Dudeman41

Ok so I have a 94 z28 and I have recently been having a rough idle and hesitation problem. It seems to be running decent for the first 5-15 minutes but then it gets worse. After it warms up it really seems to hesitate and stumble under acceleration. It acts like its running on 6 cylinders but all 8 are still firing. I have a scan tool and I do not have any codes showing. I really thought it was the O2 sensors but Im not sure. It has been running rich for some time now. It will spit out some black smoke only sometimes especially if its been sitting for awhile. When I pulled out the O2 sensors they were completly black. I really dont know what is causing it to run rich. When I hook up the scan tool the readings on the 02 sensors are:

Left 02 on cold start- started at about 400mv
Right 02 on cold start- averaged anywhere from 250-500 mv

Left 02 then went down to about 60 and eventually averaged out around 150mv

Right 02 then went down to about 4mv for awhile then jumped all the way up to 700+mv and then it was just all over the place and would not stay consistent.

I dont know much about 02 readings but the right one just doesnt seem to be right.

I know it could be a number of things wrong so if anyone has some ideas I would greatly appreciate it.
Thanks

tricky69

sounds like the same problem i was having with mine would run like a beast for the first 10mins or so and then when it got to operating temperature i would get a mis when i would give it slight gas, but if i went wot it would not mis. replaced my opti and all is well now. hope this helps.

bw_hunter

First thing to try is to switch sides with the sensors. If the problems switches sides, it's the sensors. If it doesn't, well, it probably isn't.

Starting out with odd readings isn't surprising. They (the O2 sensors) don't work until they warm up. After they warm you can expect them to fluctuate up and down as the computer adjusts the fuel.

You described the response of the right side senor, what was the left doing at the time? Did it stay at 150mV the entire time? That doesn't seem correct either....

Anyway, switch 'em and tell us what you find.

BTW, what brand are they? Did you check the plugs?

henryz28

I have about the same mods as you and my car was doing the same thing. Only thing is my car would ping at wot at about 6000rpm. My problem turned out to be the opti.

Dudeman41

Ya the left side was averaging around 150-300mv i think I'm not sure what its supposed to be. I will switch them around tommorow and see what that does. They are Bosch O2 sensors and they are about 2 years old. I have only pulled 2 plugs. One was pretty black and the other looked kinda oily which i know isnt a good thing. Thanks for the help

Dudeman41

Ok so I put in new O2 sensors and that definatly seemed to help. It's running like it was before but I still think it is running rich. I think thats why the 02 sensors stopped working, maybe they got clogged up.