Rando MisFire code #3 cylinder

2003 3400 GA with 90 K on the clock that my daughter drives. It occassionally (like once every couple of weeks) throws a random miss fire on cyl #3 code. Originally was told that it was the #3 injector. Replace the injector. Couple of weeks latter, it threw the same code again. Have been swapping stuff and clearing codes but an down to the point nothing left to swap. After replacing the injector, swapped #2 and #3 plugs, then the 2/5 and 6/3 coil packs, then the #3 and #5 plug wires. The codes have been cleared after each swap but always returns with in a week or so of average driving. Because, no matter what I do (swap) the code always points to cylinder 3. A top end cleaning was done by pontiac about 18 months ago and I have cleaned the top end with seafoam through the vaccum intake line and then the fuel system. Fuel filter has been replaced recently also.
The car drives fine, no miss at any speed, no loss of power. Not sure where to go from here.
THANKS.

First check your vacuum lines. If you search this forum, go back 2 months or so and you will find my post about P0300 problem but only when the car is in P or N. My misfires were on cylinders #3 and #4 (75% #3. 25% #4) Yes I have 3.1 and you have 3.4, but it is the same engine design just bigger pistons.
You will really have a hard time to diagnose this one, I am warning you. Do not waste too much money on parts that you assume could be faulty. Chances are those parts are all good. I had the exact same problem, my first problem that I had was p0303 solved that with changing injector #3, but still that cylinder continued to misfire slightly when in P or N. Changing fuel pressure regulator will not help just like new wires, plugs, iac, maf, coils, egr, icm, fuel filter, and I can not remember what else I tried. Let me guess your car is shaking a bit when in Park am I right? Whatever new I try works for a short period but comes back again. The only other thing that it could be is small head gasket leak, bad camshaft lobe or burned exhaoust valve. Other posabillities are: since our type of engines get very hot, without much air circulating unde the hood engine gets very hot and #3 cylinder in in the hottest spot suraunded by other cylinders and firewall. It is very possible that the injector resistance changes off specs due to high heat, or camshaft lobes are pressed on the camshaft's hollow tube and for some reason they could be sliping off. Next thing find electrical diagram for your GA, disconnect your big injector connector, engine off, test for continuity between #3 injector ground and disconnected pcm connector that contains that wire there should be continuity, try wire wigle test also. This test does not cost you anything. The second one will cost you some time and uper plenum gaskets which are $7 make sure all your injector wires between your lower intake manifold and uper plenum are all good, first visual inspection then check for continuity and wigle test. While you are there check all your injector's resistances I think they should be around 12.6 ohms check specs first because I am not sure for your car. I do have one very nice procedure to test your injectors spray patern on car with uper plenum removed and eliminating the odd ball, experienced guys will know what I am talking about but I am afraid to post it because if proper care is not taken it is very dangerous task.

Welcome to AF 03blackburb.
This realy isnt that hard to isolate.

I would look at #3 wiring harness.
These sometimes get breaks and will short out causing
a misfire.
Esspecailly since someone was in there cleaning,
could have easily damaged the wires.

I too was pleuged with this a few years ago.
Replaced all the injectors, & wiring harness and havent had any mosfires since.
As the coils, plug&wires were all in great shape.

This is a commen problem with the 99+ 3.4L's :grinyes:

Good Hunting

ok i have a very similiar problem with the #3 misfire, before i was about to pull the phenum off and check the harness and injectors, i put a meter on several spark plugs, the one in number 3 read 3 ohms all the others read 18 ohms, these are bosch platinum 2. i went to the parts store and they pulled the same plugs off the shelf the first 2 did not have any continuity at all the 3rd one read 18 ohms. i bought a set of ac platinums and the motor is running perfect, i have not had a chace to reset the codes yet or drive it for an extended period. but this is the last time that i will ever use bosch plugs