Mazda - Tribute :: 2002 V6 - Releasing Throttle Does Not Result In Engine Decelerating Or Returning To Idle

We've owned this 2002 Mazda Tribute v6 for six years. Recently, the car has become somewhat dangerous to drive. While driving down the road at any speed, releasing the throttle does not result in the engine decelerating or returning to idle. Instead, it continues to hold the speed steady for 3-5 seconds before the RPMs drop. At lower speeds (~5mph or so) the car will even accelerate a bit, which can be a problem in tight parking situations. At higher speeds (e.g. on the freeway) releasing the throttle has no effect for a few seconds (ie it does not decelerate), which can be very surprising if you're responding to suddenly slowing traffic. The brakes can, of course, overpower the engine, but you do have to push harder to get the same braking effect as if you were not fighting against the engine. Releasing the throttle and shifting into neutral while driving causes the engine to rev quite high (3,000rpm) for the same 3-5 seconds before dropping to a steady idle, but it does allow you to brake normally. Shifting back into Drive while in motion but not touching the throttle causes the engine to decelerate the car normally. Touch the gas even briefly, though, and the engine will once again surge.

Oh, and turning the steering wheel from side to side, as you might do while parking, also causes the engine to surge, though not as much as it does when I touch the gas pedal. Also, I've disconnected the throttle and cruise control cables and worked the throttle plate by hand. Doing so causes the RPM to rise, as you expect, but it does not immediately return to idle when the throttle plate snaps shut. Instead, the RPMs stay around 12~1500 for 3-5 seconds before it drops to idle.

Over the years, I've replaced the throttle position sensor, idle air control valve, and other normal maintenance items. I used a meter to troubleshoot the IAC and TPS and used propane to look for vacuum leaks, but everything is within spec. There are recall notices related to unintended acceleration for these vehicles, but those are reportedly caused by interference between the engine cover and cruise control. I do my own maintenance and don't like engine covers, so I removed the cover from this car shortly after buying it; the engine cover cannot be causing this problem. I have disconnected the battery for an hour to reset the computer, but that has no effect. There are no trouble codes that show up on a OBD-II scanner, and the CEL is not illuminated. There are many internet reports of this problem, but thus far I've found none that indicate a solution.