Mufflers Sucking Back Air

Ive Got Flow Masters 40 Series On My 98 Z71 5.7l And When I Rev The Engine....as The Rpm's Come Back Down, Its Making A Heavy Sucking Sound From The Tail Pipe........before I Had Flowmasters Put On It Didnt Make The Suction Sound.....it Also Didnt Have Cats On It.....when The Flows Were Put On Cats Were Put On With Them For The State I Live In......can The Cats Create The Sucking Noise......if You Put Paper Up To The Tailpipes The Paper Sucks To The Tail Pipe..........to Get An Idea Of How It Sounds....put Water In Your Mouth, Lean Over And Start Sucking In Thru Your Mouth.......if That Makes Anysince, But Thats What It Pretty Much Sounds Like...any Help Would Be Great

Dude, its called backpressure and is pretty important to have. Almost every vehicle i know of has that to some degree or other. It is rare but possible to have to much back pressure tho.

If the paper is literally being "sucked" to the tailpipe you have too much back pressure. Back pressure really is only measured at idle or 1000 rpm, or some set idle speed for diagnostics, so in teory there is no "measurement" for back pressure during decel. but it should never be able to actually suck anything back into it from the tailpipe.

Two things to check:

1- Are these aftermarket hi-flow cats. that you had installed, or GM replacements? The cat' is such a dense material that any backpressure beyond them should be almost nil as it is "absorbed" by the air flow changes in the cat and muffler. (Even with straight pipes I've never seen anything suck in like that.

2- Check your timing, and set properly. If you are getting that much back pressure, it sounds like your timing might be too advanced. (Have you by chance changed the lifters, rockers, or the cam?)

Hope this helps.

No Change On The Lifters Rockers Or Cams, The Cats Are Really Cheap The Dealer Sent Me To Their Muffler Shop And Had Them Put On.....could They Be To Restrictive.....and Can I Make Valve Adjustments On This Engine
5.7l (r Code)...............thanks For The Info..... Im Prob. Going To Just Cut Them Off, Ive Heard The Same Year Truck With The Same 40 Series Flowmasters On Them And Mine Sounds Nothing Like It......thanks Agian

Cam Not Cams.......... Im Not Retarted Really

how many miles ? if you let it idle and put a piece of paper over the tail pipe and it is sucking it back into the pipe it sounds like its time for a valve job ( not seating right and the piston is pulling air by the valve thus the suction)

122k....... I Didnt Want To Hear That