Finally solved backfire problem

[sammyg2]

Ever since I bought my 911 three years ago it had a subtle backfire type of gurgle out the exhaust when decelerating.
It's drove me crazy enough to try everything to get rid of it. You name it, I tried it. I could make a list as long as my arm of all the stuff I tried.
The car always ran fine, I just hated that sound.
Well, today it went away.
I had a bad connector for the O2 sensor (the insulator had crumbled) but I was making good contact so I ignored it until now.
i finally got in there and cut the wires and crimped in a new weathertite connector and went for a drive, wow!
Smooth, stable, no gurgling. it idles and drives like a modern fuel injected car now instead of a 25 year old CIS car.
Yep, I'm a happy camper.

[kach22i]

You are champion of the world...............until the next little problem pops up.

I filled this one, never know when it will come in handy.

[hobieboy]

Padron my ignorance - but what does the O2 sensor connection have to do with backfire?

In my case, I don't even have one but would also like to get rid of that gurgling. Can it be decel valve? Suggestions?

[yelcab1]

My guess is the uninsulated O2 sensor wires were picking up electrical noises in the engine compartment, thereby fooling the computer into thinking the emission was too lean. The ECU would richen out the engine unnessarily, and boom, back fire.

[Mysterytrain]

Insulation won't prevent the pickup of stray EMI or RF... sheilding does that.

[targa80]

A lousy connection would degrade the signal being sent to the Lambda brain and cause over compensation whether rich or lean.

[sammyg2]

That's the part I can't figure out, and why it took me sooooo long to find:
I ran for a while with the O2 sensor functioing, it gurgled. I replaced the O2 sensor, it gurgled. I ran for a long time without the O2 sensor hooked up o the lambda brain, the exhaust gurgled on decell. I removed the decell valve, it still did it. I hooked the decell valve back up, it still gurgled. I played with the mixture every way till Sunday, it still did it. I replaced plug wires, cap and rotor, injectors, did full diagnostics on all pressures and they were in spec.
I eliminated every possible vacuum leak, no joy.
I replaced every exhaust gasket, it still did it. I did a top end rebuild, it still gurgled.

Then..... I replaced the connector, the gurgling went away. I noticed something else last night, the decell valve seems to be working better also i.e. the engine rpm seems to drop slower. Coincidence? I dunno. Maybe the decell valve suddenly started working again, even though I tested it prior and it appeared to be functioning correctly according to my mighty-vac.

The only other thing I did was to throw some Techron in the tank but I did that after I noticed the gurgling had gone away. Doesn't add up, but it is what it is.
One thing that bothered me before that should have been a red flag:
when the lambda brain was hooked up there was a slight surging which was verified with an Air Fuel mixture gauge I hooked up.
The lambda brain was swinging and over-compesating which was why I ran with it disconnected. DUH! It doesn't do that anymore.
If everything else is eliminated, the obvious has to be the solution, or something like that.
At this point it doesn't make sense but unless the gurgling and popping on decell comes back I am not going to question my dumb luck.

[jester911]

Sammyg2,
When you say you replaced the connector, do you mean the one in the engine compartment?

[sammyg2]

Yes, The one in the engine compartment. The bakelite insulator had crumbled which is not unusual.
The wire the goes through the insulator was still intact, but most of the strands inside the insulation had broken from getting bent back and forth, prolly due to not being supported properly, (the insultor thing).

So I cut about 1" off the wire that goes to the lambda brain, and cut a little off the wire that goes to the O2 sensor.
I crimped new connectors on the two wires and connected them.