Slant Six Forum :: When does too rich start to look like too lean?

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When does too rich start to look like too lean? https://www.slantsix.org/forum/viewtopic.php?t=21919	Page 1 of 1

Author:	Sam Powell [ Sun Feb 18, 2007 4:57 am ]
Post subject:	When does too rich start to look like too lean?
I am told that too rich can actually look like too lean on the O2 readout. Does anybody know why this happens, and how you recognize it? I have had Eli telling me that too rich will blow out the spark flame, and yet the O2 sensor is sending redings of .02 volts. Is it possible that too rich is mimicking too lean at a healthy O2 sensor for some very objective, technical, scientific reason? If so, how rich does it have to go before it loks like too lean in error? Sam

Author:	mpgmike [ Sun Feb 18, 2007 6:19 am ]
Post subject:
I would think that at the very least you would see the spark plugs telling you that things are too rich before you experience this level. I would also suspect that the black smoke coming from the tail pipe would be a good clue. Aren't you running a wide band? I remember you talking about it and thought you had one. If you had an overly rich mixture that put out the flame, creating a "lean" reading on a NB O2, then I would suspect your ignition system is lacking. Indexing the plugs will minimize such things. Mike

Author:	Sam Powell [ Sun Feb 18, 2007 6:44 am ]
Post subject:
Thanks for the feedback MIke. The implication when this has been talked about in the past has been that this can happen with a healthy system. Also, we are talking spots here and there in the map, under certain specific situations, such as when you lift the throttle after acceleration, not accross the board. I certainly will check the plugs when I get things running. I remember a plug reading web site someone posted here. I think I have it bookmarked. Does anyone remember hearing of this? What conditions in the exhaust deterimine the O2 reading, and how could going too far one way mimic the opposite condition? This is a very basic question. How does reading O2 content indicate A/F ratio in the first place. Why does the voltage go up as the mixture gets richer? Is there more O2 in the pipe, or less? I think maybe someone has talked about this before, and I am just getting too old to remember things like this anymore. Sam

Author:	Matt Cramer [ Sun Feb 18, 2007 9:59 am ]
Post subject:
Normally, the richer the mixture, the more oxygen is used up in the incoming air, so there will be a lower oxygen content. However, if the mixture becomes so rich you have a missfire, the oxygen in the incoming air gets dumped right into the exhaust, making it look like you have more oxygen and hence less fuel. You may also see other signs of a missfire when this happens, and the mixture needs to be very rich, like around 9:1, for this to happen.

Author:	Sam Powell [ Sun Feb 18, 2007 11:21 am ]
Post subject:
That was the explanation I remember hearing before. Maybe this is exactly what was happening when the engine began to misfire at 150 degrees ECT. Because the O2 reading was dropping off the scale, I thought it was going lean, and riched up the mixture with the warm up adjustments. Maybe that was exactly the opposite of what I should have done, thus making the problem worse. I would see theO2 reading go down to .02 as the engine was missing and bucking, and barely running. Once it warmed up to 170, things smoothed out. My reaction would be to go back to the garage and increase the enrichemnt coefficient at 150 ECT. Once I get this going again, I will try going the other way. If the base AF target ratio is 14.7;1 and you have dialed in 1.4 coefficient for warm up, it is possible that if the incoming air is warming up faster than the coefficient is dropped off, you could get a 9:1 ratio. I remember when first tuning up the Mega Squirt, if it got down to 10:1 at warmup it would misfire. Sam

Author:	mpgmike [ Sun Feb 18, 2007 12:10 pm ]
Post subject:
Are you using a heated O2 sensor? It might be possible that you aren't able to maintain the 660* needed to keep the O2 sensor active. As you increase the oxygen content in the exhaust, the voltage goes down (lean condition). In the total absense of oxygen, at proper temperature, the sensor will put out over 2 volts (I tried it with a single wire sensor). Mike

Author:	Sam Powell [ Sun Feb 18, 2007 2:39 pm ]
Post subject:
I am using a heated O2 sensor. In fact the Accel software refers to it as the HEGO.

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