First Thread! Hesitation Fix...Running Cool?

Hi guys,
My first thread so don't kill me. I've got a 98 4.0. It was hesitating and backfiring through the intake on hot or cold start up for about 2 minutes, then it would run fine. Searched and searched but couldn't really find a consensus on what the problem was. I decided to replace the upstream 02 sensor. Voila! hesitation cured, and I'm hoping with the mixture right I might get a little better mileage. However, I'm noticing the temp. gauge is reading high 130's low 140's. I'm not sure but my gut says the temp is still where it should be. Is it possible the o2 sensor replacement would lower the operating temp by that much? It was running 190-210. Thanks guys.

I can't really remember if the 98 has a distributor or not? I'd check the orange wire (with it connected) at the CPS for 5 volts ( I don't have a 98 FSM, so the color may be different, but is likely the same, orange). Much less than 5 volts and it's likely something is shorting out and sucking down the common sensor (5V ) supply voltage. The 5 volt power supply is *common* to many sensors. I may be off base here, but I'm trying to think what would be common for the coolant temp sensor and the other sensors like the CPS, TPS and sync sensor (if you have one on the 98).
An O2 sensor wire cooking on the exhaust someplace (the rear is most common), would likely affect the 5 volt supply.

It may be unrelated and it's just the temp sender (or connections) thats acting up.

Thanks, I'll see if I can check the voltage. If you get a chance, could you tell me a little more specifically about how to do it? I'm pretty new with electrical stuff. Also, the 98 does have a distributor.

When you say the wire may be cooking somewhere on the rear of the exhaust, do you mean the downstream o2 sensor, or just somewhere towards the rear of the front?

What confuses me is that the temp reading was probably accurate before the o2 replacement. If a wire was cooking it seems like it would be the one i just replaced, but that wire seems ok to me. BTW, thanks again for the suggestion.

The downstream O2 sensor was a known trouble spot the wires would cook on the exhaust or get ripped out by the roots offroading. They rerouted them in the mid 90's and they seem to do better. But the holddown bolt for the downstream O2 sensor is one of those use once types, if the downstream O2 sensor has ever been changed, the mechanic may have left the holddown for the wires hanging loose and the wires are cooking on the exhaust periodically.
Many of the sensors share a common 5 volt power supply, the reference voltage before it is reduced by the sensor and sent back to the ECM. The only thing I can think of that my be common to the temp sensor and most any other sensors is possibly the 5 volt reference voltage.
I've seen a bad CPS and a Bad sync sensor suck the voltage down to around 2.5 volts or so, caused instrument malfunctions, trouble lights not working etc.
Though it usually messes with more than one instrument. I had an ASD relay wire making partial contact the other day, temp. and volt gage was acting up, the Gen, MIL, and ABS lights refused to come on.
There is likely a simpler explanation, like a temp. sensor wire or connector making an iffy connection or the sensor going out (which is fairly common).
If you stick a half paperclip into the back of the CPS connector, with it still connected, at the orange wire (likely though I don't have your year wiring diagram), with the key in run and test with a volt meter to ground it should read 5 volts +/- a volt or two.

I took a quick look. I don't see anything getting fried, but I haven't checked the voltage. Where are the temp sensors? I think there are two. Maybe I could check or replace them?

Looks like the temp problem fixed itself! Yay