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I put in a new exhaust and it sounds great but when at idle at a stop light it starts to rumble and it sounds like its gonna stall although its not. I simply press down the gas to 1000 rpm (rather than a notch below) and it solves the problem. So I have a few questions

Why does this solve the problem?
Why did it start doing this?
Can I buy something to change the idle, if so what is cost and difficulty?
Thanks

 
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If the exhaustt is too big i.e. 3 inch or something like that it is a lack of back pressure and you may be causeing permenent damage to your vehicle. Your exhaust valves will wear out twice as fast as normal due to excess heat not being pushed out of the exhaust.

 
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yeah probably 3 in is to big for a n/a car
i have the hp racing catback with 2.5 piping i havent had any problems with it but i think is 2.5 is a little big for our cars

try putting the stock one back on if its not welde and see if it does the same thing if it does get 2an a quarter pipe