So Im trying to put this catch can on my S and when I crank the car with the can installed it revs to about 3500 instantly then drops to 2000 and revs between 2-3k constantly....at first I thought my hoses were bad and there was a leak so I replaced them and the problem persisted.

doesnt the RPM oscilation mean the emmisions control is sensing a leak in the vacume system?

Has anyone had this problem and how do I fix it?

 

You have a vacuum leak somewhere. How did you hook up the catch can? Nobody will be able to help you until you give more details.

 

brand new vac line from check valve on valve cover to catch can, vac line from catch can to first nipple on intake mani. I held my thumb over one of the hoses and blew into the other to see if there was a leak in the catch can somewhere but it held without any leaks.

 

You either have a giant vacuum leak or a stuck IAC.

 

IAC? sorry

Car runs fine when I put the stock hose back on.

 

Then you've got a vacuum leak in your can.