Quote:
Originally Posted by JimSmith
Idle speed control is pretty crude on these vehicles - they have no microprocessor involvement - so they don't respond particularly precisely to any and all conditions. In Park there is relatively low drag load on the engine from the transmission, even when the transmission fluid is cold, compared to when you are in reverse or drive. As the engine heats up it burns the fuel a bit more efficiently (slightly more torque) and the transmission fluid drag at low engine speeds drops, so the idle speed will increase. Which means your idle speed doesn't drop as much. I would guess it still drops a tad from neutral, or park speeds to drive or reverse.
Jim
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My thoughts are threefold:
1) When the governor is working correctly, the idle speed is maintained with great accuracy. During a cold start, as the engine warms in Park, one can observe the rpm's rise ever so slightly via sound.........and then immediately fall back to the predetermined idle speed. This process happens repeatedly during warmup as the governor is continually reducing the rack as the friction in the engine decreases. I'd hazard a guess that it can control rpm to an accuracy of about +/- 50 rpm.
Therefore, if the governor is working correctly, the change from Park to Drive should not result in a drop of more than 50 rpm.
2) Like any mechanical system, as age settles in, the capability of the governor to perform the desired task becomes more difficult. Therefore, certain engines may exhibit the exact behavior of the OP, whereby the governor is unable to handle the increased load of the transmission and the rpm's drop by more than 50 rpm due to insufficient fuel.
3) The range of the governor at idle is uncertain. The governor may not have sufficient range to handle the load placed by a cold transmission without an rpm drop.