P0300 Random Misfire after engine rebuild
 

Last October I blew out the bottom end on my 2003 350Z...a couple of exploded pistons, broken rods, holes through the block, crushed valves, etc...Essentially total carnage.

Anyway, I managed to get a used block, rotating assy, and cylinder heads to rebuild my motor. When I finished rebuilding it the motor fired up right away and just from listening to it seems to be running pretty smoothly. No ugly noises or vibrations at all.

Idling it in the driveway gave me a couple of P0300 random misfire codes, very sporadically and they'd go away as soon as they came up. Then I took it for a short drive down the road and under load the engine bogged down and the CEL was flashing. I checked the codes, and P0300 was back.

After blowing the motor the car sat outside for 6+ months with less than 1/4 tank of fuel. My leading theory on the cause of my P0300 misfire code is that the gas in the tank has oxidized and is no longer good enough for a high compression motor like the VQ35DE.

I've read other causes of this could be bad coil packs, improper compression, bad plugs, etc. I don't think those are that likely since I would think that those issues would give me a specific cylinder misfire code.

Thoughts?

Think you're on the right track - I would try to get what's left in your tank out and fill it up with fresh fuel before I did anything else.

So I finally got around to draining all the bad gas last night and replacing with fresh 93 octane. I threw some fuel system cleaner in there for good measure as well.

This didn't fix my problem with the P0300 misfire, but I think I've got the cause of the problem identified now. My latest theory is that plugged cats are causing too much backpressure, and resulting in misfires. The engine stumbles a little bit when rev'ed from idle to about 1700 rpm, then runs smoothly after that.

I also found bits of catalytic converter bits on the driveway behind the tail pipes...so they must be plugged and breaking up. It makes sense considering their history. I blew the motor last fall, and in the process a bunch of oil / shrapnel from the engine grenading itself blew out the exhaust ports and into the cats. Didn't think anything of it at the time, but that kind of debris in the cats would most likely destroy them.

Test pipes on the way...we'll see if that resolves the issue.

Does sound like clogged cats to me (which probably happened before you blew the old motor...clogged cats will wreak havok on your pistons/rods). My other guess would be coil packs. At idle unplug them one at a time and when you get to one that doesn't affect the idle you have your bad one.

I had thought about coil packs being a possibility, but I moved that down towards the bottom of my list of ideas because I'm only getting the P0300 code. I'm not getting any of the cylinder specific P0301 - P0306 codes that I would expect to see if I had a bad coil pack on 1 cylinder.

The problem also seems to be more prevalent above 2k RPM. It'll idle and run at low revs without throwing codes, but once I hit ~2k it starts throwing the P0300.

I got P0300 from a bad coil pack -- had it been cylinder specific codes I wouldn't have gone through the trouble of unplugging each pack. It's such a simple test; no point in putting it at the bottom of the list because of a hunch.

Hooked a PC up to the OBDII port last night to read data from the engine sensors, and it looks like there's a bad upstream o2 sensor on one bank. It wasn't responding the way it should, and the fuel trim on that bank was constantly being adjusted by the ECU in response to the bad sensor data, which is causing it to run overly rich on one bank. Hopefully replacing the faulty sensor will clear this issue up.

I had a P0300 code. Turned out to be a faulty fuel injector.

it's not responding the way it should because of the clogged cat

Check you haven't switch around the 2 coil pack connectors on the driver side of engine closest to the firewall. The length of the two connectors are close to the same. Easy to mix up, I did it when I rebuilt my engine ran like crap.

If you search in the FI forum, you will find that many built 350Z with FI got that code. No one figured out why and sometimes it just went away on it owns. I also got that code and had to flash the edu to disable the code so that I can get pass inspection.

I swapped those connectors last night, and sure enough, problem solved. I remember when I was wiring those coil packs back up that it looked like the two wires for #4 and #6 naturally wanted to cross over one another but I thought that it made more sense for the wires to be sequential, so that's how I plugged them in...turns out I was wrong. Thanks shiftdrift06!

No problem:thumbup:

genius
 

You sir have brought me an unfathomable amount of joy by commenting your all knowing knowledge. i could not find the damn misfire for the life of me and that never crossed my mind. ILOVEYOU:thumbup:

my 2007 has a random misfire P0300,also P0304,i just fixed the P0024 and the P0014=$1300 now this issue