new member misfire / hesitation help

I have a 1998 Sunfire GT 2.4l, 138,000 miles. I have been having a fluctuating idle problem, and slight loss of power that until just recently had NO code. i finally threw a p0300 misfire code, and am frustrated as to the problem.

i have replaced the TPS, MAP sensor, IAT sensor, and spark plugs, all within the last week...NO help.

I replaced the Idle air control valve, fuel pressure regulator, within the last 6 months.

and replaced the coil packs, O2 sensor, coil boots & plugs, cleaned the EGR valve (and replaced the gasket), within the last year.

i can't detect for a vacuum leak, and have run a few bottles of fuel injection cleaner through the system.

PROBLEM:
When i start the car and "coast" down the drive the idle fluctuates and then either "fixes" itself or coughs / sputters and dies, but i was not throwing a code. this happens when the car is cold AND sometimes when it is warm.

i also have been having an issue with hesitation / loss of power while driving...if i give it a little gas (gradual) it increases in speed. If i give it half pedal or more, it wants to bog down, then it may or may not kick in and go. this is not cool when trying to merge into traffic, or when that street light goes to yellow...

I am investigating the possibility of a bad ignition control module, clogged fuel injectors, a fuel pressure test, and i may clean out the EGR valve again, i also seem to get a lot of carbon in the carburetor area for some reason.

I also noticed that there was just a SMALL "puddle" of oil in the well of #1 spark plug when i replaced it this week...but that is near the oil fill cap, and maybe the oil change place spilled a little while filling.(hopefully NOT a head gasket)

i don't want to keep replacing parts that aren't solving the problem, just asking for any help / suggestions / ideas as to what to do, based on my symptoms

THANKS

have you gone through the list on this link?
http://www.obd-codes.com/p0300

thanks for the response & link...I did pull a P0300 code, drove the car today and it didn't fluctuate the idle at all. I did however research some more to find that my coil HOUSING may need replacement, I also picked up spark plug boots, another set of coil packs, and an air filter. I will replace the coil housing & boots and see if that helps. (I don't think my coil packs are bad)

i also found in researching, that the oil in my spark plug well may be from a leaking valve cover gasket, which in turn would foul out the spark plugs and cause my misfire.

As for the throttle response, it appears that my grommet at the throttle body is cracked...not broken off, but the added slack, might be the cause of the hesitation.

any ideas as to where i might find JUST the throttle cable grommet, instead of replacing the entire throttle cable ?

I was goung to sugest the housing. Have you cleaned the IAC?

just replaced the housing...didn't seam to make a difference...auto parts store suggested the crank position sensor, since the bogging happens when i give it half pedal or more. i think i located the sensor, but i don't know how i can even get my hands in there to get the bolt out. my repair manual shows it as quite easy access...so maybe i'm looking in the wrong spot. when looking from underneath i'm seeing the sensor to the left of the oil filter...

oh and i did replace the idle air control, and cleaned it anyway when this started happening...thanks again for responding

have you considered a clogged converter? It's on the list for P0300

i read in a blog that if you hit the cat with a hammer and it makes noise, that the insides are "bad" or coming apart. i tried that and it seems solid...no rattles. is there a way to have a muffler place check for this ? or are they just going to hit it with a hammer also ?...i also read that you can check the temperature before cat and after with a inferred thermometer, of which i don't have. I am not even really sure what this would do.

I also have heard the idea that my fuel pump might be going bad...

i am continuing to do more research, but i really appreciate your responses.

a low vacuum reading is one sign of a clogged cat, if possible unbolt the cat and see if it makes a difference

i checked with a mechanic at Farm and Fleet, who had a grand am with the same motor, and he said that if the cat was clogged, the car wouldn't even start...

I also found something interesting : when i give it 1/2 pedal, that's when i bog...but if i squash the gas, i get up and go. Very slight hesitation, but it GOES. I think the slight hesitation is just the computer realizing, hey this car wants to go fast.

i tested the TPS again today, and i don't seem to have any gaps in voltage when i "sweep" the sensor.

I am kind of leaning towards my fuel filter (which i will replace today or tomorrow) OR a problem with one my injectors. but I am really confused as to why then do i have enough power at full throttle ?

going to try a bit of sea foam through the vacuum line also today, to hopefully "melt" away any carbon on the injectors.

on another note, I noticed that on cold start ups, that the misfire is also happening, but just for a bit, then the engine smooths out...but when i give it a little gas (still in Park), it seems that my exhaust, "rumbles" or "sputters" as if there isn't a good flow under the load or pressure. almost like a back pressure thing.

update on vehicle : went to change out the fuel filter... when blowing into the NEW fuel filter, it was like blowing through a straw. after removing the OLD filter and trying this, it was like blowing through a STICK. then lumps of "particles" came out.

EVEN THOUGH i changed my filter about 2 years ago, i STILL had some blockage.

the vehicle is accelerating better...but i still have a bit of hesitation.

I also had a fuel pressure test done today, sitting about 40 - 45 psi, and relatively slow on the drop down (taped into the fuel filter, as we don't have a schrader valve on the 2.4)

going to test the coil packs with spark tester "boots" to see if they are causing my hesitation...running out of ideas, but issue IS getting better.

any other suggestions would be greatly appreciated

i have owned 3 sunfires : 03, 02, and a 96. which I currently drive. the half pedal bog is something I have had. It was as simple as fuel filter and air filter.
problem solved, however I never did have an engine code.