Question about idling
 

Hi all,

During the first start of the day, my friend's 1984 300td idles regularly in park, but then when he shifts into reverse or drive, the idle lowers very noticeably prior to actually moving the car. This only occurs during cold starting. What causes this?

Thanks in advance for any replies.

The Engine Oil and Transmission Fluid are thicker when they are cold and it takes more energy from your Engine to circulate them.

The above added to the fact that with a cold Engine your Fuel is not burnig as well and a lot of the heat from what is burning is going to warm up you Engine and Coolant instead of being turned into powere that rotates your Engine.

If your Injectors are not within specs that causes them not to atomize as well you may see a noticalbe differance of better performance when the Engine is up to operating temp.

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

The changes in idle are much more evident if the idle speed is set on the lower end. Mine I have set around 750 and it doesn't change much even if its cold/warm. Its lower when it is REAL cold out, thats it.

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.