How to: reduce the rich condition of a cold start. - Fuel Economy, Hypermiling, EcoModding News and Forum

C3H8 · 07-30-2010, 03:28 PM

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.

Phantom · 07-30-2010, 05:29 PM

I just have a quick question the 3min warm up for the car is that from a cold start and idling?

Why I bring this up is my Grand Prix also takes about 3min to change over to Closed Loop, but I have noticed that if I start driving right away it changes over much quicker.

C3H8 · 07-30-2010, 05:44 PM

yeah, 3 minutes is what it takes from cold start and idling.

and yes if you start driving right away it will change over quicker. but a lot of people like my self tend to let the engine idle for 20-30 seconds after a cold start to give things a chance to start working, and the oil a chance to circulate before putting load on the engine.

Daox · 07-30-2010, 06:12 PM

You have oil pressure almost immediately after starting an engine. I see no reason to idle. I just start and go almost immediately.

RobertSmalls · 07-30-2010, 07:03 PM

Simple, clever, probably effective. I may have to try something similar.

How much did it increase your temperature reading when hot?

My radiator fan kicks on and off based on the coolant temperature reading. I only have a 5-10°F window between the radiator fan and the thermostat.

ShadeTreeMech · 07-30-2010, 07:16 PM

I'd just start driving. A modern engine wil be ready for a load in the time it takes to put it in gear after starting. If you're worried about it much, just be easy on it for the first bit.

My car reads closed loop in a matter of about 30 seconds. Is your o2 sensor a wideband or narrowband? IIRC the wideband is a bit quicker to start reading.

Phantom · 07-30-2010, 07:36 PM

Ok I just checked my car on the way home after sitting for 6hrs the car was reading the ECT (Engine coolant temp) at 100F the air temp was ~93F. I started driving as soon as I started the car it took 46.4sec to enter Closed Loop.

I'm not saying that this will be the same for every car but I do know that the fastest way to warm a car up it to drive it. In the winter the car can idle for ~13 min to get warmish or I can get heat out by driving 3mi.

Thanks for the info and remember the OL/CL stuff I'm going by is for OBDII it could be completely different on OBD1 or OBD1.5

Laurentiu · 07-30-2010, 09:34 PM

Quote:

Originally Posted by RobertSmalls

Simple, clever, probably effective. I may have to try something similar.

How much did it increase your temperature reading when hot?

My radiator fan kicks on and off based on the coolant temperature reading. I only have a 5-10°F window between the radiator fan and the thermostat.

Well, the device could be rigged to a button ON/OFF, it would be possible , no ? Also , even better I would think you could put a timer in the circuit, one that could be set to let's say 3 minutes. You start the car, the timer starts as well (like staircase-lights) and then it stays on for that initial minutes you need a leaner burn, then turns itself off to avoid starting the radiator-fan at lower than needed temps.
I don't know, I'm not mechanically inclined so my idea might not work..

RobertSmalls · 07-30-2010, 10:41 PM

The Insight loves the heat, and I'd like to know why. You can get into lean burn much sooner in 90°F weather than 70°F, and I see an improvement in FE that air density alone can't explain.

I've been thinking about replacing my ambient air temperature sensor (an IMA system input) with a resistor corresponding to ~150°F. I'd like to find out if a coolant temperature sensor lie-box would also be helpful.

Now, Propane, if your 3kOhms adds 30°F during the summer, what's it going to do in the winter?

I'm also very curious about how rough my precious, sensitive Insight will run when it misjudges the coolant temperature. I guess we're going to find out.

P.S. 0 Ohm pot in parallel = instant CEL, yes? Coolant temperature sensor reporting absolute zero.