'89- stumble at 2-3k rpm

So yesterday I passed the emissions test, it was close but I passed, so the 198,000 mile 3s-fe is in pretty good tune.
Last winter the car developed a stumble or slight miss at part and heavy throttle between 2-3k after that it would smooth out. So after tinkering and trail and error, i finally discovered that when I unhooked the o2 sensor, it ran fine, so I left it.. I figured i would not be able to pass emissions like this, so I put in a new o2 figuring my old one was done. When I hooked it up, the stumble came back although not as bad. Aside from that it actually idled and pulled better after 3.5k. So my question is what could cause a miss or stumble in lower rpms at mid to heavy throttle only when the o2 sensor is hooked up?

Thanks

It sounds like your engine runs richer when you unhook the HO2 sensor (running in open-loop mode). This would indicate that whatever is causing the stumble is making your car run lean, since it is fixed by unhooking the HO2 sensor.

Running lean can be caused by many things, partially plugged injectors, poor fuel pressure from a bad pump, or plugged fuel filter, etc.

A sensor problem could also cause this, such as a dirty MAF or bad MAP or IAT sensor, ECT sensor, etc. See the post in the FAQ about the EFI feedback loop and the sensors which feedback to the ECM in a OBDI engine. One of them may be out of spec.

A lean condition is what I suspected as well, I'll take a look at the post.

http://www.autoshop101.com/forms/h20.pdf
http://www.autoshop101.com/forms/h21.pdf
http://www.autoshop101.com/forms/h22.pdf
http://www.autoshop101.com/forms/h23.pdf
http://www.autoshop101.com/forms/h24.pdf
http://www.autoshop101.com/forms/h25.pdf
http://www.autoshop101.com/forms/h26.pdf
http://www.autoshop101.com/forms/h27.pdf

Thanks Brian.
Would an adjustment of the AFM vane do the trick? I only plan on getting one more winter out of the car, so I'd rather not spend a lot of time and money trying to pinpoint the exact culprit.

It might. Easy enough to try

It look simple enough to do, is the cog a ratchet type system if I turn CCW to richen up will there be a prong or something I have to release?

See the post in the FAQ thread.

My 88 Camry had a hesitation around that rpm and stumbles on acceleration once in awhile. I've replaced the coil in the distributor and problem went away. The coil looks ok from the outside but when removed there were one big crack and a small hairline crack on the inside mounting face. You can check that also.

My 88 Camry had a hesitation around that rpm and stumbles on acceleration once in awhile. I've replaced the coil in the distributor and problem went away. The coil looks ok from the outside but when removed there were one big crack and a small hairline crack on the inside mounting face. You can check that also.


That very well may be it. I've leaned it out, richened it, pulled the 02, no change. I'll take a look. I blew a rear brake line Friday, so that takes priority for now.