coolant sensor mod

[MetroMPG]

<img src="http://www.injectionlogic.com/images/CTS.jpg" width="200" align="right">

here's a mod that offers a small potential mileage improvement under engine warm-up conditions: spoofing the signal from the water temp sensor (WTS) / engine coolant temp (ECT) sensor.

The coolant sensor is often called the “master” sensor because the powertrain control module (PCM) uses the sensor’s input along with that from the oxygen sensor to go into the “closed loop” mode of controlling the fuel mixture.

The coolant sensor also controls a number of other systems, including:

- Start-up fuel enrichment on fuel-injected engines.
- Spark advance. Spark advance is often limited until the engine reaches normal operating temperature.
- automatic transmission torque converter clutch lockup

... and more. (source: You Can Master Coolant Sensors)

the theory behind this mod is that the manufacturer has played it conservative in programming the PCM's response to engine temperature during warm-up, i.e. erring on the side of drivability.

the goal of the mod is to alter the WTS signal to "hurry up" the warm-up process as far as the PCM is concerned, while still maintaining satisfactory engine performance.

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the mod requires a basic understanding of electrical resistors (the WTS is a heat variable resistor after all - a thermistor): the PCM feeds the WTS a fixed reference voltage in one side, and reads the return voltage from the other wire.

most coolant sensors' resistance decreases as they warm up.

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an elegant approach to this mod is described at teamswift.net: one metro owner attached a second WTS - externally - beside the original one. it's in direct contact with the housing where the factory one is installed, so they are heated together (though obviously not to the same extent).

he then wired them in parallel, effectively cutting the resistance in half - the PCM sees his engine warming up apparently twice as fast as it actually is.

he claims no drivability problems. (he actually says the amount of soot around his tailpipe has decreased since adding this mod.) he lives in a cold climate (germany) and his commute is short (15 km), so he says he has seen a positive mileage improvement - though I suspect the improvement would be quite small).

to prevent the ECM from reading a false overheating situation once the engine actually reaches normal operating temps, he wired an appropriate value resistor in series with the additional WTS. this retains enough resistance in the modified circuit once warm and the temp gauge reads "normal".

here's the metro schematic:

<img src="http://metrompg.com/offsite/ECT-Mod-01.gif">