engine revs to 6000 rpm on start up and won't idle down

That is more that a vacuum leak. More likely the throttle body butterfly stuck open. Remove air filter assembly and close it manually, then if it happens again you may need a new throttle body. Allowing it to tach at 6000 rpm not under load will blow your engine

Years ago, my Dad was trading in a Buick, and the salesman wanted to have it inspected before offering a trade-in value. The mechanic - while in Park, revved it up to a unreal rpm and it threw a rod. It was running fine before that but then the dealer said the engine was bad.. well sure it was bad after that, I was standing there watching him. I have no idea why he felt the need to do that, but it was absolutely screaming but end of story is the dealer gave him the trade-in value of the car in good running condition, but not until after a heated argument

My GMC will not idle down, I changed the idle control valve, and the throttle body is not stuck open