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Old 02-01-2006, 10:01 PM   #1
mr. ed
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Default Open-Closed Loop Question

I had my mechanic scan my car today. He did a very thorough job, spending at least 20 minutes in the process. The vehicle is a 2000 SL2 with approx. 95k miles. All parameters checked out very good but there was something he wasn't too sure about. Therefore I ask you gurus on the Forum to enlighten him (us). The question is:

When the car is idling, the scan tool shows that the system is in open loop. When the RPM's are raised to about 3,000 the computer shows that the system is going into closed loop. Upon starting to come back down to idle, it's open loop again. My mechanic feels this may be normal for the Saturn (he works on all brands), but he isn't sure. He thinks it should be in closed loop at much lower RPM's. What do you think?

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Old 02-01-2006, 10:24 PM   #2
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Default Re: Open-Closed Loop Question

If the car is STATIONARY, then YES it is normal.

If the car is MOVING, then no it is not.

(The PCM software does not have fuel mixture etc... built into it for engine speeds of over ~1500 rpm when the vehicle is STATIONARY, so it goes to closed loop---best guess)

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Old 02-02-2006, 12:44 AM   #3
mjo
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Default Re: Open-Closed Loop Question

You must mean the scan tool showed closed loop at idle and open loop above a certain maximum RPM.

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Old 02-02-2006, 11:08 AM   #4
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Default Re: Open-Closed Loop Question

May I assume that the above responses are relative to OBD-II systems
For my understanding, relative to an OBD-1 system, the above reponses do not apply.

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Old 02-02-2006, 01:16 PM   #5
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Default Re: Open-Closed Loop Question

For a car to go into closed loop. the engine must be running for a pre determined amount of time, the coolant has to be a certain tempurature, and the oxygen sensor must be at 572 degress F. it will go into open loop at a prolonged idle, and at WOT. You get the best gas mileage in closed loop.

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Old 02-02-2006, 02:12 PM   #6
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Default Re: Open-Closed Loop Question

I can attest to the above (^^^).

Our 2002 SC2 (manual) was getting pretty crappy fuel economy and my job is so close that the engine barely warms up before it gets shut off. We have been getting roughly 23mpg since I started this job.

We took a one hour road trip on Sunday and filled the car back up on Monday. That tank of fuel worked out to be above 30mpg right where it should be using the winter mix.

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Old 02-02-2006, 07:35 PM   #7
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Default Re: Open-Closed Loop Question

Quote:
Originally Posted by mjo
You must mean the scan tool showed closed loop at idle and open loop above a certain maximum RPM.
No, it's as I stated: open loop at idle and closed when reaching upper RPM's. The engine was warm, as attested by rhe fact that the fan came on frequently.

WOLFMAN: Thanks for the explanation about the car having to be driven for tests to be meaningful. I'll educate my mechanic on that point .

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Old 02-02-2006, 10:03 PM   #8
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Default Re: Open-Closed Loop Question

Quote:
Originally Posted by mr. ed
No, it's as I stated: open loop at idle and closed when reaching upper RPM's. The engine was warm, as attested by rhe fact that the fan came on frequently.

WOLFMAN: Thanks for the explanation about the car having to be driven for tests to be meaningful. I'll educate my mechanic on that point .
Few even "seasoned" mechanics realize that with ECM/PCM controlled powertrains, MOTION is everything. That is why States like California use Dynos that simulate the car being driven for emissions inspections.

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Old 02-03-2006, 12:24 PM   #9
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Default Re: Open-Closed Loop Question

having seen many S-series on a scan tool, some will go into open loop when stationary due to the exhaust temps dropping and the 02 sensor cooling off. Once the car is in motion again (higher engine rpm) the sensor will heat up and the PCM (upon seeing the renewed 02 activity) will go back into closed loop.

Most cars I have seen with a properly functioning 02 sensor(s) will stay in closed loop even at idle - this may be an indication that your 95k old o2 sensor is starting to get a tad lazy. completely normal, and if the 02 signal degrades enough, the MIL light will come on to let you know.

Cheers.

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Old 02-03-2006, 06:13 PM   #10
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Default Re: Open-Closed Loop Question

Quote:
Originally Posted by Saturn-Eh!
having seen many S-series on a scan tool, some will go into open loop when stationary due to the exhaust temps dropping and the 02 sensor cooling off. Once the car is in motion again (higher engine rpm) the sensor will heat up and the PCM (upon seeing the renewed 02 activity) will go back into closed loop.

Most cars I have seen with a properly functioning 02 sensor(s) will stay in closed loop even at idle - this may be an indication that your 95k old o2 sensor is starting to get a tad lazy. completely normal, and if the 02 signal degrades enough, the MIL light will come on to let you know.

Cheers.
We watched the scan tool as it was checking both 02 sensors and they appeared to be operating just fine (readings were varying rapidly as engine was being accelerated). BTW, the front 02 sensor was replaced about 2 or 3 years ago. The other one is original. If my check engine light comes on again, I'll certainly take your advice and have the diagnostics done again to verify the problem Thanks.....

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Old 02-03-2006, 06:31 PM   #11
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Default Re: Open-Closed Loop Question

The B1S1 sensor, the one you replaced, is the one that controls your fuel trim. The rear, B1S2, just measures catalytic converter efficiency. A good sensor will sweep at least six times per second. Kind of hard to read the sweeps and sweep ranges in a datastream, but easy with a lab scope. When they are getting old, the sweep range and frequency declines which means that the feedback to fuel trim tends not to be as accurate.

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Old 02-04-2006, 04:28 PM   #12
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Default Re: Open-Closed Loop Question

I'm with saturn-eh, my obdii saturns hit closed loop within 2 minutes of idling after cold startup. Might be worth throwing a front o2 sensor on just for grins...

GM "allows" a car to drop to open-loop at idle; they mention it in their service manuals, but my scan tool indicates closed no matter what, once it goes into that mode.

Are you encountering some driveability/emissions/economy problems? Did the data stream of all sensors seem acceptable?

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Old 02-06-2006, 05:25 PM   #13
mr. ed
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Default Re: Open-Closed Loop Question

Quote:
Originally Posted by eljefino
Are you encountering some driveability/emissions/economy problems? Did the data stream of all sensors seem acceptable?
No, the car seems to drive just fine (except for a somewhat rough idle, which has been there for quite some time). Mileage seems good and I don't know about emissions. I plan a move to So. Dakota, so I don't think I'll be finding out about emissions results any time soon (no emmision tests in SD)

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Old 02-06-2006, 06:28 PM   #14
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Default Re: Open-Closed Loop Question

I hope the downstream doesnt fluctuate much when warmed up. If so u may have issues. if its following the upstream on readings the cat is junk.
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Old 02-07-2006, 01:45 PM   #15
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Default Re: Open-Closed Loop Question

Quote:
Originally Posted by mjo
You must mean the scan tool showed closed loop at idle and open loop above a certain maximum RPM.
that's what i would have thought. the key here may be that the car was not moving. maybe??


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