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Old 03-26-2009, 10:05 PM   #1
w chapman
Scooby Newbie
Member#: 75673
Join Date: Nov 2004
Angry 1999 NA 2.5L going into Open Loop at idle

On my 1999 2.5RS engine....a NA 2.5L, no mods...stock ECU.

My ECU is switching from closed loop to openloop after about 25 seconds at idle.

When the switch occurs the engine stumbles then surges up and down a few times....then settles into a (somewhat) stable open loop idle....although not nearly as smooth as the closed loop idle w/ the O2 sensor.

An OBD-II scanner shows it going from "openloop O2" to "closeloop drive".

Immediately when the I let up on the gas the O2 sensor voltage stops fluctuating and flattens at .01v or less....then in about 25 seconds it goes open loop...accompanied by a stumble and a few surges while the ECU figures it all out.

OBD-II reader shows (Absolute Throttle Position = 0.0%" at idle

and Short Term Fuel Trim also goes to 0.0% at idle.

Have installed new O2 sensor .....no joy.

Very frustrated.....need help.

Warren C.
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Old 04-01-2009, 10:38 PM   #2
w chapman
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Bump......come on ....someone HAS to have some helpful info on this


Warren C.
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Old 04-02-2009, 08:36 PM   #3
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have you tried the ECU forum? they might be able to help, too. Sorry, i couldn't give you a credible guess.
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Old 04-02-2009, 08:41 PM   #4
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2003 toyota corolla
RIP wrx, damn u bambi!


Have you reset the ecu?

Maybe take it in for a reflash (obviously stock map) and see if that works.

Other then that...you already replaced the o2 sensor so
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Old 04-03-2009, 05:25 PM   #5
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05 Corn Burner
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o2 sensors will cause this issue. I have had this happen to me before. I would start there. Try the rear sensor first.
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Old 04-03-2009, 05:50 PM   #6
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1994 Acura Integra
Milano Red


Which 02 did you replace? The fronts are whats need for open-loop....the rears are for fuel trims from what I know of Fords. Also the front and rears are different as the Fronts are "heated" and the rears not...i think its a load of crap since both are in an exhaust pipe...but start with the fronts and then the rears, i know it gets kinda spendy...my 98 contour did this to me and that solved my issue... theres my 2 cents. hope it helps
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