O2 sensor questions
11-30-2012, 02:12 PM
#1
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O2 sensor questions
Have long tube Pacesetters. If I start the engine from dead cold and just let it idle the O2 sensors read rich, and just stay there. If I bring the RPM up to about 2500 and hold it there for a few seconds the O2 sensor voltages start toggling as I would expect them to. If I then let it idle they keep toggling.
BTW: When I first start it and as long as I don't rev it up the fuel trims are 128, when I rev it they start bouncing around.
Is this considered normal? What should the fuel trim be or do they jump around at steady rpm?
The sensors are AC and the heaters are working.
Can someone point me to a place where I can find some PCM files to compare to mine? I'm using TunerPro RT. Trying to get the fans working the way I want. Have a TH400 and no speed sensor. Right now they go on high as soon as the engine starts.
Al 95 Z28
BTW: When I first start it and as long as I don't rev it up the fuel trims are 128, when I rev it they start bouncing around.
Is this considered normal? What should the fuel trim be or do they jump around at steady rpm?
The sensors are AC and the heaters are working.
Can someone point me to a place where I can find some PCM files to compare to mine? I'm using TunerPro RT. Trying to get the fans working the way I want. Have a TH400 and no speed sensor. Right now they go on high as soon as the engine starts.
Al 95 Z28
11-30-2012, 04:19 PM
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Location: Hell was full so they sent me to NJ
Posts: 70,647
Re: O2 sensor questions
Sounds normal.
When you start the engine, the PCM is in "open loop".... the O2 sensors are not working and not providing feedback to the PCM. The intial reading will be about 450mV. The PCM is ignoring the sensors. The SHORT TERM fuel trims will be 128. The LONG TERM fuel corrections will be whatever is stored in PCM, based on past engine operation. If you reset the PCM by pulling power, the long terms will reset to 128.
The short terms will stay at 128 until the PCM goes into "closed loop". That means that three things have happened - 1) the O2 sensors are hot enough to start working; 2) the coolant temp indicates the engine is warming up (usually in the 120-140deg range); and 3) a timer has timed out (usually 205 seconds). When all those conditions are met, the PCM goes into closed loop, accepts the feedback from the O2 sensors, and starts toggling the A/F ratio back and forth between slightly rich and slightly lean, because that needs to happen for the cat to work.
So far sounds normal. Not sure how "revving it" comes into the picture. That might indicate the heaters are not working, and the extra load produces enough heat to push the temperature of the sensors beyond the threshold for closed loop.
When you start the engine cold, the PCM richens the A/F mixture for cold start, just like the choke on a carb. That would explain the rich O2 readings on startup. It would also indicate you don't have a working AIR pump, since if it was working, the O2 sensors would show 450mV at startup, and as they heated up, they would drop slowly to 0xxmV because of the air being blown into the exhaust by the AIR pump. Only when the PCM enters closed loop will the sensor readings start to toggle back and forth, in response to the PCM toggling the A/F ratio.
The fans will come on at cold start if any of several trouble codes are set. If the AIR pump is not there, and has not been correctly eliminated (needs a good fuse in the circuit), it will set DTC 29 and cause the fans to come on.
A faulty VSS sets a code, but does not turn on the SES light, and does not cause the fans to run.
When you start the engine, the PCM is in "open loop".... the O2 sensors are not working and not providing feedback to the PCM. The intial reading will be about 450mV. The PCM is ignoring the sensors. The SHORT TERM fuel trims will be 128. The LONG TERM fuel corrections will be whatever is stored in PCM, based on past engine operation. If you reset the PCM by pulling power, the long terms will reset to 128.
The short terms will stay at 128 until the PCM goes into "closed loop". That means that three things have happened - 1) the O2 sensors are hot enough to start working; 2) the coolant temp indicates the engine is warming up (usually in the 120-140deg range); and 3) a timer has timed out (usually 205 seconds). When all those conditions are met, the PCM goes into closed loop, accepts the feedback from the O2 sensors, and starts toggling the A/F ratio back and forth between slightly rich and slightly lean, because that needs to happen for the cat to work.
So far sounds normal. Not sure how "revving it" comes into the picture. That might indicate the heaters are not working, and the extra load produces enough heat to push the temperature of the sensors beyond the threshold for closed loop.
When you start the engine cold, the PCM richens the A/F mixture for cold start, just like the choke on a carb. That would explain the rich O2 readings on startup. It would also indicate you don't have a working AIR pump, since if it was working, the O2 sensors would show 450mV at startup, and as they heated up, they would drop slowly to 0xxmV because of the air being blown into the exhaust by the AIR pump. Only when the PCM enters closed loop will the sensor readings start to toggle back and forth, in response to the PCM toggling the A/F ratio.
The fans will come on at cold start if any of several trouble codes are set. If the AIR pump is not there, and has not been correctly eliminated (needs a good fuse in the circuit), it will set DTC 29 and cause the fans to come on.
A faulty VSS sets a code, but does not turn on the SES light, and does not cause the fans to run.
Last edited by Injuneer; 11-30-2012 at 04:21 PM.
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