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SN95 Cobras
Running Lean At Startup
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<blockquote data-quote="mwolson" data-source="post: 11588439" data-attributes="member: 16006"><p>You got it.</p><p></p><p>When it is in closed loop peration, the PCM watches the O2 sensors and adds or subtracts fuel until it sees stoich, so it is constantly trying to compensate for the leak. Once that compensation hits a pre-defined threshold (25% in a stock tune), it sets a DTC.</p><p></p><p>When it is in open loop operation, typically during startup and at WOT, it runs out of tables, rather than looking at the O2 sensors. So it assumes the correct amount of air is coming in. A vacuum leak allows extra air air to come in during open loop startup, causing it to run lean.</p><p></p><p>However, on startup the pcm normally runs the engine lean for a couple of minutes in order to heat up the cats faster. That is when the SAI runs, too.</p></blockquote><p></p>
[QUOTE="mwolson, post: 11588439, member: 16006"] You got it. When it is in closed loop peration, the PCM watches the O2 sensors and adds or subtracts fuel until it sees stoich, so it is constantly trying to compensate for the leak. Once that compensation hits a pre-defined threshold (25% in a stock tune), it sets a DTC. When it is in open loop operation, typically during startup and at WOT, it runs out of tables, rather than looking at the O2 sensors. So it assumes the correct amount of air is coming in. A vacuum leak allows extra air air to come in during open loop startup, causing it to run lean. However, on startup the pcm normally runs the engine lean for a couple of minutes in order to heat up the cats faster. That is when the SAI runs, too. [/QUOTE]
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Running Lean At Startup
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