MD10 Captain reports manifold #2 fail alert passing 15000 FT in climb. Thrust is reduced to idle in accordance with procedures; fuel is dumped and uneventful landing ensues. Post-flight reveals faulty sensor.

Narrative

We were negotiating a north/south line of thunderstorms westbound when we received a Level 3 Manifold #2 Fail alert. Level 3 is the highest level of warning on the MD10/MD11. It was the First Officer's leg (Pilot Flying) so I gave him the radios and consulted our QRH (Emergency/Abnormal procedures). It directed us to bring the affected throttle to idle and land at nearest suitable airport. We declared an emergency and made a right 180 degree turn back to ZZZ. We had an off-duty MD11 Captain aboard and asked him to come to the flight deck and assist. We dumped between 40;000 - 50;000 LBS of fuel to be a suitable landing weight for a 35-degree flap landing (versus 50 degrees of flaps) as the winds at ZZZ were strong and gusty out of the south. We landed uneventfully on Runway XXC. Crash Fire Rescue crews examined the aircraft at our request and found no outward signs of any problems. A later examination by our maintenance crew showed it was faulty detector and there was no actual failure. ATC; my crew and Crash Fire Rescue did an outstanding job in support. Post incident observations: A more rapid approach to ZZZ would have been advantageous. I did not properly evaluate the strong southerly winds which delayed our arrival somewhat. I also directed my First Officer to slow to allow the fuel dump to be completed. Worrying about an overweight landing now seems unimportant compared with the potential of hot air leaking in the aircraft. My First Officer indicated that shutting down the engine would have been a consideration for him. Initially we were very heavy for an MD10 (which improved with the fuel dump) and while at low altitudes 2-engine operations would have been within our envelope. I have never been one to shut down a perfectly good engine without a very good reason. Had the hot air leak persisted; it certainly would have been an option. The autothrottles kept pulling the affected #2 engine power back up. My (our) reliance on autothrottles made it somewhat uncomfortable to consider flying without them. I am evaluating those implications as well.