audijim113

A few months ago I posted about surging idle during cold start up. It would surge 3-4 times for 30 seconds, then settle down with no adverse affect afterward when driving.

There were lots of people who chimed in with advice but no real solution. It wasn't a vacuum leak, MAF or apparently a dirty butterfly valve on TB. As the weather got colder (now about 30 degrees here in Michigan), my problem "went away". Since it went away on it's own, there's a good chance it will come back on it's own. I'm positive it has something to do with whatever causes higher idle on start up based on temperature.

A few decades ago, carburetors had "automatic chokes" which was essentially a coil spring heated with a tube from exhaust. As spring expanded, it mechanically let the idle slow down. Since we've come a long way from a carburetors or automatic chokes, I don't think I'll find a carburetor under the hood.

I'm assuming the idle control is somehow tied to a temperature sensor and to throttle positioning. However, I can't find any kind of explanation for this. Can someone enlighten me? Is it a function of the ECU? Is there a solenoid somewhere? Other than directly connected to the gas pedal and my big foot, how exactly does throttle positioning work?

twentysevenlitres

Fuel injection basics...
ECU's operate in two basic modes:
1. Open Loop
2. Closed Loop

Open Loop is basically the 'choke' setting.
The ECU operates on a program which will run the engine with very limited sensor inputs. Crank and Cam Position Sensors, Throttle Position Sensor, and not much else...
On some engines it can also limit revs, timing and maximum fuel so as to limit engine power when cold to reduce potential wear.

Closed Loop is the regular running program.
It's cuts in at a pre-determined temperature setting and starts getting feeds from the MAF and O2 sensors to better manage the fuel ratio.

The fly by wire throttle on the D2 engines (post Facelift) uses a potentiometer on the accelerator pedal to tell the ECU how much throttle you are requesting. The ECU then activates a motor on the throttlebody to (in most cases) provide the requested amount of throttle. The ECU also controls idle via minute movements of this throttle (thus the recommendation to clean the throttle plate - dirt can reduce airflow below tolerance).
Cable throttles differ in this regard as they have an Idle bypass tube and control valve.