If your ECT is bad, you'll be in open loop all the time, and the computer will ignore lambda sensor readings. Think of the ECT as a switch that tells the computer "OK, start reading the sensors and tweak fuel trim now!" once it gets hot enough. The computer doesn't go into closed loop until the 02 sensor reaches 600 degrees F and the ECT hits 160 or so. Your car will run rich when you're in open loop, but it shouldn't screw with your idle I don't belive, UNLESS you're already running too rich from something else (restricted return line/bad FPR/leaking injector ETC.) OR the ECT isn't reading anything at all. Usually the car will stall when cold when the ECT is shot.
Not to insult anyone, but I wouldn't have replaced the 02 right away if I were you. You should TEST something before you replace it. Backprobe the 02 sensor with a multimeter on a 0-1v scale. Spray some starting fluid or propane into the intake and see if the voltage jumps up. It should go up to about 800-900mv. If it passes this test, then try unplugging a vacuum line and see if the voltage drops to below 450mv. It should. If it passes both tests, chances are you have a good working 02 sensor that is responding to oxygen readings well. Remember, the HIGHER the voltage, the more RICH you are running and vise versa. These are actual tests people do in the automotive field.
After these tests, you need to check your fuel pressure, then check your manifold vacuum with a vacuum guage. Check your spark plugs. Find out if you're rich or lean and we can go from there. Rough idle problems are rarely caused by a bad 02 sensor ESPECIALLY if it's in open loop (basically, when the car is cold)!
OBDII systems use at least 2 02 sensors. The first one is for fuel trim, and the second one measures catalytic converter efficiency. Don't think many OBDI cars have 2 02 sensors? Not sure on this one...
Hope this helps!
Last edited by and0pand0; 04-24-2006 at 11:43 PM.