coolant temp sensor tweaking?

[MetroMPG]

we all agree that warmer engines are more efficient (thus block heaters, grille blocking, higher temp thermostats, etc.).

ODB2 systems also use the signal from the coolant temp sensor to manage A/F mixtures during engine warm-up:

Injector pulse width is heavily dependent on this sensor, causing engines to run very rich at startup (remember the manual choke?). Fuel enrichment is usually tapered off around 150°F. (source)

we need a certain amount of enrichment for cold starts, but we may not all need as much enrichment as the computer demands. so what about spoofing this sensor?

has this been discussed here before? has anyone tried it?

the benefits of this mod would be similar to synthetic oil: sensor spoofing or partial-spoofing would only reduce consumption between a cold start and normal operating temp.

a variation of this would also be doable on mechanical/vacuum operated systems.

[CosmicMC]

I don't know for sure if this will help anything or not, but I'm leaning on it won't help anything. Here's why:

If you lean out the fuel delivery rate between startup temp and 150*, the car may not start or run as well as it should. That's why chokes were invented. Let's say you do lean it out a little bit. If it takes longer to start the engine, there is more drain on the battery and more wear and tear on the starter. The engine will have to work a little harder to charge up the battery, and that may eliminate any MPG gains right there. More starter crank time means more wear on the starter. If the starter goes out sooner, you've wiped out any other gains you could potentially get by leaning out the mix for a few minutes at every cold start. It takes more energy to get the materials to make the starter, engineer it, manufacture it, deliver it, and market it. What is the cost of that energy? More pollution? Overall more fuel consumed in general, even though it may not be your car consuming it?

Let's say the start time is the same with the leaner mix. What if the car doesn't run as well as it should? An occasional misfire due to a lean A/F ratio will make the car lose power. If that happens, the instinct most people have is to push the pedal down to get more power to make up the difference. There goes your efficiency gain. What about extra wear and tear on the engine from all this? There goes some more.

It's also possible that the car will seem to run fine with a slightly leaner mix. The question then is, was it worth making the change? What if your modification has an unintended consequence, such as when the engine is overheating, say at 260 degrees, the cooling temp sensor still thinks it's 220? You continue to drive the car while it's overheating and destroy the engine.

In my mind at least, the potential, theoretical rewards do not outweigh the risks, but of course I'm open to other ideas and actual scientific explanation.

[Matt Timion]

CosmicMC, you hit the nail on the head with something I was thinking of before. I was tempted to get one of those Zemco Fuel Saver devices. They killed your engine when you idled and started it up again when you pushed on the gas. In theory it makes sense, but when you factor in the cost of a new starter every year it really adds up.

I would much rather just have an engine that could switch to 8 valves when I was idling, or have one of those cool hybrid systems that use the electric motor during idle.

[diamondlarry]

It's also possible that the car will seem to run fine with a slightly leaner mix. The question then is, was it worth making the change? What if your modification has an unintended consequence, such as when the engine is overheating, say at 260 degrees, the cooling temp sensor still thinks it's 220? You continue to drive the car while it's overheating and destroy the engine.

I would say that if you did a sensor tweak like that it might be a good idea to put two sensors in and wire one of them to a seperate temp guage. That way, you could tell when the engine did overheat. I was thinking about the coolant sensor mod but didn't want to do that because of your above concern. The problem with my idea would be trying to figure out how to get two sensors installed. Maybe a tee and some fittings?