Odd backfire problems

[NH_RPS13Drifter]

I did some searching and couldnt find what i was looking for, so here is my question.

About 4 weeks ago I gutted my cat, I could only get the exhaust side of the cat off because the bolts on the manifold side where heavily rusted on, and broke right off. When I put it back togeather, the exhause side bolted right back up wnd worked ok for about a week, then the rust that was holding the cat to the manifold started to break up, and the cat seperated from it. So I have the manifold that comes down to the cat, then a break, and then the cat bolted to the exhaust. They are seperated about 1.5 inches, so very little to no exhaust auctually gets to the muffler.

I have 3 questions:1. How long can I run like this before I damage my exhaust valves2. I have seen a huge drop in gas milage (I now get 10MPG), could this be the cause?3. My car backfires alot, maybe 10-30 times a minute. It happens all the time; accelerating, decelerating, and at idle. Its really annoying, cause when under heavy acceleration, the car jerks badly everytime it backfires.

Sorry for the long post, I'm just trying to give an accurate description. Thanks!

[URM S14]

1. not sure but you dont want to find out the hard way, do you. get it fixed ASAP2.This IS the problem, especially if your car is OBD-II and your seconday O2 is not getting exhaust to read causing management problems3. Once again, same as the last answer. Also ,the backpressure that your exhaust creates is all considered when the manufacturer creates the ECU maps, etc. So your exhaust exiting from your header collecter creates problemshope i helped

-Chris

[DAEDALUS]

Not sure how having a broken exhaust pipe will damage the valves. The exhaust pulses act to scavenge the chamber by creating a slight drop in pressure behind the valves, but removing the pressure drop shouldn't have any effect on the parts. Combustion pressures exceed 600psi, atmospheric is almost 15. A >50% drop in mileage is not reasonable. The 2nd O2 (I believe exists only on '95 and newer 240sx') is purely for emissions monitoring to tell the ECU (and the EPA) whether the cat is doing a good job. The 1st O2 is the one that is a fuel trim. Having the 2nd one out of the flow might eventually throw a code.Exhaust backfiring is the combustion of unburnt fuel in the pipe--usually not a problem on fuel-injected cars. The mileage is horrible. There is much better access to air with the gap in the exhaust. This all indicates a leaking injector or seal. Does the exhaust smell really rich with gas fumes? Pull your plugs and inspect them, they might tell you which cylinder is suspect. If it is a leak, the injector could be just fine but needs a new seal.

[eddiec]

my car backfires semi regularly to. my problem is related to an exhaust with little to no backpressure. i get backfires when decellerating and sometimes when changing gears.