Amenite

Hello All,

I have a 2008 350Z HR with 80200 miles on it. It is misfiring (I think?) but only when the throttle is pressed lightly. IF you depress the accelerator pedal and let the RPM rise, you can surpass that misfire (exhaust note changes, lack of power for a moment even though I know its not a misfire because I know what a misfire sounds like but this definitely isn't it), and the car purrs like a kitten. Now the weird thing is that the car starts up just fine, odes at 638-650 rpm smooth as silk.
The issue only happens when the oil pressure reaches around 18-25 psi, aka car is fully warm. it doesn't happen when the car is still warming up or oil pressure is high. For example, if I am at a stoplight and then i slowly accelerate, like how a normal civilian should drive, rpms rise to a thousand and then I hear the note change, car hesitates, jerks and then it overcomes it and all is normal again. This will happen every time I step on the gas no matter if its 60mph or 90mph. I have no mods except a magnaflow exhaust and I just had a cat and bank 2 VTC cover w/pulley changed at the dealer so maybe that? Since they didn't do a relearn after they changed it?. I also cleaned the MAFs and that didn't fix it. Going to go check the coil packs and spark plugs this weekend and clean the throttle body. Do you guys think it is the right path for trouble shooting?
Oh and another thing, when this issue started happening about 2 days after the VTC cover/Cat change, I got the P0300 code. I took it to a different dealer and the code wasn't found in the system. All systems had no DTC so they just gave me my diagnostic fees back and told me if I was running some sort of tune then I should tell em and all the mods, then only they will look into the problem. Now the MIL hasn't turned on since. I regularly (daily) check the ECU for codes but nothing. Yesterday I found the P0300 code as "Pending" so I guess it hadn't happened that many times for the ECU to store it in its internal memory and then it went away after 5-ish miles of driving.
The fuel trims are fine. LTs jog between 5 and 9.8 and STs between 1 and 3. Last night I was going up and down the street from red light to red light doing 0-80 and then stop to 0, and when I got back home I checked the ECU with an OBD II monitor and the ST fuel trims were high, between 7 and 10 (negative and positive values) but the LTs were fine.
Also, I have always done an oil change at 2000 miles with Mobil 1 Full synthetic 5w-30, if that relevant.

If you can, please guide me guys. parents are already advising me to get another car since the 350 is giving so much trouble but I wanna be a rebel

SQuaLZ

I would think it would have something to do with what was changed considering the issue happened right after you had the work done.

Typically a misfire is caused by an issue with air, fuel, or spark.

In my personal experience, my B13 was misfiring quite similar to what you are explaining. It idled fine, and was fine at full throttle, but at partial throttle it was misfire like crazy. Turned out one of the injectors went bad.

How did you clean the MAF? Certain sprays are not safe on the MAFs and can ruin the sensor.

Amenite

That is my thinking as well. Something that has to do with the two things that were changed. I got the new Cat checked at a renowned exhaust shop around me (they build stuff for SEMA and what not) and it is fine they say. Also checked for exhaust leaks and nothing. Only thing I am sure is that a releran wasn't performed. Exhaust valve timing cover works properly as well as the VVT solenoid. I guess I'll add the injectors to my self diagnostic as well
Also, isn't a misfire supposed to have a different tone to it? like a tik tik tik ? or am I confusing that with a knock?

Do you think taking the car to a dyno and get data pullouts would help? like a compression test/power balance or leak test logs?

I used the CRC MAF cleaner spray. Carefully removed the MAFs and then sprayed it on and let it dry and put em back on. No CEL so I am assuming its fine.

SQuaLZ

The weirdest thing to me is that you dont have a CEL.

shadow1384

Make sure there are no leaks in the air intake line between the MAF sensor and the throttle body. Loose clamps or cracks in the air intake snorkel can cause some amazing ghost problems that only happen at certain load/ RPM ranges. Also every time you clear the codes from a PCM the computer also goes back to "factory base values" and has to relearn every time how your particular car is tuned/set up. Check fuel pressure as excessively high or low pressure can cause issues and cause random miss fires. And lastly take the car to a shop that has a gas analyzer to tell you what the fuel mixture is at ide, off idle, mid range and high RPM.