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Old 06-04-2009, 12:44 PM   #1 (permalink)
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More oxygen sensor problems

So I recently posted a thread asking for help removing my oxygen sensor, and I did manage to get it off and the new one installed. But this hasn't fixed a few problems that I thought it would.

1. When cruising at steady speed, the Scan Gauge will read something like 1.5 GPH. When I lift off the gas and slip it into neutral, the GPH will slowly tick downwards, like 1.5 GPH...0.75 GPH...0.50 GPH...0.35 GPH...0.25 GPH. It probably takes about 2 or 3 seconds after I lift my foot to reach the idle GPH.

2. Sometimes, instead of the above problem, when I let off the gas and coast in neutral, the car will go open loop for a few seconds. The Scan Gauge shows DFCO, but this can't be the case because I'm coasting in neutral with the engine on. It usually only stays open loop for 2 or 3 seconds, and then returns to the normal idle fuel flow.

Any thoughts or help on this would be great. Thanks.

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Old 06-04-2009, 12:57 PM   #2 (permalink)
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#1 is probably normal engine operation. I've noticed my matrix will idle at about 1100 rpm in neutral most of the time. If I stop at a light for a while, it kicks down to 750.

#2 is probably a scangauge quirk. The engine probably does go into open loop for a fraction of a second. So, the scangauge updates and sees its in open loop. The engine quickly changes back to closed, but you have to wait for the scangauge to update again to see that its back in closed loop. There is a setting somewhere in it to alter refresh time. By default its set to slow or normal. There is a fast option, but honestly it doesn't seem to make a huge difference.
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Old 06-04-2009, 09:41 PM   #3 (permalink)
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Thanks for the tip on changing the scan gauge speed. Changed it to fast, and it has made a difference. Problem #1 is no longer a problem, as it is now clear that the fuel flow drops as the rpms drop back to idle speed. Before, the fuel flow seemed to lag a lot.

I'm still concerned about problem #2. Is it normal for my car to go open loop for a few seconds or even a fraction of a second while coasting in neutral?
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Old 06-04-2009, 11:44 PM   #4 (permalink)
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Yeah, if the rpms are falling there is no need to inject fuel until the rpms come close to idle rpm.
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Old 06-05-2009, 12:18 PM   #5 (permalink)
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Very interesting, I hadn't even thought of that. Makes total sense, no need to inject fuel while the RPMs are falling. So both of my perceived problems weren't problems at all. I guess that's good. Thanks for the help Daox.

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