Steve,
Are you finding that the pop near idle is happening when you remove your foot from the accelerator again ?
I have a gut feeling that it's to do with the idle ignition timing. There is SO MUCH air available at idle without driving the DBW to close the throttle a little (which we cannot do at the moment due to the faulty sensor I told you about) that the only way to keep the idle low enough is to run with very little timing. There are two ignition control strategies at idle :
- There is a closed loop base idle ignition adder; this is applied whenever the ECU enters closed loop idle. It is a function of coolant temperature and engine run time. For warm idle there will be a large correction, I don't recall the numbers off the top of my head, but I'de say somewhere around 20 degrees. It is this correction that gives you the primary drop in idle RPM below 3 MPH....
- There is also proportional ignition adder, which will advance or retard the ignition based on how far away from the desired idle RPM you are. If the idle is too high, the timing is retarded, if it's too low the timing is advanced.
What you may be observing is the idle strategy retarding beyond the base adder, into the territory that would be classified as Anti Lag ignition timing; it is so retarded that it causes explosions in the exhaust. Imagine the idle is at 1600 RPM and it needs to be at 1000, then at the very instant the closed loop kicks in it has a 600 RPM error, which is "huge", so it reacts to it quite aggressively (yet also effectively, because it DOES reduce the engine torque to bring the idle down to 1000). If we could control the idle air then the base offset may only be 5 or 10 degrees, which means when the proportional correction is applied, the timing does still get retarded further, but not out of the realm of "normal" running and into the realm of Anti Lag...
Hope that makes sense,
Pat.
