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So I was going out to do some formation flying today. I started the plane up and taxied off the ramp and towards the taxiway. As I was approaching the runup area, the engine just quit. I "caught" it with the primer switch and it kept going. But if I released the switch, it would die. The 252's have the Klixon switches across the bottom of the left side panel. There is a switch for Low Boost and a switch for High Boost (it has a safety tab that has to be rotated out of the way to activate the switch). These switches are on the left side if the panel. The last switch to the right, is a Primer switch. It's spring loaded and will automatically return to the off position as soon as released. Throttle was just a bit above idle and RPM's at about 1000. I also turned on the Low Boost, but that made no difference. I quit holding the Primer switch and the engine quit. As if I'd pulled the mixture. I noticed as soon as the engine died, that it sounded like the Primer pump was still on. I turned off the Master and it quit. But if I turned the Master on, it came back on. I verified all switches were off. But the Master seemed to activate the boost pump. I got out and we towed the plane back to the SWTA shop. It's nice to be on the field with JD and his shop. JD was away but Dan was there and we started troubleshooting. He crawled under the panel with a multi-meter and determined that the High Boost switch was in the "On" position even though it was physically in the "Off" position. He disconnected the High Boost switch, and then everything worked as expected. It seems that the High Boost switch had failed to the On position. I've only used the High Boost switch once in the two years I've owned the plane and that was more than a year ago. So it basically never gets used. Evidently when it failed "On", it was flooding the engine. At idle, it was just too much fuel and wouldn't run. I immediately thought what would have happened if it had failed 5 minutes later, either on the takeoff roll or while climbing out. The guys tell me that it would have run fine at full power/throttle. But at idle power it flooded and killed the engine. After disconnecting the offending High Boost switch, the engine and plane seemed to run fine. Laura called Mooney and we'll have a replacement switch tomorrow.