All replies are appreciated. My son has a 92 f250 with a 460 engine. it was running great until yesterday when he was on his way home from work. The truck started losing power and then died. when he open the hood he could hear air escaping. After a while he determined that the engine is building up pressure in the crankcase and after it builds up so much it kills the truck. after about 5 minutes it will clear up and the truck will start and run fine for another 2 or 3 miles. You can actually hear the air escaping from the engine and when it quits escaping the truck will start and run again. When it starts the engine sounds great. Has anyone heard of thihttp://www.fordforums.com/images/smilies/crybaby2.gifs?
That's probably a vacuum you are hearing. Sounds like you have a vent line plugged off. I'm pretty sure there's a vent that runs from the valve cover to the air filter box. Check that. You have to have a vent so the pcv valve can't pull the crankcase into a vacuum.
Next time you hear that sound, pull the pcv valve. If you hear a swoop sound its pulling a vacuum.
The hissing probably isn't related to the engine stall, but compare all the vacuum lines to the map on the VECI label under the hood and make sure all are connected properly, not leaking, and unobstructed.
This caption may help you understand crankcase pressure:
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