always get addressed before we would release a design for tooling and mass production. How this gets through Nissan engineering knowing that the failure would cause issues with engine operation is what i would consider ABSURD.

.. that being said, it lasted 12 years and 120,000+ miles under hard use so i suppose we should grant Nissan a reprieve. I just hope somebody with these two codes one day sees this and knows it's a failure mode before they go through the whole litany of system checks.

 

That was prolly the funniest most ridiculous fix I've seen. Thanks for a good laugh and I do mean gooood laugh.

 

Hadn't seen this thread before. Great SIMPLE solution. Stickied in Maintenance and Repair as a possible fix for P0300 that's not ignition related (and driving the trouble shooter to day drink).