what you are experiencing on the gt is a lack of back pressure and will eventually cause the vavle seats to "burn" then you will lose compression, but that may not happen for a few years. all that is happening is when you let off the throttle the raw unspent fuel if being dumped in to the header/ exhaust mainfold and igniting there. if you were to have the car tuned it may help or find a way to add some back pressure to the exhaust system. as for the gxp if it is still stock you may want to have that looked at.
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