Fluctuating idle on deceleration in 3.8 V6 auto (ECOTEC and non-ECOTEC)

[cliffo78]

Can anybody shed some light on this:

I have noticed in all of my past and present Commodores equipped with V6's (VR, VT, VX, VY) that when taking my foot off the accelerator when cruising at 80 (for example), the tacho indicates the engine speed seems to drop and then momentarily increase and the drop and then increase again, and this continues to occur until I slow to around 60. The speed ranges by only a small amount, maybe 300-400rpm's at the most, and unless you're watching the tacho you wouldn't notice, except in the case of my VR which had a 2.5" exhaust and you could actually hear the engine "revving" itself.

I'm sure that this happens for a good reason, the ECU must purposefully cause this fluctuation to overcome some other fuel/air or exhaust/vacuum pressure issue that must be inherent with the pre-Alloytec V6 setup.

It appears to only happen in auto's, as my brother's VS manual V6 doesn't appear to do it.

Does anyone know what's going on here?

[Dowling_s]

I had the same problem with my VS and it seemed to me to be the lock up torque converter causing this.

[drift-it]

well i had the same when i had a 2.5" lukey with 1 resinator at the rear!!! however problem dissapeared once i fit extra muffler...i thought it might have beens something to do with back pressure???