I have a P0303, when the car is started cold it is smooth as silk can barley hear it run, no miss. When it reaches a certain temp.(normal) there is a dead miss on #3 always ,like clockwork. I have done compression test, nothing under 174psi. I have moved all plugs and coils from bank to bank, still P0303. My coils are aftermarket with about 4k miles, Harness maybe, but what does temp. have to do with it. Replaced ecu, no help I'm dying a slow death. I'm ready to buy 8 new OEM coils but will that be my problem????

I'm sorry 1997 Q45 with 144k, I call it the black hole.

.....injectors maybey??

Injector's are new, I just put old coil's back on an started cold. Ran great until temp started to come up( never ran hot at anytime) miss came back, un-plugged #3 coil an it made no difference. I did notice a tapping right before the miss started, front left of engine #3 i'm sure. Is it possible to have a bad or sticking valve an have good compression check? Can you clean them somehow without removing the head? I ordered the damn coils, I'm at the point of no return!

Seafoam in engine and through brake booster no change.

.........a bent valve maybe?...if that has anything to do with it....are you using OEM plugs and packs?

I put the OEM coils back in but i'm not sure of the age, they test ok. Plugs are ngk iridium, they worked fine for 3k or miles. Even took the thing to dealership, they told me I had a misfire on #3, an to buy new coils, huge help. I guess i'll pull valve cover an look. This miss started on my way home from paint shop, she looks good but......

thats a good start, pull the valve covers and see whats going on, thats also a good time to pull and clean the valves

Start with the ignition. unlikely to be a bad valve.
ignition components FREQUENTLY change characteristics with temperature.
NGK good, iridium, not so much so (it is iridium/copper) . research thoroughly. no one has developed a superior plug to the NGK double platinum for this engine in over 25 years.
coil packs likely. I replaced all 8 to cure an issue similar to what you describe.

Agree with 3q Jay.The factory double platinum plugs are ideal for this engine,and last quite a long time before showing any performance issues.......currently have 110,000 miles on the current set,and went 130,000 miles on the previous set all without any performance issues.Coil packs and their harness are most likely causes to your issues.

Thank to all, when I install new coils I will swap plugs also. I will up-date.

Found this interesting.......

Q had a long standing problem with a misfire. I got lots of suggestions on
resolving this online in multiple forums and from Infiniti mechanics. Worked
through them all. In the end my diagnosis pointed to coil(s). But, diagnostic codes
couldn't pinpoint the cylinder(s) misfiring, so it was trial and error swapping with
a new coil trying to find the bad coil(s). Then I found out the OEM coils had been
superseded with a new part number because of an intermittent-misfire-when-hot
problem. This convinced me that I probably had multiple bad coils and my single coil
swapping approach was not going to work out. The Infiniti and Nissan mechanics I
know told me that the OEM and cheaper aftermarket coils tend to misfire when they
get hot (especially as they age), so I should stick with Hitachi coils on this car.
Dealer price was $120 each and they couldn't confirm whether they'd be Hitachi until
they were ordered and arrived--plus, I had to prepay. Found an online source
(PartsGeek.com) with the right Hitachi part number where I could order just the
bodies for $60, and reuse the original wires. At that price I gave up the coil
swapping experiment and replaced them all. Engine runs smooth again. Was hoping
this might help improve Q's MPG. It's only been 60 days of driving with all new
coils, so limited data, but mileage seems to be up to 15-16. Still well below what
Q use to get.

INF#22448-3H000
-6p000 was the factory installed part. 3H000 is the supersede.
yes, they are Hitachi.
keep in mind that a non-Nissan part may be a Hitachi 'second'.

Well,
I have been working on my misfire for a bit now. No code. But runs with less than ideal power. And this is how I fixed it.
Took the boot off of the coil. Streched the spring.
Shrink wrapped the plastic tube. And wrapped the coil in electrical tape.
Working now.
Also look for a carbon trail( looks like a pencil mark)

And also you can compression test the cylinder when hot. To see if you are losing compression.
Hope this helps.
Cheers

Hello to all who suffer a miss-fire, I changed plugs to OEM double platinum and install 8 OEM coils FROM infiniti (give them the VIN#) and my miss and my P0303 code are gone. I cannot know for sure if it was plugs (had iridum installed) or coils but my best guess is it was the coils. My problem had nothing to do with #3 cylinder which drove me insane (ohm'ed everything, installed injectors,compression check,moved coils and plugs around) If your coils have age on them, replace them and be done,let my experience help you, it's only money.