JayBoCC2

i enabled my chip to enter back into closed loop just recently and now it backfires badly when i let off the gas to just idle down.

i have logs and bins from before closed loop chip and after closed loop chip.

another thing, my o2s when in open loop start at about 350mV and drop down to 120ish at running temp and bounce between 90mV and 120mV.

once i enable closed loop it will bounce radically between 90mV and 600mV, usually in the 400-600mV range though. I thought this range was good in terms of a stoich AFR but it causes my care to backfire pretty badly even with over 10 deg of timing pulled across the board!

i am also getting ~20 knock counts in a 10-20 minute drive, and 0 knock counts at WOT (closed or open loop!) but 1-7 degrees of knock retard (depending on stock vette or TA timing)!

im afraid to even drive it out of my driveway nowdays.

JayBoCC2

i heard from another individual that my glasspacks may be the cause of the backfiring and that glasspacks have limited lifetime?

RBob

iTrader: (1)

Back firing out the exhaust? I would look for an exhaust leak. That causes extra air to be pulled in which when mixed with the fuel in the exhaust lights off.

According to your NB O2, it is lean in open loop, and correct in closed loop. Which has closed loop richer then open loop.

RBob.

RodsRideSS

My Monte SS was backfiring in closed loop after I installed a new billit aluminum distributor. After going through all the elecronic trouble shooting...I pulled the new dist out and this is what I discovered:
With the rotor pointed at the #1post...the reluctor and magetic pickup did not line up! I wanted to see what this OUT OF PHASE condition would look like under the timing light!
NEXT...
1/ Pull the cap and cut a hole or slot at the #1 post. this is to see the rotor as it sweeps past with the engine running
2/ Install the cap start the engine
3/ Shine your timing light at the slot
The ignition should be firing exactly on the post..not in front of or behind. If it's not.. you have a condition that is Reluctor/Magnet Phase or it's OUT OF PHASE!
I fixed it but pulling the dist and the guts out of it. Redrilling the holes that secure the magnet down and aligning the reluctor to magnet and the #1 post! If you have a dial back timing light make sure it's set to 0deg. You can measure how far off the post it is by adjusting the dial until the rotor aligns with the post. My billet aluminum distributor was out of phase by 33*advanced!! The pickup and the reluctor were off by 1/16". The spark had to jump from the rotor tip to the post about 1"...when the computer would advance the timing in closed loop and light throttle..it would backfire because{I think} the spark was jumping to the next post.

Anyway....That fixed my problem...running great!!

JayBoCC2

used an 5spd man 305 timing base and smoothed out the timing (its more radical than stock 350 / auto timing but not as radical as stock vette timing) and she's got even more bite with no backfire when i let out the exhaust. it seems that when it was backfiring it blew out one of my exhaust gaskets (between header and pipes) which i can fix easily.