bad fuel mileage and exaust popping

[95civicEX]

I just got my 89 240 a little over a month and a half ago, and have already replaced the cat back with an apexi n1 unit, before I replaced the exaust, and now that it's been done, if I rev the motor, it will randomly pop, the car also does this on occasion if I'm cruising and let off the gas and just let the car slow down in gear...is this a bad thing and could this be related to my next problem?

My fuel mileage is terrible, I just filled the car up today - 10 gallons at 170 miles, I know I'm ragging on the car a bit, but this is horrible, my civic used to get 300 miles per fillup, and that was only around 10 or 11 gallons...

any ideas, I've changed the oil, and used fuel injector cleaner for the past two tanks, and have noticed no difference at all...

thanks for any help in advance

oh yeah, almost forgot, the car is an 89, original motor with 130,000+ miles, the sr20det will go in eventually but for now I've gotta get this motor running better

[Tyler Durdan]

As for the popping, it could be a crack in your exhaust manifold.  Someone else might know better though.

As for the bad fuel mileage...try replacing the fuel filter.  I've heard on occasion of this fixing the problem.

[tnord]

you're running too rich. why? could be hundreds of things, but at least you have a starting point.

[HKrpS13]

popping sound is back-fire. Good, bad, I don't know.
Please do not compare with Civics on the gas mileage because they are very high.

[tnord]

hmmm.......i suppose i should've read your post instead of just the title. Apexi N1 duals have been known to backfire. it's because their piping is too large.........i think. but yeah, drive normally for the next 10 gallons and you should be in the mid 20mpg range.

[RoNx777]

I'm having the same problem w/ my N1 cat-back, when i cruise @ 2500 - 3000 RPM, then pull off from the gas, i hear the back fire. However i had the exhaust for about 3-4 months, and i jus notice it now, not too sure was it there from the beginning.

[Da808aZnRaVr]

actually yeah i have the apex n1 and my exhaust makes that poppin noise too when i let off the gas...have had the exhaust for bout a year now and its been doin that ever since...i dont think its a problem or anythin..actually i kinda like that noise...my friends also have n1's on thier cars and they too have that poppin noise...so i wouldnt worry bout the noise...

[95civicEX]

alright, I've checked the car out, and after seeing the air filter,which was disgusting, I've replaced it, the car is running rich as there is a nice black spot on the bottom of my bumper where the exaust comes out, so what can cause this and how can I fix it? thanks for any help in advance.

[sykikchimp]

o2 sensor, and spark plugs would be were I would start..

I started getting crappy mpg after I installed my exhaust and then I realized I was just running the car a lot harder b/c it's fun to listen to the exhaust...

[bing]

same problem...

did the o2 sensor, and sparks and wires recently. no difference. i'll try the fuel filter.

also, i like the noise kinda, but the exhaust sounds better when it doesnt pop.

[98SilviaS14]

what's all this i always hear about the backfiring on the dual n1's. i have had mine on for a while now and it runs smooth as silk, so to speak.  actually i was pleasantly surprised cuz i thought that mine was gonna do that when i did the install, but so far so good.