Hey Arb. I think u r right. You are saying that it must be the o2 side that is leaking. evause the only way for the ignition ("backfire", which isnt really a backfire) in the cat or muffler is if there is extra FUEL added not Oxygen right?

At least I think that is your question and loint. And, I think you are right!

If this is what is causing it, then yes the Leak must be on O2 sensor side, which causes an icorrect lean condition to be read which causes the ecm to add fuel which leads to some amount of raw fuel being in the exhaust which ignites in the cat.
Is that what you meant?

cheers

 

Yes. If it was the non-detected side(assuming only a left sensor) then there would be no "extra" fuel to cause a backfire. Oxygen does not explode, which is what a backfire is - an explosion.

 

cool thanks. i thought the term backfire was when the boom came back out the intake. like from timing or a valve that does not seat.

cheers.

 

Could be in the intake too I guess, but in post #1, the OP said
"in the mufflers".

 

Gonna throw a couple of ideas out there just because I had this happen to me years ago....

As pointed out, only 1 bank of the engine is monitored by an O2 sensor -- which means fuel trims are based ONLY on that one side. If that side is lean (or was lean), fuel could be "fattened" over stock MAF tables.

I'm not totally sure how "fast" the 89 ECM is but I remember wondering if the fuel lags and sends rich pulses down the exhaust? Of course, the stock setup injects air, has cats, and will "flash" anything combustible.

As things get worse (higher amounts of fuel being dumped), I'm of the impression fuel can be ignited in the exhaust pipes. I'm betting, weaker springs on older motors can create a bit of "lag", providing more exposure to burning fumes in the cylinders. Add these together, and you can get the kind of ratty-tat-tat in your exhaust -- during decel -- that you might expect from a race car.

When it happened on mine, 2-3 injectors were failing which caused the remaining injectors to run fatter -- on the passenger side.

Just to cover all the bases, injectors have performance characteristics based on the behavior of their electrical coil, voltage in the car, and orifice size. When swapping injectors, the "performance" curve may be different -- compared to stock -- where the amount of fuel on decel is increased. (For tuners, putting voltage "curves" into the BIN helps insure the best outcome.)

I'm also thinking the AIR system "shuts down" after the car is warm -- which MIGHT mean extra air isn't "injected" into the exhaust. A loose exhaust manifold might be a source of fresh oxygen to enable combustion in the exhaust pipe.

Or, if I remember wrong and O2 is injected to the exhaust, the CAT delete mentioned on the OPs car might "push" unburnt (and oxygen enriched) fumes to the mufflers -- where they may collect just enough to be ignited.

 

if the exhaust springs are weak, could it just be that?

if it helps the op, i have two L98's with stock injectors and no cats that have no issues with backfiring.

on thing i just remembered though....
I did have some backfiring on decell only (that sounded like it was coming from the exhaust rather than out the intake) when i bumped my timing by accidenton my 87. My timing was off maybe 12degrees.

I wonder OP, have you confirmed that the timing mark on the balancer is in the correct tdc position? it may have moved? ive heard this does happen. you can buy or build a piston stop and a youtube video will walk u through it.

reason i mention it is because the can ran, but just had these popping issues.

 

Not backfiring in the exhaust. Back firing in exhaust = rich; unburned fuel.

It it's backfiring under decel, that is pretty normal until you hit DFCO. It it's sitting there backfiring audibly at idle...something ain't right.

 

That's kinda what I was I was talking about when wondering if the ECM was "fast enough" to respond to slow downs.

I looked at a stock BIN because I couldn't remember if DFCO was even active. It is -- with a 1 sec delay. IIRC, DFCO stands for "Decel Fuel Cut-Off". And, it contains a value (RPMs) representing how fast/slow it will control slow-down with fuel control.

Back when I noticed the issue in mine (circa 2005-7), I'd converted to a more agressive exhaust. I'd never noticed decel "popping" before that. I remember some members suggesting it was there the whole time and that my change of exhaust enabled me to hear it.

Because I measured and found 2 or 3 failing injectors, I bought into the theory explained in my prior post. Plus, it was hard to believe it wouldn't have heard muffled popping with the original OEM exhaust.

I didn't exactly get to proof which theory was correct. That's because my 2007 tear-down of the topend, led to research/learning that spanned a couple of years. I went from wanting a bigger intake and headers, to a cam w/hone, to a stroker. The process of slowing increasing my "enthusiasm" and budget -- combined with the backlog of the builder took nearly 3 years!!!

Anyway, long story...left long, I cured the "issue" by replacing the engine with an entirely new one! LOL

I will say on rare occasion I do hear mild/short burst of popping on decel -- but WAAAAY less than the prior incident. And, my current engine/exhaust is quite a bit more agressive than the OEM setup. I still think what I heard back then was related to fuel trims causing extra fuel to be dumped in the exhaust.

Considering the age of springs and injectors, I wouldn't be surprised if more/more owners noticed this symptom....regardless of how much "normal" might be.

If it's one/two pops, I think DFCO would account for it. If the popping is prolonged, I'm not as convinced.

 

Here we go again. No I didn't specify every condition for a lean backfire but the OP did say it occurred during deceleration. So what's the problem now? Try a Google search for lean backfire. I'm more than tired of explaining everything concept to those other than the OP. If you want argument then start your own post.

 

The O2 sensor should cycle continuously between 100mV and 900mv during closed loop only. Later EFI cars used counts and cycled between 1 and 256 counts. Now for open loop counts should fix at 128 or 450mV.

It reads like something is wrong with the O2 circuit. O2 sensor generates it own voltage and you could unplug the connector and measure voltage though it can't control closed loop with it disconnected.

Something to know about O2 sensors is they have to breath out the wire end and enough oil or sealer can prevent this. Also something i just learned recently is never test an O2 sensor for resistance. The meters voltage/power will distroy it. Also look for O2 sensor wire pinches/cuts - as this happens a lot.

Hope this can help.

 

Stating facts isn't "wanting to argue". Get over yourself.