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this post touches on a very good point/subject I want to elaborate with
On most engines, like the KA/SR/RB, there is a fuel cut when TPS goes to resting position above a specific RPM threshold. For the stand-alone ECU this is adjustable naturally, but for OEM ecu it is around 1400rpm, it depends on whether the A/C is on or not. Threshold for fuel cut is higher when A/C is on. This helps keep the engine from bogging as it returns to idle.
To paint a picture, you are driving in 4th gear for example, around 40mph, some rpm (say 2400rpm). Your throttle is slightly open for the cruise situation. Suddenly, you lift your foot from the gas pedal.
If you had an open downpipe, or any kind of non-silent exhaust system, there should almost always be one single pop as the final cylinder gets some but not quite enough fuel to burn at such a lean ratio, and this fuel enters the exhaust and makes a slight "POP" noise. The key to this, and the reason I am pointing it out, is that this POP noise only occurs ONE TIME. It signifies that the engine has entered a fuel cut mode, and that none of the injectors is leaking excess fuel. Next time you drive, pay attention when you lift your foot, there is always this one critical POP as the ecu pulls into it's fuel cut mode. Any extra POPPING is caused by fuel entering cylinders that should not be there (leaking injectors). Also note the exhaust system needs to be hot enough to burn the fuel (it might not pop on decell when cold). There is plenty of oxygen in an exhaust system when the car is in decell (the engine is turning but no fuel is going in, it is literally pumping your exhaust full of fresh air. In fact you could potentially free-air calibrate a wideband sensor by allowing the engine to decel during fuel cut, I do not recommend this but the literature on wideband sensors does mention this technique).
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