[C3H8]

most cars today upon a cold start will run the engine rich to aid in starting and perhaps warming up the cat. if you can lean out the overly rich condition to make it more along the lines of 14.7:1 you will reduce fuel consumption during the warmup phase.

for this post i will be using degrees Fahrenheit.

i have found that if you change the resistance of the coolant temp sensor you can fool the computer into running the engine at any rich or lean ratio you want while (and only while) it is in open loop mode. on my car it runs quite rich when cold starting. this rich condition stays that way until the computer switches to closed loop. using a stopwatch and a obd2 scanner that monitors open-closed loop status, i have determined this takes about 3 minutes.

what i did was add a 2000 ohm 1/4 watt resistor in PARALLEL with the coolant temp sensor. this had the effect of increasing the cold water temp read by the computer by 30 degrees. now when i start the engine, it no longer gives a 1 second puff of black smoke upon startup. the engine runs just a touch rougher during the 3 minute warmup but it isnt a big deal to me.

you are probably thinking to yourself, yeah if i increase the temp by 30 degrees when its cold, then it must increase 30 degrees when hot!. this is not true and i will explain why.
coolant temp sensors change their resistance on a non linear scale. the resistance increases or decreases exponentially depending on wither its cooling down or warming up. my 2000 ohm resistor increases the temp read by the computer by 30 degrees when the engine is cold, but it only increases the temp read by the computer by 5 degrees when the engine is fully up to operating temperature. this means i can leave the resistor connected all the time and never need to disable it. i also have not had a check engine light caused by the resistor. i have driven about 5000 miles now with the resistor hooked up and have not had any problems.

the way you find out exactly what resistor value to use is by wiring up a potentiometer in PARALLEL with the coolant temp sensor. try to use a with a value of 10Kohms full and 0 ohms when turned the other way. this way you have a lot of wiggle room. the pot is available at rat shack for a few bucks. the second half involves a obd2 scanner which is capable of showing the computers instant open loop-closed loop status as well as showing an instant readout of the coolant temp sensor in degrees.

i dont know how to measure fuel economy gains from this mod but its got to count for something.