Random backfire

[vinsanity]

I tried searching for this issue but did not come across one quiet similar to mine. My car occasionally has a backfire (usually not too loud) that I can hear from the exhaust it seems like. It happens maybe 3-5 times in an average city drive (5-10 miles). It never happens if I accelerate hard nor does it happen when im driving really slow like in residential areas (15-25mph). Basically only occurs while normal city driving from light to light (approx 35-50mph). Any ideas or suggestions as to what this is can be a result of? Thanks in advance.

89 7mGTE
150k original miles
custom intake
custom 3in IC piping
custom 3in exhaust and downpipe
no cats
magnaflow turboXL muffler
everything else stock

 

[Big Wang Bandit]

How often are you boosting in those 35-50 mph jumps...

 

[vinsanity]

im guessing twice, 2-3 and 3-4 gear, but I usually hear it when I hit the clutch shifting from 2nd to 3rd, after the stock bypass valve "swoosh".

 

[SLO-MK3]

Are you venting to atmosphere?

 

[jetjock]

Not to be picky but backfires are induction system events. What you're describing is afterfire. Also called muffler explosion. They're caused by combustion in the exhaust system, usually the result of a rich mixture coupled with a leak. Does it happen only when lifting the throttle?

 

[vinsanity]

yes I am.

 

[SLO-MK3]

That could be your problem.
Read up on 'what happens when you vent to atmosphere' and then you'll probably get a better understanding of what JJ is talking about.
http://www.supramania.com/forums/showthread.php?t=1079


BTW JJ, I just saw your new sig:rofl:

 

[slidebabyslide]

check for exhaust leaks or holes in your exhaust

 

[jetjock]

Slo: When I mentioned leaks I was referring to the potential exhaust leaks slidebaby is talking about. I completely forgot about venting to ATM so it's likely you nailed it. Proves if you'd only buy a decent mulitmeter you could be be a killer diagnostician

 

[vinsanity]

Thanks a lot for the input and suggestions. That link posted by SLO-MK3 describes my symptoms exactly. I appreciate you guys taking the time to solve my problem.

Happy boosting,

Vinsanity

 

[SLO-MK3]

Slo: When I mentioned leaks I was referring to the potential exhaust leaks slidebaby is talking about. I completely forgot about venting to ATM so it's likely you nailed it. Proves if you'd only buy a decent mulitmeter you could be be a killer diagnostician

Consider it done. First thing on the list for this weekend.
The centech has already found itself a new home in the trash:biglaugh:

 

[7MsuprO]

how are you venting a stock bypass valve to atmosphere though?

 

[vinsanity]

I took out the small hose going from the bypass valve to the intake accordian tube. Plugged the accordian tube so the air does not leak from there and kept the end of the bypass valve open so I can hear the sound. Luckily I only kept it like that for 2 days so I'm hoping I did not cause too much damage. I'm just glad I was able to get help so promptly on this site.

 

[starscream5000]

Shouldn't be any damage done, just plug it back in... Does your butt dyno feel more power when you vent the stock BPV to the atmospher???

BTW JJ: What's your choice of a good Multimeter with a decent price (as in not going to break a person)?