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Old 11-29-2018, 12:53 PM   #221 (permalink)
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coolant sensor mod



I'm going to experiment with the coolant sensor signal to dial in a "spoofed" coolant temperature.

Shown in the pic above is a 0-5K potentiometer variable resistor and a simple on/off switch. The car's coolant temperature sensor has highest resistance when cold, with values similar to this plot:




Adding my circuit in parallel with the sensor will cause the ECU see a lower resistance when the pot is turned to a lower resistance setting.

Fellow EM member mjspiess did a similar mod (with a single value resistor) and documented the effect in his '95 Metro.

Jump forward to 0:50 to see the change in idle speed and GPH fuel consumption as he repeatedly turns his circuit on / off...



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Old 11-29-2018, 04:57 PM   #222 (permalink)
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So, you plan on using this to short cut the cold start / high idle situation? Do you have to constantly mess with the potentiometer while things warm up?
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Old 11-30-2018, 04:22 PM   #223 (permalink)
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I'm not too worried about high idle, because I don't idle.

What I'm hoping to learn is whether there's significant fuel enrichment above idle (ie. while driving) when the engine is cold.

I'm wondering if a valid test is to rev it to, say 2k RPM while cold and monitor GPH with the mod active vs. not active.
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Old 12-10-2018, 02:55 PM   #224 (permalink)
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It works... but does it work?

So the switch/potentiometer on the coolant temp sensor works as expected!



But does it actually DO anything useful in terms of MPG?

I monitored fuel consumption (GPH) using this method on a cold engine:

- Starting FWT = ~35 F
- holding engine RPM steady at 2k RPM
- check GPH value
- activate mod so FWT value is ~160 F
- check GPH value
- toggle the mod on/off several times

From what I could see, there was no change in the GPH reading.

I was talking to Tim about this. I forgot that the ScanGauge doesn't read fuel consumption directly from injector pulses. It's estimating it from air consumption. And if I keep the RPM steady, the air consumption isn't changing.

So really I needed to be using an MPGuino for the test.

It's definitely having some effect: if I let the (cold) engine go to idle when the mod is switched on (spoof the computer to think it's warm), it stumbles and dies. The mod cancels fuel enrichment (how much??) and fast idle.
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Old 12-11-2018, 06:09 PM   #225 (permalink)
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I did that on my saturn a few years ago, well maybe 10, but it was running too rich when cold, open loop. The diode on the map sensor also helped. I put another themistor ( temp sensor) in series with it on the head , and maybe a resistor or diode in parallel with that.

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Last edited by arcosine; 12-11-2018 at 06:15 PM..
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