AN IMC C182 PVT PLT NOTES HIS ALTIMETER DSNDING FROM 7000 FT TOWARDS 6000 FT AND NOTIFIES MKE APCH OF THE ANOMALY E OF BJB; WI.

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WE WERE WITH MKE; RADAR VECTORS TO CGX. QUITE SUDDENLY; WE WERE LOSING ALT. TEMP WAS 7 DEGS C. WE WERE IMC FOR THE MOST PART; BUT AT TIMES WE HAD A MISTY 'VIEW' OF LAND (SHORELINE). WE WERE E OF COURSE (WITH CLRNC) FOR WX. WE WERE STAYING 30-50 MI FOR TSTM READINGS ON OUR STORMSCOPE. I HAD BEEN WATCHING A THIN; MOIST BAND ON THE LEADING EDGE OF THE WINGS. IT WAS NOT ICE THAT I COULD IDENT. I CONTACTED AND ADVISED MKE APCH OF OUR ALT LOSS. HE TURNED US W TOWARD WEST BEND; INQUIRED AS TO THE NUMBER OF SOULS (2); FUEL (40 GALLONS); WITH THE INTENT TO TAKE US TOWARD WEST BEND. IN LESS THAN 1000 FT LOSS; WE GAINED 2 DEGS C OF OUTSIDE AIR TEMP. JUST AS QUICKLY; THE ALT RETURNED. I CHKED EVERYTHING THAT MY TRAINING TAUGHT ME. NOTHING WAS OUT OF ORDER (EXCEPT THE ALT LOSS). THIS NEW CESSNA IS FUEL INJECTED; SO THERE IS NO CARB HEAT TO APPLY. THE GAIN OF ALT WAS NOT ACCOMPANIED BY THE 'CHOCKING; COUGHING' SOUNDS OF CARB ICING. THE 'FILM-LIKE' MOISTURE ON THE LEADING EDGE ALSO DISAPPEARED AS THE TEMP INCREASED. I NOTIFIED MKE APCH THAT EVERYTHING WAS BACK TO NORMAL. HE TURNED ME BACK ON COURSE AND TURNED ME OVER TO CHICAGO APCH FOR OUR VISUAL APCH INTO MEIGS. I HAVE NO IDEA WHAT CAUSED THE PROB. I'VE SPOKEN WITH MY LONG TIME MECH. HE'S NOT SURE WHAT HAPPENED. I ALSO SPOKE WITH AN AIR SAFETY COUNSELOR. HE ALSO WAS QUESTIONING WHAT TYPE OF AIR FILTER THE C182 HAS. IT IS PAPER. HE THOUGHT THAT IF IT WEREN'T; THAT MIGHT HAVE BEEN THE SOURCE OF THE APPARENT ICING; SO HE'S MYSTIFIED. THIS PROB DID NOT CAUSE ANY PROBS TO OTHER ACFT. THE PROB DID NOT RECUR. ANY INSIGHT WOULD BE APPRECIATED.