| Message |
The O2 Sensors are used as an analog ‘Switch’. They are either above .45 volts ( Lean ) or below .45 volts (Rich). AT .45 volts the Mixture is Perfect ( in theory).. The Cat Converter Needs Fuel and Air to keep it’s fire lit. If you remember, the early 70’s Carbed Cars, had a smog pump to blow air into the Cat Converter. That is so it could burn the rich mixture from the Carbed Engine.
The Cat wants the Mixture to be Perfect ( 14.7:1 AFR ) for it to do it’s Magic. To accomplish this task without a Smog Pump, the ECU will swing the Fuel Mixture slightly rich, to increase the HC level in the Exhaust to put fuel in the Cat. Then the ECU will swing the mixture Slightly Lean to feed excess air into the Converter to make the fire stay lit. The Average of the ‘AFR Swing is 14.7:1.
The End result is a ‘Gulping’ of Fuel and then Air to the Cat, by swinging the fuel mixture lean then rich.. the ‘Swing Rate’ is about every 2 seconds from lean to rich, then back.. this is the ‘Dithering’ or ‘Hunting’ you see in the voltage coming from the O2 Sensors.. the ECU is ONLY looking at .45 volt, and the mixture is either greater than or less than that number. The actual AFR numbers that the ‘Swing’ goes to is about 13.8 to 15.5..The 300’s ECU will read the actual voltage, and do some calculations on the ‘Other than .45 Volts’.. but for simplicity sake, you can think of it as above or below the .45 value. There have been discussions as to what it actually does with the Voltage Numbers.. Ash might know, but I doubt he’s telling. Most things discovered about the ECU PROM are no longer talked about.. MOST cars will keep the ‘O2 Sensor Swing’ during Idle, as this keeps the Cats Happy.. the 300 does not.. It puts the Fuel System into ‘Open Loop’ and will park the fuel mixture at some fixed number, but not check it with the O2 Sensors.. If you look at the Voltage coming off the O2’s at this time, it will be parked about .1 volt or lower, and generally not the same left to right. My NA Mixtures run about 15 left bank, and 16/17 right bank at idle.. subject to variations in weather and temp. My theory is that the Early 300’s used the AIV’s to feed the Air to the Cats at Idle ( Only at Idle ) and they did not need the AFR’s to Swing to feed the Air.. they just fed excess fuel during Idle mode, and the AIV were feeding the Air. Then Someone in the factory figured the 300 would pass Idle emissions testing if they just kept the mixture lean to feed a little air to the already hot ‘New Design’ of Cats.. and the HC’s were already in the exhaust, so they could make the cars simpler by taking out the AIV’s.. Just my Random Ass Guess.. I do not think that was such a great plan.. but it mostly worked.. However you do get the random 300 that Will Not Pass Smog at Idle.. I’m thinking if the ECU was made to do the AFR ‘Swing’, it would feed the Air and Fuel into the Cats and keep the car clean at idle.. Ash has not responded back to me on this yet.. ‘Cause I figure he’d know.. Cj
|
 |
