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I have a 00 Civic Si with a greddy turbo+intercooler, aem fuel modifications, walbro fuel pump, greddy catback exhaust and a straight pipe that replaced my cat. My engine is throwing these codes: fuel system too rich, and fuel trim malfunction. My car has started to backfire slightly with change in rpms usually between gears or downshifting. I also have a VAFC II installed, but not tuned at all yet. Anyone know why this is happening? Thanks.

 
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My 02 civic with headers intake exhaust hi flow cat is doing the same thing just use that v-afc to tune the car so it's not runing so rich and i think with the backfiring it's just straight through with no back pressure so thats normal. My car was throughing the code too rich too. Hope this helped

 
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most likely cuz of your fuel mod and fuel pump.. thats why your running rich.. plus you didnt tune with the VAFC yet.. the backfire is cause by not having a cat.. I kno my buddies with subarus backfire.. with their turboback exhaust.

 
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Sounds like you are running too rich. All your fuel mods need to be re-adjusted. If you know how to adjust them lean it out a little bit.

 
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I've heard that you can't really tune the vafc without a dyno very effectively, so I'm gonna try to find a shop that has a dyno. Is it possible that the backfiring might stop if I get my fuel leaned out a little bit and possibly less airflow with the vafc? Thanks for all your advice, it really helped a lot...

 
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I thought the VAFC was just for v-tec adj. If there are fuel ratios in there as well then yeah that needs to be looked at too. I dunno if the back firing is due to not having a cat. Thats usually occurs if the car is running too rich, which it obviously is.

Why cars backfire - A car backfires because unburned fuel is getting to the muffler. The heat of the exhause system causes the fuel to compust, thus a backfire.

Just looked that up.

 
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i knew the loud noises coming out the back end had somthing to do with gas......... AAAHAHAHAHAHAHHAHAHAHA

ps.. that was my first car joke.. i think its great

 
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I never actually knew why a car backfired. I assumed it was from built-up back pressure or because the catalytic converter was fried. That was very helpful. Thanks again everyone!

 
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moved to the Vehicle Issues / Problems forum

 
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technically this should go into the Previous Gen Civic's forum, but i put it here to get more exposure

 
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the apex'i Vafc controls vtec, and air fuel.. Vafc= Vtec air fuel controller.. i think lol but it makes sense.. haha