| 'Stock' Narrow Band Sensors work great at the range they are intended. They will not give you as accurate of a reading Below about 13.5 or above 15.5.. But the information is there to be seen and used. The 'Problem' at Idle, is the Nissan Engineers. They built a really screwy system, that Mimics a Carburated Idle. At running the Fuel Mixture is controlled very well by Feedback of the O2's and the Maps.. Ignition Advance is derived from the Maps as well. During Idle for the '1990-1992, the Idle Mixture will go to a Slightly Rich Position to keep fuel in the Cats, and the Ignition Advance will go to 'Base' Time 15 Deg. The IAV's will get called open to add 'Air' to the exhaust Pipes leaving the Engine, but Before the Cats. This is exactally what the Late 70' Carburated Cars were doing. Except the 300's do not have an 'Air Pump' Line like in the '70's cars..
The IAV's use Exhaust Puffs through a Check Valve to pull air in during the Negative Pressure times of the Exhaust. So this system will Mimic the Carburetor System Nissan used in the late 80's on the cars. The '93 and later 300's used a simpler system. The AIV's are gone, the Idle Mixture goes to ~14.5 ( With Limited Ability to Correct it ), and the Ignition Timing goes to 'Base'.. The Problem is, in order to keep the Cats 'Lit Off', the Fuel Mixture needs to swing 'Lean/Rich' like it does at Crusing.. This Lean Rich 'Swing', 'Gulps' Fuel, then Air into the Cats to give them Fuel and Air to burn. At Idle, the 300 has VERY LIMITED ability to correct AFR's.. So, depending where your Mixture is sitting, the Cat will or wont stay lit off. All OBD2 Idle Systems use a Very Active Idle Lean Rich 'Swing' to keep fuel and air in the Cats. Because of the Limited AFR Mixture Adjustment at Idle, and the Fact that Fuel Mixtures have changed over the Years, and Geography, Some Nissans will have a Hard Time Idling and Running Smooth and Passing Smog.. New Cats will pas better than Old Cats.. ( No Kidding, thank you Captain Obvious ! ) Generally if you put the Late 8 Bit ECU into a Early Nissan, it will Idle Better. As it doesn't rely on the IAV's and the Mixture is programmed to be better.. But Conzult will tell you how good the Mixture is at Idle now. Cj
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15:22:57 10/25/22
