I'm currently haveing issues with my 2004 G35 sedan 6mt rwd. It is currently throwing a P0300 fault code for multiple/random cylinder misfire, around 3000 rpm the ses light will begin to flash. Symptoms other than the misfire are high idle black smoke from unburnt fuel, and lastly if i am driving say around 2000 rpm and push the clutch in the idle will drop to 1000 rpm bounce back up to 1500 rpm and the settle back at 1000 which is high(car used to idle around 700rpw). I have recently changed driverside valve cover (leaking tube gaskets) intake gasket and center plenum gasket and all 6 spark plugs (used NGK). Had the car diagnosed and is saying that the misfire is coming from the number 6 cylinder so that coil has been replaced and still no difference in how the car runs. I am new to the nissan/infiniti world and have no ideas of where to look next any ideas would help. Thanks, Sam

 

Did you check the gap on your spark plug? What spark plugs did you get - models would be helpful. If they're the platinum plugs like recommended in the manual (PLFR4A-11, PLFR5A-11, or PLFR6A-11) you should be gapping them to 0.044". They are supposed to come pre-gapped (to 11mm or 0.433 inches, as is shown by the model number) but sometimes when they're being moved the gap gets adjusted.

That would be the first thing I'd check. Secondly, remove your engine cover and intake and check to make sure you have the cylinders corrected in the right firing order! You may not think you could actually mess this up but when I changed my spark plugs I did switch 4 and 6 on accident as I was talking to a friend of mine. Started it up and it immediately felt rough and went ahead to switch them.

If you look at the harnesses going to the ignition coils, there will be two wires with the same color and the wire that is on top (when the harness is connected to the coil) should be different on each one. This is the ignition signal wire going to your ECU. If everything is wired up correctly, cylinders 4 and 6 should have the same color wire, but 6 should have a red tracer and 4 should not. If you see a tracer on the harness connected to 4, switch them. Because of the way the connections come off of the main harness, it can be easy to switch the two if you're not paying much attention.

Lastly - try not to drive your car much in this condition. Misfires and can be detrimental to the life of your engine. Try to diagnose it before you continue driving.

 

Thank you for the response. I know for a fact coils are correctly connected (my harness is numbered did notice that 4 and 6 could be easy to mix up) plus the car ran fine one day parked it and then it acted up the next day. however i did not check the gap on the plugs, but like i said it ran fine for a week or two after plug change.

 

Any symptoms leading up to this or was there anything different about the day it was parked before it had this problem?

 

make sure the pins and connectors are in good shape. a p0300 is a multi-cylinder misfire so if you KNOW #6 is missing, there has to be another one or two gone too. double check the plugs. oem pn? gap? you replaced a coil.....aftermarket, oem, used?

 

Try putting in an old plug or try swapping out the #6 for the #4

 

Also, is it P0300 you're getting or P0306? If it was a misfire on just cylinder 6 your ECU should be throwing a P0306. P0300 is multiple cylinder misfire, meaning it could be occurring on any of the cylinders, or multiple. If that's the cause, I would definitely look at your plenum gasket and throttle body as well. Could have a plenum leak from the gasket not being seated correctly causing improper A/F ratios. Make sure you properly reset the ECU to relearn proper A/F ratios. Playing with the plenum and throttle bodies on these cars is a pain, they will act up on you.

The rest, listen to the_coupe. Give us the model number of your new plugs, pull them and check the gap on them, and check the coil.

 

Car began to run rough noticed oil in number 4 and 6 spark plug tubes replaced valve cover and gasket intake gasket and plenum gasket car ran fine right after fix next day it went back to missing and running rough. I know p0300 is multiple cylinder and no its not throwing p0306 for number 6 cylinder but when i drove the vehicle hooked up to a computer and shut off each cylinder individually the misfire got worse with cylinders 1 through 5 and stayed the same when number 6 cylinder was off.

 

plugs were ngk laser platinum PLFR5A-11 coil was OEM nissan intake gaskets and VC were oem nissan valve cover base gasket was fel-pro

 

Well that's a good way to diagnose it. If it's just cylinder 6 there's definitely something going on with the coil or plug. Have you double checked again to see that there's no oil in the spark plug tube or around the seal with the new valve cover in place? Also pull that plug again to make sure that residual oil left on the threads from the previous valve cover didn't foul the new plug.

 

after replacing the valve cover i replaced the drivers side plugs again to make sure i didnt have that problem only 1000 or so miles since the first change before misfire occured. I havent checked the plug again since new valve cover i will have to do that tonight. Will touching the throttle plate or unplugging throttle body cause a misfire code or just the rough/high idle?

 

If you touch the throttle plate with the harness connected, you may actually have to replace the entire throttle body. I doubt it would cause a misfire, just a high idle caused by the plate being more open than it should be. Plenum gasket I could see causing a misfire depending on how bad it is because it is leaving an opening for air that doesn't travel through the MAF. Putting it this way, if you have a leak somewhere that will allow air to still pass through your mass air flow sensor, your ECU should compensate for the increased (or decreased) air flow and run decently well. If it's after the mass air flow sensor, then you'll run into issues because the ECU can't compensate the fuel for the increased or decreased air flow since it doesn't know.

But none of this is relevant if you're getting a cylinder 6 misfire. If it was anything generic like a plenum gasket it would be a multiple/random misfire or just general roughness.

 

I understand all of that learned alot about mass air sensors and air/intake/vacuum leaks with my last project (1.8t) i replaced center gasket only gaskets i didnt replace are the intake gaskets to the head. The idle was fine after i had plenum off however the car was back at the dealer for the coil replacement (just got it a month ago) and idle was high ever since ive gotten it back.

 

If the dealer did the coil replacement for you I believe they have a 12 month/12,000 mile warranty on parts and some other warranty on their own labor. Bring it back to them and tell them you have had a high idle and now you have a misfire, see what they say?

 

Ive had the misfire before coil replacement they GUESSED on a coil issue after diagnosis, i have yet to tell them about the high idle....needless to say im not very pleased with said dealer.