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[Bernd]

While our 1994 Caldina 3S-FE injected engine idled above 1500 while cold, our 2005 Noah direct injection 1AZ-FSE engine idles well above 2000. I find it annoying, although the transmission in either case engages rather softly, a firm foot on the brake is needed, and the exhaust smells like a lot of unburned gas.

It may have to do with passing a stricter the emission testing cycle for the type approval. However, in the EU the testing omits the first 40 seconds after a cold start and the US test switches the exhaust gas collection bag after 505 sec. (Source: Bosch Automoitive Handbook, 22nd edition) Therefore a faster warm-up cycle may be better for passing, however, idling at 2400 pushes out at least 3 times as much exhaust gas than idling at 800. I am not sure it shortens the warm-up time to 1/3, though. Anyway I drive off with a moderate foot as soon as possible, that really shortens the warm-up time.

On the Caldina, I half-fixed this hi-idle by bridging the temperature sensor (THW) with a small 2.2 kOhm resistor, right at the connector in the engine room. After a winter garage start, high idle was just above 1000, but when the car was frosty outside, the cold engine would not start until I unhooked the parallel resistor.

On the Noah, I just recently installed a makeshift "choke" (ha!) by connecting a 10 kOhm potentiometer (volume knob) in series with a 1kOhm resistor parallel to the THW input at the engine computer. To start the engine, I turn the choke all the way up (highest resistance), the computer sees the cold engine and adds the fuel it needs to start. As the engine idles, I turn down this "choke", tricking the computer to see a higher engine temperature. I can nicely bring down the RPM below 1000 and just leave it there. Then I put it in D and drive off, leaving much less of a stink in the area. This control can bring the idle all the way down to 800, but then I see surging, as the mixture is really too lean.

When the engine is hot, the resistance of the stock temperature sensor is below 200 Ohms, so my add-on has no significant influence. It is active mainly during the "open loop" cold phase when the computer does not yet have a signal from the oxygen sensors and eyeballs the fuel amount anyway.