The situation: When the cold engine is started, the fuel/air mixture is enrichened by adding extra fuel. The colder, the richer. This is to make the car more driveable, so it won't sputter and shake, and most important for emissions, to get the catalyst to working temp as quick as possible. The superfluous fuel burns inside the catalyst and that heats it up real quick.
There is generally very much more enrichment than really necessary. A bit less would save lots of fuel during warm-up.
Now, we can't re-program the ECU for that, but there is something else that can be done! If the ECU gets informed by the coolant sensor that the engine is a little warmer than it actually is, it will produce less enrichment and save gas. However, when the car is warmed-up, the sensor must tell the ECU the correct temperature in order for the engine to operate properly.
How is this done?
The coolant temperature sensor is a simple NTC. Most cars use this method. The resistance of the NTC increases when cold, and decreases when hot.
The Mirage sensor has about 2500 Ohms at 20′C / 72′F, around 180 Ohms at boiling point, and about 15 000 Ohms (15K Ohms) at -20′C.
With this mod, a second sensor is added parallel to the first via a series resistor of 2700 Ohms. The series resistor is of similar resistance as the temp sensor at room temperature. This resistor is to make the sensor-circuit report the correct temperature when the engine is approaching, and when it has attained normal warmed-up state. Important: The ECU must receive correct temp notification at operating temperature.
See circuit diagram for details. The two added parts are on the top part of the diagram.
I got the second sensor at a junk yard. It is electrically identical to the original sensor, but mechanically of somewhat different shape and size.
Below is a photo of the secondary sensor with plug and wires, soldered onto a copper bracket. The copper was a junked piece of a copper water pipe, soldered onto it for the sake of better heat conductivity:
Ideal would be to use the same type coolant temperature sensor the Mirage came with, but this one perfectly serves the purpose.
The original sensor is installed on the left side of the cylinder head, and although it is fastened on the head, most of it is immersed in coolant. You can find it between the engine and the battery. There is one brown, and one sky blue wire attached to it by a removable plug. In the following photo, it would be in the far right, but can not be seen here because there is other stuff in front of it. The new second sensor is in the middle of the photo.
The second sensor was soldered to a copper bracket and bolted onto the cylinder head. That ensures ideal heat conductivity, because the second sensor needs to report engine temperature correctly, just as the original one.
Note: a strong and heavy soldering iron is needed for soldering the sensor to a copper bracket! That way, the sensor does not have to suffer from high soldering temperature too long. After soldering, be sure to check the sensor electrically, because too much soldering heat can wreck it. Check there is no internal conductivity between the connectors and the case.
It was then fastened to the cylinder head where it seemed convenient. Thus it can be assumed it has and reports the same temperature as the coolant and the original sensor.
The second sensor is connected by a plug/jack, its two wires leading to the two wires of the original sensor via the 2700 Ohms series resistor. See circuit diagram. All wiring was put neatly into the cable assembly. All of the new electrical connections and the resistor were perfectly insulated. The cable assembly looks like it has been manufacturered that way, and not been touched since.
Note: There has been a recall in the USA to reprogramme the ECU for reduceing the way too excessive enrichment of the cold engine. If they offer it for your car, go get it done! This mod goes a bit further.
This mod will not just reduce enrichment a bit when cold, but also further reduces excessive enrichment during the entire warm-up period and thus saves fuel. I have have not had any driveability issues whatsoever. In Europe, the above mentioned recall was not offered.
The fine print:
1) As usual, everything at your own risk.
2) If you are not used to work with a soldering iron, or not used to working with electrics/electronics, or have not much notion or little experience with car electrics, please don't do this yourself!
FYI here the electrical curve of the second temp sensor, being electrically identical to the OEM sensor.
Based on the info from above, a knowledgeable person could if they wanted recalculate this mod and exactly understand what it does:
See the table Eggman calculated and added in post number 18.
It clearly shows the modified coolant temp as reported to the ECU.
Tried different values for the series resistor in my 1.0L Mirage, and this is what happens:
At 2.500 Ohms everything is fine.
2000 Ohms also works well.
1750 Ohms or less is no good, because the radiator fan will come on too early during warm-up with the radiator still being cool and the thermostat closed. Remember, this car has only one single temperature sensor for everything!
To sum it up:
1) any value between about 2500 and 2000 Ohms is fine.
2) This mod goes much further than the reprogramming of the ECU (USA recall) for enrichment reduction.
3) My car is a 1.0L without(!) EGR valve. Yes, the 1.0L engine without AS&G is sold in Europe without EGR, but the 1.0L with AS&G does have EGR.
I think using valve-overlap is maybe how they keep NOX down at testing-relevant revs and load levels, unless they VW their way out to get better figures...