F100 CREW TAXIED OVERWT AND FILLED THE CABIN WITH JET EXHAUST FUMES FROM FOLLOWING AN MD80 ACFT TOO CLOSE.

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OVERWT TAXI; FUMES IN AIRPLANE; OVERWT LNDG. PRIOR TO BEFORE STARTING ENGS CHKLIST; FO SENT ACARS MESSAGE WITH FUEL ON BOARD OF 23000 LBS. RELEASE FUEL WAS 22200 LBS. THE CREW CHIEF TOLD US WE WERE UNDER ON CARGO WT. GATE AGENT CALLED LOAD CTL TO GET EXTRA PAX ON BOARD; INCREASING PAX COUNT TO 84 FROM OUR WT RESTR LIMIT OF 83. REALIZED WE WERE OVERWT FOR TAXI AND RETURNED TO GATE. ACCOMPLISHED OVERWT TAXI INSPECTION. DURING TKOF ROLL; WE WERE CLOSE TO AN S80 AND BELIEVED WE WERE SMELLING HIS EXHAUST. FUMES INCREASED DURING CLBOUT AND WE GOT SMOKE AFT R TOILER ALERT. COMPLETED THE ABNORMAL CHKLIST AND THE ABNORMAL CLRED ON THE RMFDU; BUT FUMES REMAINED STRONG. DECLARED EMER AND LANDED OVERWT AT ORD AT 95200 LBS VERSUS MAX 88000 LBS. TAXIED TO GATE WITH ARFF ESCORT; SINCE FUMES HAD BEGUN TO DISSIPATE. ARFF BOARDED THE AIRPLANE WITH INFRARED CAMERA; BUT COULD NOT LOCATE HEAT SOURCE. ENTERED BOTH PROBS IN LOGBOOK AND CHANGED TO ANOTHER ACFT. EVEN THOUGH WE HAD SENT AN ACARS FUEL RPT AND HAD BEEN ASSURED THAT WE COULD TAKE THE EXTRA PAX AT OUR CURRENT WT; I FEEL I COULD HAVE PREVENTED THE OVERWT TAXI BY BEING MORE INQUISITIVE AND WAITING UNTIL RECEIVING THE CLOSEOUT BEFORE PUSHING BACK. SUPPLEMENTAL INFO FROM ACN 563244: IN THE PROCESS OF PUSHING; WE NOTICED THAT WE WERE OVERWT. WE HAD STARTED #1 AND WERE IN THE PROCESS OF STARTING #2; BUT LEFT THE STARTER ON MORE THAN THE LIMITS. LESSON LEARNED: ANY TIME IN THE MIDDLE OF A CHKLIST; DON'T GET DISTR -- FINISH THE CHKLIST.