I have a couple tech issue's I'm trying to deal with here. the vehicle is a Jeep Grand Cherokee 1998 with the 5.9L.

The first question has to do with the downstream heated oxygen sensor. I have been fighting a high idle condition it seems. I had been driving around with the rear o2 sensor plugged in and zip tied up by the transfer case (threaded in a spare to block the empty hole in the cat) with no MIL and no high idle. To try it out, last night I put the O2S back in the hole and noticed the high idle was back after driving around and then parking before dinner. When I start the car it doesn't seem idle high, its after I've been driving. It did it again this morning when I pulled into work, putting it in park or kicking it in neutral its up around 1200 rpms.

Come home, zip tie the o2 back by the t-case and plug the hole. No high idle issues, and again, no MIL.

The FSM seems pretty mum about what EXACTLY the rear O2S does. Why is taking it out of the exhaust stream not setting an MIL. It seems that on my car it is causing a high idle, but not setting a code. Shouldn't I get a code for the high idle itself?

My second question has to do with Deceleration Fuel Shut Off.

When I reset my battery, it seems to do this just fine for a few days. (Let off gas on freeway, you can feel the fuel shut off, and car begins engine braking with the torque converter locked and rpm's come down a bit, the instant mpg read out can hit 99mpg).

Now it acts like it won't do this. The best I can get coasting was 41mpg today. This tells me it's still putting fuel in.

It is supposed to sense high vacuum, low load and go into this I believe. Do you know what the values on the MAP/TPS etc have to be to activate this. Why would it happen when I reset the battery and not after it has been driven for awhile.

I found this on a document titled "PCM modes of operation"

"If the vehicle is under hard deceleration with the proper rpm and closed throttle conditions, the PCM will
ignore the oxygen sensor input signal. The PCM will enter a fuel cut-off strategy in which it will not supply
a ground to the injectors. If a hard deceleration does not exist, the PCM will determine the proper injector
pulse width and continue injection."

thanks for any help!