scj

iTrader: (89)

ok, so here's my problem...


i have a 1997 z28, and it's misfiring.
it runs and drives great, the only time while driving that i can feel the misfire is when i'm at low rpm in 5-6th gear as it vibrates a little. other than that it drives fine.I know I've got a problem with my IAC valve cause the car idles at 1.5k and is throwing a code for it, so that's in need of replacing... idk if that would cause the misfire, but that's just a bonus to the problem. As far as the misfire goes i'm throwing P0300, i replaced my plugs with NGK tr55's, and replaced my spark plug wires with factory replacement wires, and the misfire's still there. none of my wires are touching anything, and nothing is arching off anything either. the thing that's really throwing me for a loop is this: when i start the motor cold, there's no misfire for at least 2-5 mins, then once it gets a little warm it starts to misfire. i can hear a popping sound through the exhaust and once the engine's warm it keeps missing, even after killed then started again.

i was going to replace the coil next, and if that doesn't fix it i was going to replace the ICM and so on with the optispark, but before i spend any more money on this thing, i want some input.

what the hell's going on? haha.
i'm clueless.

scj

iTrader: (89)

anyone?

moehorsepower

iTrader: (3)

OK, First sign you say after it warms up it starts to misfire, if this happens only when it hits closed loop then it could be O2's, do you have a scanner to view the movement?, Second what did the old plugs look like, black? Usually the coil will react with heat but it pretty much will just kill the motor, the ICM will not let the car start and of course it could be the good ole opti. Remember codes will usually not be thrown by items such as the O2's unless they are dead,

scj

iTrader: (89)

ok, then that rules out the coil and icm because the car stays started and i don't have any issues with it not starting any time.

as far as the scanner to view the movement, what do you mean?
an OBDII scanner?

as far as the OBDII scanner goes i've gotten it scanned, and the codes it's throwing are
P0300-multiple cylinder misfire.
p0400-EGR malfunction
P1657-IAC valve Bad.

as far as the old plugs went they looked pretty worn out, and black.
what's this mean?

after the plug and wire replacement the car seems to run better, but still has the miss after warm up.

hmmm.

ALEXZ28

my brothers camaro was doing that with bad 02 sensors. id try to replace them see what happens.

scj

iTrader: (89)

well i was wanting to delete my cat to begin with, how would i go about doing this without throwing off the o2 sensors?

could i unplug them and short them out, so the computer thinks they're reading right?

i'm completely lost, and i'm really fed up with trying to fix this car. haha.

i do have a cut-out after the cat, could this cause anything?

moehorsepower

iTrader: (3)

What I meant by scanning the O2's is looking at the movement of the millivolts, they should be jumping all around, you say that the plugs look black, So now I am inclined to say that it is leading to your O2s's. When it warms up it goes into closed loop which the fueling relies on feed back from the O2 sensors, does it drive rough as low speeds but fairly well at WOT? is so then it is definetly your sensors, Change them out and you should be OK, but you will probably have to change the plugs again, NEVER clean fuel fouled plugs even it they were only in for a short while, Not a good idea...

sdm1234

iTrader: (8)

I had this very problem with my car and it ended up to be the EGR valve.

scj

iTrader: (89)

well i'm getting a code for the egr valve, but i was just thinking that the reason it was throwing the code was because of the misfire.

as far as the o2's go, i have all stock exhaust except for a cutout right under the reat seats, would this cause anything extra? the cutout seems to be putting out alot of black ****, not smoke, just **** on the bottom of the car, is this normal exhaust **** or what?

i was thinking of cutting out the cat though, how would i go about doing so with the o2 sensors reading that it was lean?

ss.slp.ls1

iTrader: (36)

Seems like you have good troubleshooting skills, you are replacing parts in the correct order.
A coil is cheap, if it's got some decent mileage, might as well throw a new one on.
If you have a code for the EGR valve, I would remove it and clean it thoroughly and put it back on.
Like someone else mentioned, it sounds like you have an open loop/closed loop issue. That would make me suspect the MAF or the o2's. I would replace the o2's and clean the MAF.
Another thing to consider is cold/hot issue. I would perform the ICM cooling mod at the same time as you replace your o2's and see if your symptoms disappear.

Breathing Fire

For your post cat sensor question you have to either delete the codes in the computer or buy an o2 sim that fools the computer into thinking the cat is still there. Done both and both work well.

sdm1234

iTrader: (8)

Usually, it's the other way around.

scj

iTrader: (89)

regarding the coil, i'm pretty sure it's working good only like 50,104 miles on the motor, and i don't have any problems with it starting, which leads me to rule the coil out of the suspected problem... although i figured doing the icm cooling mod wouldn't hurt anything just in case.

as far as the o2's go they have suspected 148k on them, which is probably my problem, but which one's should i replace, there are four. two before the cat, and two after the cat, should i replace them all or just the two after the cat? they're 50-80 bucks a pop.

and regarding the MAF, it's brand new. i just replaced it last week, so that can't be the problem.
and last but not least, how do i go about removing and cleaning the EGR valve?

how do i scan for movements of the millivolts for the o2's
and as far as driving goes, the car does drive rough at low speeds around town, but a WOT it drives fine.
again, which o2's should i change out, all of the four, or the ones after the cat?

how do i go about deleting the codes in the computer and if not that, where can i get an o2 simulator for 1997 z28?

ss.slp.ls1

iTrader: (36)

You should definitely change your o2's if they have that much mileage. You need two AC Delco o2 sensors (flat 4 prong style) for the front two oxygen sensors. The front two are the ones that actually affect performance. The rear two only monitor the catalytic converters.

As far as monitoring them goes, you need a scanner that can read real time data. Once in closed loop, the o2's should be oscillating between 0-900mV consistently.

With 50k on the coil, and taking into account that they are only like 30 bucks or whatever, I'd replace it.

You can tune out your codes (emissions codes and other unnecessary codes) with a mail order tune from MadTuner.com. It's also beneficial to gain some horsepower and make your car run better.

scj

iTrader: (89)

okay, so here's the deal...

i'm only pulling codes.
p0400-egr
p0300-misfire.

if it was an o2 sensor causing this issue, wouldn't it be throwing a code?

moehorsepower

iTrader: (3)

[QUOTE=scj;11526685]
can for movements of the millivolts for the o2's
and as far as driving goes, the car does drive rough at low speeds around town, but a WOT it drives fine.
again, which o2's should i change out, all of the four, or the ones after the cat?


Like I said, Please read the post of info, it at low speeds it runs rough and at WOT its fine then is is most definetly your two "FRONT" O2's, Change the front ones, replace the plugs and you should be done...