My 95 Z28 started sputtering/misfiring and idling rough yesterday. Also started what I believe fuel in the exhaust like I'm running rich. Pulled the plugs on the drivers side and they're all black. Got 2 off the passenger's side before I called it quits for the day, and those 2 are gray and looking good.
I've done a bunch of searching, haven't found a post with the same findings with regard to the plugs. Running rich seems most commonly caused by exhaust leaks before the o2 sensors. My father doesn't think its that - says it would have to be pretty big. Another common cause seems to be loss of vacuum pressure to the fuel pressure regulator. But, all the plugs would be black then right?
I'm hoping that it's a bad o2 sensor. How would I check that? I don't have scanning software.
What would cause black plugs on one side?
Also, preliminary inspection of the wires is that they seem alright. Thanks in advance for any input.

 

The question is, which came first.... the misfire or the black plugs? A misfire will cause that bank of the engine to run rich. So will a faulty O2 sensor or wire on that side. And an exhaust leak doesn't have to be very big. The fast moving exhaust gasses in the primaries will "educt" air into the exhaust stream. It takes very little air in the exhaust to screw up the O2 sensor readings.

Swap the O2 sensors side to side, and see if the problem follows the sensor. That would confirm a faulty sensor.

Might be time to pick up the scan software and a cable, and start learning how to read the data.

 

Thank you very much Injuneer.

 

i have something close to this happening to mine only the plugs on cyls# 1, 2, 3, and 4 look fine and the plugs on cyls# 5, 6, 7, and 8 are black. any idea as to what would cause this? i have pictures of the plugs if needed? sorry to thread jack lol just figured this my help me and the op out. thanks

 

Are you using the conventional numbering system of 1/3/5/7 on the driver's side?

 

yes sir, 1 and 3 looking good with 5 and 7 looking black on the drivers side an 2 and 4 looking good with 6 and 8 looking black on the pass side. car seems to run good at wot. hell i made the numbers in my sig with the plugs looking like this BUT it is one hell of a job to drive when its cold. the o2 sensor reading stay above 980 till it warms up. i have a scanmaster thats how im seeing the o2 readings and it only goes up to 999 on the scale an till the car warms up its pretty much maxed on 999 while im driving. once it warms up its a lil easier to drive but while cruising i can hear/feel it missing. but like i said this doesnt happen while under wot. any idea of what it is?

 

Have you checked the vacuum compensation line for the fuel pressure regulator? If the FPR diaphragm leaks, the fuel ends up in the back of the manifold.

 

the line running from the fpr to the intake mani? it is hooked up unless it is leaking... i did have to replace the fuel pressure regulator not to long ago but im pretty sure its not leaking as i cant smell any gas or anything from under the hood...

 

Why did you have to replace the FPR? Have you monitored fuel pressure right after the engine is shut off?

 

I guess pulling the vacuum line off the side of the intake manifold, and checking it for wet fuel is out of the question?

 

not to sure what happened with it but under wot the fuel pressure would drop below 25psi so i put a new fuel pump in, didnt fix it so i replace the fpr and that fixed it. not to sure what was wrong with the fpr but its good now. i stays between about 42-48psi now with no problems at wot. as for the pressure after it shuts off im not to sure, what should it be at with the motor off?

 

oh no no i can do that, i didnt know that what you meant for me to do...

 

so just a update on my case, i ended up checking some things out and found that 3 of the 4 bolts to hold my throttle body to the intake manifold were so loose that i could almost pull them out by hand!!! well tightened them up and now the plugs are lookin good except the number 8 plug it looks kinda orangeish??? but man the car drives SO MUCH BETTER NOW.