I have a 93 z28 and i have noticed a huge power loss.

I am also hearing a hissing noise but i cant realy tell where it is comming from inside of the car. from what i think if a exhaust gasket becouse of the presure.

Right now i am guessing that my cat is pluged. I have disconected my smog pump and have done a seafoam treatment recent and I have a feeling all the carbon pluged my cat.

Does this sound like it could be the problem.

 

Disconnecting the smog pump does nothing for your car. But take a look at the exhaust, and as you mentioned, make sure the cat is not clogged or that some exhaust pipe is not crushed. Also check the EGR valve.

 

Is the hissing noise constant? If it's ticking and moves with engine RPM you're probably on the right track with the exhaust (check the cat either way). If not check for vacuum leaks.

 

ok thanks i plan on taking out the cat today.

and yes i only hear it when i am under a heavy load.
what size is the stock size pipe that i will have to buy?

 

I took out the cat and it fixed the problem 100%.....

I think i gained 20HP or so it feels. and it sounds mean. but i hope that is the reason i got 15MPG.