3.1 L Engine intake or exhaust backfire only in Park or Neutral ! HELP !!!

Hi Everyone! 94 Cutlass Supreme 3.1 L V6. The engine sputters ONLY when in PARK OR NEUTRAL. Sounds like an intake or exhaust backfire. It's not SUPER loud but it is very very noticeable.
Sometimes it won't do it but most of the time it does. If you give it a quick rev and let off the accelerator, it will ALWAYS do it (but ONLY in park or neutral) and much more loudly / violently. At first I suspected bent or sticking valves but it stops immediately when you put it into ANY gear (auto tranny). It will not do it under any conditions when in gear, i.e., in gear at idle speed, accelerating, decelerating, any constant speed, heavy load, hot or cold. But as soon as it goes back into neutral or park, it does it immediatley. Due to that, I figure it is not sticky valves.??? It runs VERY well when in gear under all conditions. Has about 129K miles on it with 80% highway miles. Has always had Mobil 1 synthetic oil. I took off the ignition computer and took it to NAPA. They tested it 3 times consecutively (to make it get hotter each time) and it passed each test. Is there a different operating 'mode', via computer, that runs the engine when in park or neutral versus when it is in gear? Any help would be much appreciated. Thanks in advance!
Mechanical Engineer and Backyard Mechanic
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How many rpms does this occur at in neutral?

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Hi Shep, Thanks for responding. It does the same thing in park as it does in neutral. It will do it at idle speed. On the few occasions when it is in park or neutral at idle speed and NOT sputtering, if you give it a quick shot of the accelerator and let off, it will always sputter (intake or exhaust backfire?) bigtime. No, it is not a rev limiter doing it because you can just raise the RPM to 1500 or so and let off and it will sputter. Usually I just start the car and immediately put it into gear because 95% of the time it backfires at idle speed in park but NEVER if it is in ANY gear. There is nothing you can do to make it misbehave when it is in gear...... It runs like a champ when in gear. There must be a different mode that the engine runs in when some computer gets the signal from the transmission that it is not in gear???
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Possible when the engine torques in gear a harness may be pulling apart or grounding, say maybe around the ign module or any pcm related ign harness.

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Thanks Shep, I'll go move some harnesses around while it's running in gear and see if I can get it to sputter.
Any other Ideas out there???
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On Jul 2, 2:34 pm, snipped-for-privacy@comcast.net wrote:

I ran the car in gear with parking brake on and tried to get it to sputter by moving harnesses, etc., around but failed to make it misbehave. Then I ran it in park and moved the harnesses, etc., around to try and make it NOT backfire but failed to make it stop this sputtering.
Any more ideas would be greatly appreciated!
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