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wastegate

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    05 Legacy GT limited

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  1. I have followed the rough idle and misfire postings for a long time now since my '05 has been rough nearly all its 37K miles. Dealer has replaced MAF sensor, injector, O2 sensor, 2 computers, coil packs, and other items I don't even remember with only short term or limited success. My car has consistently had #3 misfire codes thrown, after warm-up at idle, and up to about 3000 rpm when under no load. I did a plug replacement on #3 with no success, and then swapped injectors hoping that the misfire would move. Eureka! it didn't move, but the problem is fixed! Here is my theory: (I do know my way around engines & have been building and tuning them for 25+ years & have had a car featured in Hot Rod mag.) The Theory --- Fuel injectors don't all flow the same, particularly at low duty cycle (idle and light load) so in my case I had one cylinder (#3) that liked a little more fuel than some of the others (cam lobe variation, combustion chamber shape, port shape in intake, etc.) and unfortunately it was paired up with a fuel injector that had lower than average idle flow. When the motor is cold it runs extra fuel so all cylinders were happy, but once warm, it kicks into closed loop mode and the O2 sensor looks at OVERALL A/F ratio (not the ratio for #3 cylinder) and leans things out. #3 got too lean and could not fire consistently. When I moved the injector (which just happened to be a higher flowing one by chance) over to #3, the fuel demands of #3 cylinder were met, and the old #3 injector flowed enough to meet the demands of the other less demanding cylinder. When you have Exhaust Gas Temperature probes on an engine you can easily see how much cylinders vary from one another - and tune accordingly, but unfortunately we don't have that luxury with these factory engines. Hope this makes some sence and can help some of you fellow rough idlers!
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