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Hi i have a 2007 cobalt ss sc completely stock other than a catless down pipe, when im driving and give about more than 25% throttle in any gear it bucks/surges badly untill it gets to about 3800-4200 rpm and smooths out but if i putt around with not much throttle its fine, even revving it out. At idle it also seems a little off. Ive changed spark plugs twice, cleaned the throttle body, replaced the fuel filter, replaced the maf and the pre cat o2 sensor, tested the fuel pump pressure, compression and smoke tested for vacuum leaks. No codes are present. I used misfire monitor and it is indeed missing at idle and when accelerating not like a granny, misfire is not specific to any cylinder its all over the place. Has anyone else ever ran into this issue and figured it out? Any advice is appreciated, thanks!
 

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Check the vent solenoid and the purge solenoid - both of those acting up can definitely lead to this kind of behavior.
 
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