does anyone know what would cause this?

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White '92 Firebird w/ 305 TBI. Open element w/ non drop base, increased fuel pressure, TBI spacer, Hedman Hedders & y-pipe, no cat, cat-back exhaust

 

just a WAG but i'd look at the o2 senser.

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-=ICON MOTORSPORTS=-

 

My firebird did this when the IAC was too high (45 at idle).

 

TAB,

Ed is correct. Unless there is sufficient exhaust flow to keep the oxygen sensor hot, the ECM will drop into open loop at idle. Mine does this all the time when at hot idle. Wick the throttle up a couple of times and the hot exhaust should produce a good temperature at the sensor for a minute or two.

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Later,
Vader
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"Make Me Bad"
Adobe Acrobat Reader 4.0

 

Or you could just buy a heated oxygen sensor and dont have this problem anymore

 

Would this cause a bad idle? also, in case I do decide to get one, where is the best place to wire the 12 volt lead to?

Thanks,
Brandon

 

A heated O2 sensor will not cause the computer to remain in closed loop during prolonged idling. I have one and it still will go to into open loop. I would have to monitor it again to see the exact amount of time it takes to go into open loop when idling, but I think it was around 2-3 minutes.

It will also do this on prolong deceleration down a long hill. This is all part of the normal operation of the ECM and there is no need to worry about it or be concerned. The engine runs leaner and I suspect it is part of the computer to conserve fuel.

FYI, it has done this when I had stock manifolds and when I installed the headers with a heated O2 sensor.

But, remember this if you are taking it in for an emission test...as the leaner condition may cause higher NOx readings. Just blip the throttle every few minutes.