I'm fussing around with a 91 L98 that runs good, but not perfect. It has a slight misfire when warm. Sometimes at idle, sometimes when holding the throttle open a hair (~1000 RPM). I tried unplugging the O2 sensor to see if that cured the misfire, and while it might have, there was no check engine light with the O2 sensor unplugged. This didn't make sense to me as normally any sensor unhooked causes an SES light.

Any input on this, or while I'm at it, the warm misfire?

 

Yea, it should definitely throw a code after it's been running a while.

 

O2 is unplugged, I let the car idle 15 minutes, got up to temp, fan cam on, fan went off, I drove around for a while, engine is as hot as it's going to get. Still no SES light.

The light works. I get a light with the EST unhooked, I got a light with the knock sensor unhooked, and it gives me a quick little blink on key-on.

What's up with this? Car not going into closed loop? Wire grounded out somewhere?

 

I lost my o2 sensor ground at the ecm a while ago. With no ground for it, its like the o2 sensor is unplugged. My ses light wouldnt come on for a while and sometimes not at all. Had to be driving it for the light to come on. The ses light would come on for a while then go out, but it only did it when I was throttling up and down. Found out I lost the ground for the o2 sensor at the ecm, fixed the ground and the ses light hasnt come back on since.

 

Sounds like the O2 sensor is bad? But it should still throw a code 13. Mine did when my three-wire O2 sensor stopped working due to an open power wire to the heating element.