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Put the resistor on a switch, go out for a drive, cruising in closed loop. Now look at your A/F meter and flip the switch. Nothing will happen to the A/F since we're in closed loop right? You won't see it jump and then be corrected right?
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It will jump because it's working off of tables that have been confirmed by the O2 sensor. Then it will lean out for a second or so, then quickly be corrected by the O2 sensor back to normal. When you take the resistor out, it will go rich for a second or two, then it will go back to normal. Let me illustrate what happens when you do the resistor trick...
ECU: "Intake air is 90 degrees, I'm switching between lean and rich."
O2: "Mixture is lean."
O2: "Mixture is rich."
O2: "Mixture is lean."
O2: "Mixture is rich."
O2: "Mixture is lean."
O2: "Mixture is rich."
IAT: "Air coming into the engine just got 100 degrees hotter."
ECU: "Holy #$*#, I'd better cut back on fuel fast!"
ECU: "Let's see what my tables say I should be injecting based on the last time I dealt with 190 degree air..."
ECU: "OK, I'm cutting fuel delivery by 20%"
O2: "Mixture is lean."
O2: "Mixture is lean."
O2: "Mixture is lean."
O2: "Mixture is lean."
ECU: "I've over compensated, adjust the trim tables and increase the fuel a bit."
O2: "Mixture is lean."
O2: "Mixture is lean."
O2: "Mixture is lean."
O2: "Mixture is lean."
O2: "Mixture is lean."
ECU: "I'm still off. Adjust the tables some more and increase the fuel."
O2: "Mixture is lean."
O2: "Mixture is lean."
O2: "Mixture is lean."
O2: "Mixture is lean."
ECU: "Wow, I'm really off. Injecting even more fuel."
O2: "Mixture is rich."
ECU: "OK, we're back to normal again."
O2: "Mixture is lean."
O2: "Mixture is rich."
O2: "Mixture is lean."
O2: "Mixture is rich."
Everything above happens within about 2 seconds.