I have a 97 Civic hybrid:
-swapped in 2000 civic si and everything.
-running a 98 ITR P73 ecu
-I/H/E
-test pipe
-ITR intake manifold
-B16A2 fuel pump

it's been running fine for 4 months till today. it's loud as hell just started today. The idle is fine, but it seems like there is an unsual backfire?

 

Are you running a chipped ECU? With H/E and test pipe your going to have a little back fire since there is nothing there to restric flow. Has the car been sitting, maybe take it around the block and see if it goes back to 'normal'. HTH

 

Mine seemed to get louder too over time with my test-pipe and started back firing more... don't know why...

 

no..i know something is not right here. it's LOUD..... my car sounds like there's no exhaust system.

 

i had the bolts between the header and my exhaust on my 92 sable 3.8L work their way loose and that shiet was loud loud loud. tightened all the exhaust bolts...problem solved

 

problem fixed. 5 zigen exhausts suck. snapped at the resonator....it was only 4 months old.

 

check for exhaust leaks, i had one on the mustang, right where the header and hpipe meet. it would backfire cus there wasnt enough back pressure.