Narrative:

Aircraft was in cruise at 5500 ft. The engine fuel mixture had been leaned to 50 degrees rich of peak egt. When beginning descent to my destination airport, I began to enrich the mixture. I noticed no drop in the egt. As descent progressed egt began to rise. I moved the mixture control lever to full rich but egt still continued to rise with descent. I arrested the descent and performed a slow climb back to 5500 ft without adding power. Egt returned to 50 degrees rich of peak. This was consistent with a broken mixture cable. I decided to return to teb. I turned to a new course direct to teb and descended to 4500 ft. Prior to entering the class B airspace overlying teb, I contacted new york approach, explained that I had lost mixture control with the engine fuel mixture leaned for cruise at 5500 ft. I requested entry into the class B at 4500 ft and a low power descent when closer to teb. I also asked if new york approach could arrange with the teb tower controller, a straight-in approach to runway 24 which was in use at that time. This he was able to do and shortly I was told to contact teb tower. The approach and landing were achieved without any problems or excessive egt. Some might argue that I should have landed at the nearest airport. I don't think that the problem put me or the aircraft in any immediate danger. I felt the long, low power approach would not pose any risk of damage due to excessive egt.

Related reports:

Google
 

Original NASA ASRS Text

Title: A MOONEY PLT EXPERIENCED A FAILURE OF THE FUEL MIXTURE WHILE IN N90'S AIRSPACE AND RETURNED TO TEB.

Narrative: ACFT WAS IN CRUISE AT 5500 FT. THE ENG FUEL MIXTURE HAD BEEN LEANED TO 50 DEGS RICH OF PEAK EGT. WHEN BEGINNING DSCNT TO MY DEST ARPT, I BEGAN TO ENRICH THE MIXTURE. I NOTICED NO DROP IN THE EGT. AS DSCNT PROGRESSED EGT BEGAN TO RISE. I MOVED THE MIXTURE CTL LEVER TO FULL RICH BUT EGT STILL CONTINUED TO RISE WITH DSCNT. I ARRESTED THE DSCNT AND PERFORMED A SLOW CLB BACK TO 5500 FT WITHOUT ADDING PWR. EGT RETURNED TO 50 DEGS RICH OF PEAK. THIS WAS CONSISTENT WITH A BROKEN MIXTURE CABLE. I DECIDED TO RETURN TO TEB. I TURNED TO A NEW COURSE DIRECT TO TEB AND DSNDED TO 4500 FT. PRIOR TO ENTERING THE CLASS B AIRSPACE OVERLYING TEB, I CONTACTED NEW YORK APCH, EXPLAINED THAT I HAD LOST MIXTURE CTL WITH THE ENG FUEL MIXTURE LEANED FOR CRUISE AT 5500 FT. I REQUESTED ENTRY INTO THE CLASS B AT 4500 FT AND A LOW PWR DSCNT WHEN CLOSER TO TEB. I ALSO ASKED IF NEW YORK APCH COULD ARRANGE WITH THE TEB TWR CTLR, A STRAIGHT-IN APCH TO RWY 24 WHICH WAS IN USE AT THAT TIME. THIS HE WAS ABLE TO DO AND SHORTLY I WAS TOLD TO CONTACT TEB TWR. THE APCH AND LNDG WERE ACHIEVED WITHOUT ANY PROBS OR EXCESSIVE EGT. SOME MIGHT ARGUE THAT I SHOULD HAVE LANDED AT THE NEAREST ARPT. I DON'T THINK THAT THE PROB PUT ME OR THE ACFT IN ANY IMMEDIATE DANGER. I FELT THE LONG, LOW PWR APCH WOULD NOT POSE ANY RISK OF DAMAGE DUE TO EXCESSIVE EGT.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.