Hello, iam a new member to the site although i've been on here gathering information for months lol. My mustang idles great when i first start it up around 750 RPM'S with my new underdrive pulleys, but after a while of driving around to where engine is warmed up my idle tends to sit around 850/900 RPM's and wont drop back to my likings of 750 RPM'S. My motor has 80 000km's on the rebuild. TPS is set at 97 volts/IAC is clean/ECT has been sanded clean because there was signs of rust before/ACT had gunk on it so i took carb cleaner to it and made it look brand new again/ took some electrical contact cleaner to my MAF sensor and its clean as hell. Only engine upgrades would be Taurus electric fan with 130 amp 3g alternator/Mac underdrive pulleys. I have also unhooked negative battery cable for 30 minutes everytime i had adjusted something electrical. P.S when i installed my underdrive pulleys i thought my idle would drop about 100 RPM'S and it did but still goes up to 850/900 RPM'S when engine warms up. Could this be a fuel system issue? I cant hear any vaccum leaks as i checked all vaccums and the only one that looks no good and cracked up is the one that connects to the charcoal canister but i dont think that sucks too much vaccum, could be wrong tho. oh and my fuel line that is comming out of the gas tank that leads past the exhaust pipe has been burnt a bit from rubbing on exhaust pipe, didnt change hose yet because it doesnt leak. ANY HELP WOULD BE NICE.