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Old 05-09-2011, 08:18 PM   #37
the suicidal eggroll
Scooby Guru
Member#: 51961
Join Date: Jan 2004
Chapter/Region: RMIC
Location: Broomfield, CO
2005 STi
2012 WRX


Originally Posted by Black94Snake View Post
Also, O2's dont always fail in a dead rich or dean lean report... As the element begins to get fouled, and resolution degrades... the reporting slows down. SO now you have a car that is potentially having large swings in AFR (causing a idle surge), because the O2's can't properly report back what the ECM needs to see.
I know, the bottom line is that when they fail they either don't read at all, or they read the wrong AFR. If they read the wrong AFR, the ECU adds the wrong correction to hit target, and regardless of whether the o2 is reading rich or lean, the actual AFR will be thrown off because it's based off of an incorrect measurement.

Originally Posted by Black94Snake View Post
As for forceing the car into OL at idle, it should only be done to negate large cams, poor header selection, and bad O2 placement. Making the car run in an OL table might not make his problem go away, if the targetet AFR of the OL table, is where his car is running currently. Which is another reason why I asked what his idle AFR was
What? The idle AFR target will always be 14.7, whether you're in closed loop or open loop, unless the OP changed this target in his ECU. The difference is in open loop the ECU won't make fueling corrections based on the output from the front o2. If this fixes the problem, it's pretty obvious that the corrections coming from the front o2 are wrong. I never said unplugging the o2 would fix the problem, I said that it would tell him whether or not the problem was related to the o2 sensor BEFORE he goes and dumps $150 on a new one just to find out the problem was something else. I didn't suggest it as a permanent solution, I suggested it as a debugging tool, because blindly replacing a $150 sensor (as you told him to do) when you have absolutely no reason to believe it is the problem, and can determine whether or not it is the problem in 15 minutes, is just dumb.

Unplug front o2, if the situation doesn't change or gets worse, the o2 is not the problem. If the uneasy idle and hesitation is gone, the idle smooths out, and the wideband reads the target AFR (14.7), then the o2 was the problem and he should replace it. It doesn't get much simpler than that.

You can do similar debugging by unplugging the MAF sensor. It forces the car into a rudimentary speed density calculation. If this fixes the problem, you know the problem is the MAF scaling or a leak somewhere in the intake tract or manifold. If this doesn't fix the problem, then there isn't a leak and the problem is coming from the fuel system or closed loop system (o2 sensor or exhaust leak).
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Last edited by the suicidal eggroll; 05-09-2011 at 08:36 PM.
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