my z28 has 3'' exhaust with a flowmaster muffler and the cat is gutted. every time i rev up my car it always has a few small backfires, nothing major like when the opti was bad, just small pops as the motor winds back down. what causes this?

 

excess fuel left in the pipes is igniting. u are probly running rich. are there more power adders? whens the last time the 02 sensors were checked or changed?

 

it definitely smells like im running rich. the only other power adders that i know of are a cold air intake and a k&n filter. the o2 sensors havent been checked or changed in at least a couple years. keep in mind that i was having the problem with the fuel pressure leak/evap system. but yeah im sure its running rich.

 

ok if it was running rich becuase of other problems the 02 sensors can get fouled and tell the computer theres not as much fuel as there really is. this creates a cycle that ends with some really dirty 02 sensors and fouled spark plugs.

 

not to jack this thread, but i have the same problem, and smell fuel like crazy sometimes when it backfires letting off the gas pedal. not too sure if could be running rich though, i usually put 89 octane (midgrade) fuel in it is that hurting me? the manual says 91 octane premium only, but we dont have 91, only 83, 89, and 93. i heard somewhere that the octane has a + or - 2 octane rating so i figured midgrade would be ok for a while since trying to afford 93 premium twice a week gets pretty pricey lol

 

stock lt1 will run fine with 89 octane fuel. u need to make sure u dont have a fuel leak, 02 sensors will wear/foul out over time so if they are old, changing or at least testing them is a good idea, newer 02 sensors can be cleaned with a propane torch. check the fuel pressure regulator (pull vacuum hose off fpr check for gas) and check and test your evap syste.

 

so were are the o2 sensors on my car and how do i change them? and honestly about the evap system...i've never done anything like that and i would have no idea where to even start, even though you put that diagram up its like trying to read chinese to me lol


imalixz28: does your car build up an excess amount of pressure in the fuel tank??

 

The cat does act like a resonator. I've gutted one before, and the exhaust had a hollow can sound and did rap back as well. I have my own theory about how the exhaust flows as it expands into the open unrestricted cat. I liken it to sticking your head into a metal garbage can and yelling. I try not to do that very often any more!
For best exhaust flow, the way to open a cat and still keep it on the car for looks, would be to run a section of exhaust pipe through it (providing the inlet and outlet lines up), and tack weld it in place.

 

passenger side 02 sensor is in the exhaust header pipe by the starter, driver side one is in the header pipe just below the oil filter. 94 should have the one wire sensor, they are not to hard to change, getting them unscrewed its the hardest part.

 

Welcome to an LT1. My car does this too (usually when it's cold). Like Craby said, it's excess fuel igniting in the exhaust. If it only happens when the you're letting the rpm drop from a rev it's nothing to be concerned about. The LT1 PCM doesn't quite keep up with the RPM dropping and thus it runs rich on its way down if it's not under load.