Narrative:

Flew a practice approach into ZZZ 1. Did a pre-flight; everything looked normal; no leaks; fuel was as expected; oil at 7 quarts. Did a full run-up at ZZZ 1. Departed left cross-wind; ZZZ 1 did a 'point out;' and then transferred us over to ZZZ tower. ZZZ cleared us to transition the airspace and advised we could change frequencies at 2;500 feet (we were climbing to 3;500 feet). We leveled off at 3;500 feet over ZZZ and I configured for cruise flight; bringing the rpms back to approximately 2;550-2;600 rpm (placarded to avoid continuous operation between 2;350 and 2;550 above 24' manifold pressure); and leaned the mixture to approximately 12 gph. A couple of minutes later; the engine suddenly lost power. We turned the electric fuel pump on and switched tanks which had no effect. My passenger told me to turn back towards ZZZ but I knew we did not have the glide range to make it back to an airport; so I pitched for best glide and started a standard rate turn towards the ocean; intending to ditch into shallow water if no better options appeared. My passenger reports getting some power from the engine after manipulating the throttle; but I don't recall that. After I made a mayday call to ZZZ; my passenger told me he had the comms; and punched 7700 into the transponder. We were not on with flight following; because ZZZ 1 cannot set that up on the ground and from prior experience I knew that we'd get dropped by TRACON almost immediately after coming into this zone; please restore ZZZ 1's ability to coordinate flight following!my passenger 'played with' the throttle and RPM controls and the mixture was full rich (I believe I set that; while I was activating the fuel pump and switching tanks; but I cannot be sure). I do know that I started pulling the mixture back (I don't know why I tried that; but it was one of the only things we hadn't tried that we could from the cockpit; and we had time to experiment). Regardless; once the mixture was pulled back about 1/2 way; the engine sprang back to life. I don't know if mechanically it was capable of making more power; but it was configured to about 22' manifold pressure and about 2;200 RPM and that was enough to arrest our descent (leveled off at approximately 2;000 feet MSL); so I left the settings unchanged. I completed the standard rate turn back to about 180 degrees opposite our previous heading; and flew up along the coast to ZZZ. We were cleared to land on runway 27; winds were 200 at 10. I kept our altitude and made no power changes until I'd reached the airport environment; at which point I started pulling the throttle back and pitched to bleed off airspeed (we'd gone from best glide; about 100 mph; to about 130 mph when we restored partial power; vle is 120 mph); and when I got below 120 mph put in 50% flaps and lowered the landing gear. We were still very high; as my passenger noted; so I put in full flaps and executed a forward slip that I carried into the flare. Once we were wheels on the ground I raised the flaps and braked aggressively (I may have put a flat spot on one tire; I think the left main); and was able to bring the aircraft to a halt just past the intersecting runway. Tower asked us to cross on the other side; which we did without incident; bringing the aircraft to a stop on the taxiway south of the runway. We shut down and exited the aircraft. I don't yet know the cause of the engine issues; but other mooney pilots with similar experiences have told me it was likely caused by a failing fuel injector servo.

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Original NASA ASRS Text

Title: M20 pilot reported engine failure with eventual partial power restored and made a successful diversion.

Narrative: Flew a practice approach into ZZZ 1. Did a pre-flight; everything looked normal; no leaks; fuel was as expected; oil at 7 quarts. Did a full run-up at ZZZ 1. Departed left cross-wind; ZZZ 1 did a 'point out;' and then transferred us over to ZZZ Tower. ZZZ cleared us to transition the airspace and advised we could change frequencies at 2;500 feet (we were climbing to 3;500 feet). We leveled off at 3;500 feet over ZZZ and I configured for cruise flight; bringing the RPMs back to approximately 2;550-2;600 rpm (placarded to avoid continuous operation between 2;350 and 2;550 above 24' manifold pressure); and leaned the mixture to approximately 12 gph. A couple of minutes later; the engine suddenly lost power. We turned the electric fuel pump on and switched tanks which had no effect. My passenger told me to turn back towards ZZZ but I knew we did not have the glide range to make it back to an airport; so I pitched for best glide and started a standard rate turn towards the ocean; intending to ditch into shallow water if no better options appeared. My passenger reports getting some power from the engine after manipulating the throttle; but I don't recall that. After I made a mayday call to ZZZ; my passenger told me he had the comms; and punched 7700 into the transponder. We were not on with flight following; because ZZZ 1 cannot set that up on the ground and from prior experience I knew that we'd get dropped by TRACON almost immediately after coming into this zone; Please restore ZZZ 1's ability to coordinate flight following!My passenger 'played with' the throttle and RPM controls and the mixture was full rich (I believe I set that; while I was activating the fuel pump and switching tanks; but I cannot be sure). I do know that I started pulling the mixture back (I don't know why I tried that; but it was one of the only things we hadn't tried that we could from the cockpit; and we had time to experiment). Regardless; once the mixture was pulled back about 1/2 way; the engine sprang back to life. I don't know if mechanically it was capable of making more power; but it was configured to about 22' manifold pressure and about 2;200 RPM and that was enough to arrest our descent (leveled off at approximately 2;000 feet MSL); so I left the settings unchanged. I completed the standard rate turn back to about 180 degrees opposite our previous heading; and flew up along the coast to ZZZ. We were cleared to land on Runway 27; winds were 200 at 10. I kept our altitude and made no power changes until I'd reached the airport environment; at which point I started pulling the throttle back and pitched to bleed off airspeed (we'd gone from best glide; about 100 mph; to about 130 mph when we restored partial power; Vle is 120 mph); and when I got below 120 mph put in 50% flaps and lowered the landing gear. We were still very high; as my passenger noted; so I put in full flaps and executed a forward slip that I carried into the flare. Once we were wheels on the ground I raised the flaps and braked aggressively (I may have put a flat spot on one tire; I think the left main); and was able to bring the aircraft to a halt just past the intersecting runway. Tower asked us to cross on the other side; which we did without incident; bringing the aircraft to a stop on the taxiway south of the runway. We shut down and exited the aircraft. I don't yet know the cause of the engine issues; but other Mooney pilots with similar experiences have told me it was likely caused by a failing fuel injector servo.

Data retrieved from NASA's ASRS site 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.