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Old 11-06-2007, 10:10 PM   #1
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O2 mv rich/lean mean threshold vs airflow tables? LT1/4

Does anyone know what these tables do and or need to be changed to for a modified engine to assist in idling?
I understand that the voltages are a little high for modified engines and need to be lowered for bigger camshafts etc, as your combustion chamber is less efficient and the tables for stock dont allow the O2's a good range to adjust air fuel ratio at idle etc.
It was suggested I go to 400mv for the "0" table?
These are my tables: And mods are in the signature.
0 579/ 597
32 522/ 557
64 548/548
96 513/513
128 495/495
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Old 11-07-2007, 08:03 PM   #2
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When the PCM is running in closed loop, it monitors the O2 sensor voltages to determine the current air fuel mixture. A properly running stock motor will show voltage oscillation from high (rich) to low (lean) as the computer is constantly enrichening and enleaning the fuel mixture to keep it at 14.7:1.

The transition point table you are referring to is the mid point where the PCM considers a rich to lean, and lean to rich transition has occurred. For example, the first table is 579. That means when the O2 sensor voltage goes from above 579 to below 579, the PCM assumes that a rich to lean transition has occurred.

If you lower this value to 400, the PCM will wait until the O2 sensor reports 400mv to assume a rich to lean transition has occurred. This has the effect of making the closed loop target AFR leaner.

A very common problem with cammed motors is they idle and cruise too rich. This happens because the computer is trying to command a 14.7:1 AFR but cam overlap can cause the O2 sensors to report the exhaust is leaner then it actually is. Many tuners have found they can lean out the closed loop operation during idle and low RPM operation by modifying this table.

If your idle AFR is not close to 14.7:1 then you might be able to fix it by lowering the values in this table (at the 0 row). The rows correspond to airflow levels - e.g. 0 is idle, and it goes up from there.

I've experimented with this value a bit on LS1 engines and found I wasn't able to make more then a .5 change in AFR. If you are too rich, but relatively close, this might help.
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Old 11-07-2007, 11:56 PM   #3
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Vince, Thanks for the great explanation. Now I get it. My Wideband shows 14.2-ish at idle, but the blms show splits (LT1 issue) and the drivers side is 108-114ish and the right side is 112-120ish and it slowly varies. And the wideband is in the right header, as I don't have a bung in the left yet.
I know some people have done the open loop idle, but haven't figured that one out yet.
I lowered these numbers by 100mv and it didnt like it, so I raised it back 50mv towards the original numbers, and it ran a little better. I will keep messing with it just for experience in being able to see what she does. I know the big camshafts in LTx engines cause problems, so I am fighting an uphill battle. But it is all about learning.
Open loop is next.
PCNforless has done a great job on this tune, and I am just tweaking to see what she does.
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Old 11-07-2007, 11:56 PM
 
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