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">

[DRW]

Hello, yes I know I'm digging up an old post, but I wanted to share my experiences with wiring a 10k pot to my ECT sensor.

I originally did this to my '92 Eagle Talon AWD because I had a cooler thermostat installed and I wanted to fool the ecu into thinking the engine was warmed up so it would stop enriching the A/F mixture all the time. I have a logger that reads ECT(engine coolant temp, aka WTS) and IAT(intake air temp), among other things. I used IAT as a reference since both would be the same when the car has been sitting overnight. I adjusted the 10k pot until the ECT read about 15 degrees higher than IAT. Then I tried to start the car. It took a couple tries to start it. Observation #1, ECT has a significant effect on startup enrichment. So I readjusted the pot to read 10 degrees over IAT.

Then I drove the car until it was warm and ECT stabilized, indicating the thermostat was open and regulating ECT normally. Normal temp was about 185 degrees, and with the 10k pot the ECT read 187! Observation #2, this is not a linear sensor curve. Eventually I had to reduce the ammount of adjustment made by the pot to the point where it wasn't really worth it. The car was still hard to start when cold, and there wasn't enough change when warmed up.

Your car appears to be quite different. Heres' your chart;
WTS resistance/temp:

F --- C -- OHMS

210 - 99 - 190
160 - 71 - 400
100 - 38 - 1250
070 - 21 - 2350
040 - 04 - 4780
020 - 07 - 8100
000 - 18 - 14650
Lets say you subtract 200 ohms with the pot. At 160 degrees actual temp your ecu would think it's about 205 degrees, a difference of 45 degrees. At 40 degrees actual, the ecu would read 43 degrees. It looks like you could adjust the resistance by 100 OHMs and see a difference. So in your case you could subtract just a little resistance without affecting startup too much, then as the car warmed up the difference would get bigger. There shouldn't be as many compromises to driveability at higher temps. Since the ammount of adjustment is small I'd recomend a pot with a smaller rating to keep it from being too sensitive to small adjustments. I'd say give it a try and see how it goes. It didn't work so well on my car, but every car is different.

[onegammyleg]

Hi DRW -&#168;Hello, yes I know I'm digging up an old post&#168;

Now thats an old post !

[DRW]

Har har har a-hawhaw