If I run her for a while around town, sometimes when I stop at a light it "breathes" the tach goes up and down between about 650rpm and 1500rpm. I do have a lean code from the O2 sensor before the cats on the drivers side. Checked all fittings on the cai and tightened them down. Bad O2 or bad MAF? Btw its a 2002 gt.
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You can use soapy water and spray around the headers. See if it bubbles. Have you tried also cleaning the MAF, Throttle Body, and IAC? Since this is an older car, it could be gunked up. Also maybe seafoam the engine. That is always fun
Had the same problem... Check throttle body, could be dirt in form of a thick ring where the flap opens up causing it to stick and as a result, the flap can't open as freely as needed, oh and while your at it clean Iac. Your problems should be gone. I know how that is. When your driving there's no problem then as soon as you stop BAM! Rpm acts up and sometimes the car jukes back and forth when the rpm goes up and down l.
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