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					Originally Posted by knucklesplitter  I don't see how a header crack would affect your idle.  The O2 sensor measures the A/F *ratio*, so even if there is a leak the ratio stays the same.  But.. I hope it fixes your problem, and if it does I would like to know how. | 
	
 At idle, you could easily draw atmosphere into the exhaust and then push it into the 02 sensor on the next pulse.