98 civic ex 5 spd. put a full exhaust system on it, one for ebay because of $$$ issues and because my stock manifold was cracked and my intermediate pipe had holes. after install the car now misfires, no codes are thrown about a misfire. its intermittent. gas mileage is also suffering from it to obviously. i do have 1 code that pops up since the install and its 1325 which is knock sensor. any ideas to why a full exhaust change would cause a knock sensor code and miss fire? again they are random the both of them. but both began when new exhaust was installed.

 

sounds like an oxygen sensor problem, dont know why its not throwing codes tho..

 

both o2 sensors are new.

 

maybe you have an exhaust leak ebay products aren't exactly known for there craftsmanship especially there donut gaskits.

 

Have you reset the ECU since installing the exhaust. I would reset it and see if the codes come back. How old is your ignition system? (cap, rotor, plugs, wires)

 

ive reset the ecu many times. the cap, rotor, distibutor, wires and plugs are all less than a year old. i have no exhaust leaks. im really at a loss of what is wrong with this car. the misfire is interemittent, gas mileage sucks, idle quality sucks. all this started after the new exhaust. could all this be related the fact the cat, is one of those supposedly "high flow" cats. but when you look through there isnt nothing in there. the exhaust shop told me without a cat there, the car could run to rich. but that might be on newer cars he thought.

 

It does work better with some back pressure. An empty cat means you're polluting unnecessarily as well.

You should also check the valve adjustment because if they are too tight it will cause misfiring especially at idle.