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 Short life on O2 sensors 
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Joined: Mon Apr 08, 2013 10:21 pm
Posts: 122
No, I'm saying there is no "correct AFR" for any circuit whether its curb idle, low load, or WOT. Yes WOT has a fairly narrow "ideal" AFR, but even that varies from engine to engine. The low load area also has a somewhat narrow "ideal range" that varies with each combo. But the curb idle has to be tuned to what makes the engine happy. I can make the idle AFR read whatever I want anywhere from 11 to 17 by trimming the mixture, but the manifold vacuum, plug readings, and engine response tell me where the best mixture is. I've seen different well tuned engines have a curb idle AFR anywhere from low 12's to mid 14's. The AFR reading is just a tuning reference for each engine combination and its various circuits. That's why in the end, main jetting is adjusted for max MPH for WOT. Once you reach that sweet spot you make note of the AFR and use that as a future reference for your particular engine. AFR is a tuning tool that is very helpful and streamlines the tuning process, but its not the end all. ;)

Wed Apr 24, 2013 4:50 pm
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Joined: Fri Feb 08, 2013 2:37 pm
Posts: 1704
barryh wrote:
So are you saying no matter how much fuel you remove from the Idle it still will idle at 13.5???

He’s saying the intake mixture which passed through the valve overlap dilutes the exhaust gas with oxygen that was not in the combustion and so causes an erroneously lean O2 reading. The oxygen sensor just senses oxygen; it doesn’t know where it came from or how it got there. The combustion can still be rich and making too much soot. The sensors don’t like soot much, especially when they are heavily covered and a sudden WOT and the associated heat sort of fires it on like a carbon glaze. That is when a careful roasting with a propane torch can help if you are careful not the heat and cool them too fast. Sensors don’t like vibration or mechanical shock, or thermal shock at all. Water splashed on an operating sensor, such as from a puddle, will kill them quick. I have seen a couple be goofy and intermittent after an external water shock, before they quit altogether.

I don’t know why you should have sensor life problems unless there is some silicone contamination from your engine. The most minuscule amount silicone you can imagine will kill a sensor. I used some silicone spray for lube to install plug wire boots on the distributor end of a new set of wires and the next track session an hour later showed a degradation of the sensor response time. I have some sensors I have used for only leaded fuel dyno and track sessions for the last several years, for - ??? - I lost track of how many hours are on them, and they still work OK.

Thu Apr 25, 2013 7:59 pm

Joined: Mon Apr 08, 2013 10:21 pm
Posts: 122
Tuner, thanks for the more technical description of the sensor's readings on contaminated idle mixture. I never know how in depth to get on replies to gentlemen I don't know. I did take the sensor out and heated it with a small butane torch and blew it out with compessed air. I did a heater calibration it and worked just fine yesterday for 30 minutes of driving and a few WOTs. While I had the sensor and cable out I went ahead and did the heat and blow on two old sensors that I thought were dead and I was able to get both of them to heater calibrate. :ummm?: I try to not introduce silicone to the fuel or induction/exhaust systems. That's pretty weird that you had a change just from spraying silicone in the distributor area. My exhaust blows condensation mostly about a minute after startup, but I would think that is mostly from everything downstream of the header. Maybe I should try starting it and letting it run a a few minutes before I power up the sensor? BTW, I changed carbs yesterday - same TB size, slightly larger venturi, same exact main jets, IFR, LSAB, HSAB as the carb that came off and it required ~ 12.8 AFR for proper idle instead of the mid 13's the other carb wanted. Imagine that ;)

Fri Apr 26, 2013 10:07 am

Joined: Mon Feb 18, 2013 5:11 pm
Posts: 18
The biggest killer I see to O2 sensors is guys running the cars without the sensor up to temp. Doesn't matter who's system you're using if the O2 is installed in the pipe you need to get it up to temp BEFORE starting your engine. Other killers are of course being WAY too rich, and oiling the engine down after a race. Using a Propane torch to clean off the sensors will bring them back in many cases. Typically I find on older sensors they get "numb" and do not respond as fast nor do they have the same swing (rich to lean) as a fresh sensor does.

Fri Apr 26, 2013 4:07 pm

Joined: Mon Apr 08, 2013 10:21 pm
Posts: 122
Been waiting to follow up on this because I wanted to make sure my finding was not temporary or a "fluke". Looks like my sensor problems were due to an inconsistent ground. In this case the battery is mounted in the trunk and grounded to the "frame" of the unibody under the trunk. I had suspected possible issues with this before and wired the LM1 directly to the battery thinking that would rule out any power circuit problems and it did help some but did not completely get rid of the problem. I noticed the problem seemed to rear its ugly head when I'd hit a hard bump. I started suspecting the ground somehow trying to travel back through a weak connection in the unibody when it flexed a little, so I ran a #2 ground cable from the battery to the starter mount. It's been almost 6 months now and several hours of run time and not one problem. :thumbl:

Sat Jan 11, 2014 10:08 am
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