The logic is not totally perverted. The ECU has the amount of air from the MAF plus readings from the front O2 sensor. The additional air from a leak will show up as a "lean" condition on the O2 sensor, so the ECU lengthens the injection time to increase the fuel. As you apply throttle, the vacuum level in the manifold decreases, and the air leak is not as bad, giving a very rich environment.
A friend of mine has a NA engine which had a very bad leak. First, the O2 sensor became very dirty and failed. The sensor was replaced, turning off the CEL, but the engine was running so rich at lower rpms (1500-2000) that the plugs fouled and caused a no start condition. All is now good with a new elbow.
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My name is Klaus and I am a Volvoholic.
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