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I replaced the exhaust and reused the existing catalytic converter, which was more work than it was worth. I had to take it out to get the heat shield and resonator pipe off so I had to disconnect the O2 sensor on it and used a torch to get the bolts off.
Now the exhaust sounds great, but after about 1 min of idle the car revs up and down when in park or neutral unless the AC is on and the fan turned up. In drive it does not rev. It revs between 1000-2000 up and down about 8 times, then stops for awhile then starts the revving cycle again. It does the revving thing hot or cold but takes about 1 min for it jump into the rev cycle. vroom vroom vroom
I have no diagnostic computer but no check engine light came on. I did however unplug the map sensor and revving stopped but of course the car idles at around 1500 without the map sensor.
Also found a crack in the radiator so I'll be replacing that tomorrow but the crack has been there most likely since I bought the car because it hasn't been used/inspected yet and it was in a minor fender bender. The car never did this until I replaced that exhaust.
So.. could this be the O2 sensor? Does the removal of the map sensor stopping the revs suggest anything? I just find it hard to believe that the car worked fine and I took off the catalytic converter and all of a sudden the next start it revs up and down.
So I was reading about O2 sensors and how they can get deposits on them. So I revved the engine up to 7000 rpm and held it there until it smoked off the glue and pieces of label from the new exhaust parts.
Let off the pedal and the RPM dropped down so far I thought it had stalled at first from being used to the up and down 1000-2000 rpm over and over. RPM dropped to about 800 and the erratic idle is gone. Let it idle for awhile and ran it around for about an hour and still no problems.
Now.. why it would go from working to not working just from being taken off to install the new exhaust I dunno. All I can guess is that when we used the torch to get the old bolts off a little bit of carbon from the torch flame fouled it up. OR perhaps just something else 'broke loose' at high rpm and started working. Probably not a common fix, but it worked great for me so I figured I'd pass it on.