Peugeot 406 major flooding

had a problem with a rattling cat, which i replaced, all seemed fine, swithched it off after running for 10mins. when i came back to car it wouldn't start, it had flooded big time! thought it could be a lambda sensor fault, so replaced it but problem still exists. on start up engine runs VERY rough and big plooms of smoke, unburned fuel smoke, engine will just about idle, but no warning light on dash so my code reader is no help!! i can't work out why just changing the exhaust would suddenly cause my car to overfuel so drastically??