wont rev past 3k, hissing noise from under hood...vacuum lines?

We were doing a normal plug change on an 01 ranger ( a.t., 4x4, 4.0L sohc) and when all the new plugs were in, started it up and there is this hissing noise under the hood, and the engine wont rev. up past 3000 rpm. it will hit 3000, cough and slow down the come right back up and repeat. we also saw some sparking around cylender 6, the tower on the plug was cracked so we replaced that one, the sparking i have not checked for again as of yet, but the rev limit and the hiss are still there, checked the plug and the plug didnt look like it cross threaded, any ideas on what it might be? i know there were a lot of vacuum lines in the way while changing them. any help is apreciated.

Hissing is usually caused by a vacuum hose that has come loose, check both ends of all the hoses that were nearby and make sure you didn't accidentally pull one off.

will check, thank you.

it also sounds like it could be an exhaust restriction that could be the hissing noise and would also limit your rpms.

well, i know in my truck, when that hissing started it was the vacume booster pump. That is where i would start looking. Good luck man, vacum lines suck.

Sparking at the wires and boots etc. can mean misfiring has occurred,which can cause failure/plugging of cat.converter,and the resulting hissing.

there is a rev limiter on your truck. in park or neutral it wont rev up past ~3grand. did you drive it? does it go past 3grand? is the CEL on?