P0300 Code Can't figure it out
 

So my car is throwing the dreaded P0300 code. Tried searching but nothing helped so far so trying this incase I missed something. Car is 99 Z28 with mods in sig... Sometimes when driving on the expressway around 3500-4000 rpms the ses would start flashing. If I shifted it would stop. Have a Snap-On MT2500 scanner and while using the misfire section every cylinder misfires. Sprayed for vacuum leaks, nothing. Occasionally at idle the rpms will jump from 800 to 1000rpm for a split second. Plug wires are good, and plugs are good. Rear o2's are deleted. Now the other o2 readings jump from 100-800 constantly. Could that mean both my o2's are bad. I'm going off the scanner. Sorry don't know much about using these scanners except for reading DTC's. I need to figure this out soon cause I have emissions due by the end of the month and I'm stumped!!!!

100 to 800 switching indicates the O2's are working.
What does batt voltage do when this happens?
" " fuel psi " " ' " ?
Battery, cables, grounds all verified as good??

I'll have to check that out later. I'm just starting to get into panic mode cause of emissions. It's not a dd so if it wasn't for emissions I wouldn't be rushing. That's why the 2 posts but I'm forgetting about the other one

Unplug your mass air meter and try again. At the rpm you are having issues at is where the ecm starts to use the maf 100%.. So it may be a bad mass air meter causing a massive lean condition.

Unplug the sensor, and go for a drive. It will throw a light of coarse, but it will prove the point.

Ok. Would that make a difference at idle. Cause like I said I get misfires on all cylinders at idle also

Just try it.

Will do as soon as I get home. Thanks

Won't prove much because the misfire monitor is disabled when the maf code is set. Have you cleaned the maf recently?

Yea I did that first

so unplugged the maf and no misfires so someone was right on that

anyone else?

So, get a new mass air meter. It is causing you to run lean.

I got a buddy that has an extra one I'm going to throw on and see if it fixes it. I'm replacing the alternator too since it squeals anyways

Do you actually feel/hear it misfire? Has the pcm been tuned for the mods? Have you replaced the fuel filter recently? Could be fuel pressure dropping underload.

I had this same problem recently, replaced my old stocker for an SLP MAF, no more P0300

Has been tuned for mods. Fuel filter replaced just before winter. I don't really hear the miss either

So an update....Got the maf from my buddy only to learn that I don't believe I have a factory maf since mine is bigger than his unless they were different sizes from 99, which is mine, to 02 which is his. Also leading me to believe my throttle body is bigger also. It is a 3 wire maf so i'm so unsure. If anyone can help i'd appreciate it. I can add a pic of my maf if it helps. New alternator showed up today so I'll throw that in sometime this week.

OK so pulled out my pcv system today and it looks like the y connector is weak and possibly collapsed under vacuum, is it possible this has anything to do with it too?

Nope.