A PA28 WHILE PRACTICING STEEP TURNS AND REDUCING PWR IT WAS DISCOVERED THE PWR COULD NOT BE REDUCED BELOW 20 INCHES OF MANIFOLD AIR PRESSURE.

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WHILE ENRTE TO THE PRACTICE AREA ON A DUAL FLT WITH A CFI APPLICANT; WE STARTED TO SET UP A SERIES OF STEEP TURNS. UPON COMPLETION OF OUR CLRING TURNS; MY APPLICANT ATTEMPTED TO REDUCE PWR IN ORDER TO SLOW TO THE PROPER MANEUVER ENTRY SPD WHEN IT WAS DISCOVERED THAT WE COULD NOT REDUCE OUR PWR BELOW 20 INCHES MANIFOLD AIR PRESSURE BECAUSE THE THROTTLE WAS BEING BLOCKED BY SOMETHING EITHER IN THE QUADRANT OR IN THE ENG. WE COULD ADD PWR BUT NOT REDUCE PWR. WE IMMEDIATELY DECIDED TO RETURN TO THE ARPT AFTER WE PRACTICED HOW WE WERE GOING TO CTL THE PWR WITH THE MIXTURE AND PROP CTL A FEW TIMES. AFTER WE TURNED BACK TO THE ARPT; I TOOK CTL OF THE AIRPLANE WHILE MY APPLICANT WENT OVER THE CHKLISTS CONFIGURING THE AIRPLANE FOR APCH AND LNDG. AT 10 NM OUT; WE CONTACTED THE TWR AND INFORMED THEM OF OUR SIT. THEY CLRED US FOR A STRAIGHT-IN APCH AND AT 5 MI CLRED US TO LAND. LNDG WAS UNEVENTFUL OTHER THAN THE FACT THAT I HAD MY APPLICANT REDUCE THE MIXTURE TO IDLE/CUT-OFF ON 1/4 MI FINAL. AFTER TOUCHDOWN WE MANAGED TO COAST OFF THE RWY AT THE CTR-FIELD TXWY. AFTERWARD; WE WERE TOWED BACK TO OUR NORMAL PARKING SPOT. WHEN OUR MAINT CONTRACTORS ARRIVED AT THE AIRPLANE THEY THEORIZED THAT SOMETHING INSIDE THE THROTTLE QUADRANT WAS JAMMING THE THROTTLE AND NOT ALLOWING IT TO BE REDUCED BEYOND A SETTING WHICH PRODUCED APPROX 20 INCHES MANIFOLD AIR PRESSURE. AS OF THIS TIME THE AIRPLANE IS STILL WITH MAINT BEING INSPECTED. I BELIEVE THAT EITHER SOMETHING VIBRATED LOOSE OR FINALLY BROKE DUE TO EXCESSIVE PRESSURE BEING APPLIED. I BELIEVE THAT THESE TYPES OF OCCURRENCES CAN BE PREVENTED WITH INCREASED VIGILANCE DURING ROUTINE INSPECTIONS OF THROTTLE ASSEMBLY INTEGRITY AND CLOSER INSPECTION OF THROTTLE ASSEMBLY FRICTION LOCKS IN THE FUTURE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE PROB WAS CAUSED BY THE THROTTLE CABLE BEING TWISTED AND THE TWISTED PORTION OF THE CABLE WOULD NOT PASS THROUGH THE FIREWALL FEED-THROUGH FITTING. THE CABLE REPLACED AND CHKED OK FOR SVC.