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This is an article I’ve been meaning to write for a while, but circumstances recently have delayed my writing due to preoccupation.

Circumstances haven’t changed, but lets get back to work anyway and que sera sera.

This article is hopefully going to steer you in the right direction regarding ways of fooling your cars computer into giving better gas mileage. I know gas prices are down right now and human nature being what it is, saving money on gas has a much lower priority. Just remember, gas prices will go back up and now is the time to make the modifications while you have spare cash available thanks to the current low prices.

These first modifications are cheap and easy to implement and are a good starting point in understanding the logic and methodology I use when dealing with modern emission control systems.

I equate working with car computers to the human brain. Brain surgery is complex and dangerous. But by modifying the signals the brain receives we can make it see thing in a different way. The sensors attached to the engine are like our eyes and ears. Put on sunglasses and things seem darker to the brain. Cover our ears and sounds are less intense. The mods to the sensors work in the same way by telling the cars computer that temperatures etc are different to what they really are.

Please bear in mind that in some countries/states any modifications are illegal due to ‘big oils’ control of government. That doesn’t mean you shouldn’t do them, just make sure they are done tidily and out of sight

Secondly, as with any modifications, do not be greedy in your search for economy. Used sensibly these mods will save you gas, but taken to excess can damage your engine by weakening the air/fuel ratio beyond the point where engine damage can occur. No economy in a burnt out motor.

The standard used by most manufacturers for air/fuel ratio is 14.7-1. This gives a reasonable balance of emissions between the hydrocarbons in the exhaust and nitrogen oxide which is generated at higher temperatures.

This standard has the added side benefit for manufacturers by giving them some leeway in the tolerances in building engines. Although current manufacturing processes are reasonably accurate, there will always be some variation in final assembly. It also makes catalytic converters a necessity to burn up the excess gasoline generated by slightly out of tolerance motors.

Although 14.7-1 ratio is the target, 2 identical cars may vary by anything from 13-1 up to 16-1 when they leave the plant. Both cars will run fine and when warmed up will have very similar fuel economy thanks to the oxygen sensor which is much more precise in its manufacture and constantly adjusts the air/fuel ratio back to 14.7-1 once the engine is warmed up.

Where the cars will differ is in the warm up period. A low air fuel ratio will use more fuel during this period than a a car with a high ratio. Just like using excessive amounts of choke on older cars.

Although it is possible to adjust the oxygen sensor values, that involves more complex electronics and will be covered in a later article. Driving style also needs to be taken into account when making modifications in that area, and also has a major impact on fuel economy in general.

For now I will just cover three simple mods that can be made by anyone with the ability to cut a wire and make a new connection in it.

The first mod will give more gains to people who make many short journeys as it only affects the warm up period before the oxygen sensor takes over.

The other two mods should be done in combination as they not only affect fuel consumption but also the ignition timing.

Modification #1 is to make the computer think that the engine temperature is higher than it really is. The engine temperature is sensed by a small unit called a thermistor that is screwed into the cylinder head, usually near the thermostat. This unit is a simple variable resistor that reacts to temperature and reduces resistance as the heat rises. There are only 2 wires and we will be adding an extra variable resistor between them, effectively reducing the resistance the computer sees. This is called a parallel connection and gives two paths for the current to flow.

For Chevrolet engines a 3.9k resistor is a good approximate value, but can lead to cold start problems in very low temperatures. For this reason I prefer to use a variable resistor or potentiometer rated at 5k ohms and dial it in according to circumstances. A switch can also be used to cut out the modification on excessively cold days, just for long enough to start the car. Fords seem to prefer a slightly higher value, around 5k ohms and for these engines an additional 2k ohm resistor can be added in series with the 5k ohm potentiometer.

By telling the computer that the temperature is higher than it really is, the computer will reduce the amount of fuel slightly to compensate and save you money. Not a big saving, maybe 1 or 2 mpg overall depending on the number of cold starts you make but a saving nevertheless.

This modification can be made by simply adding a resistor across the two wires by opening up the insulation and soldering the resistor in place. This makes the mod permanent but is easy to hide. Okay for those who never experience excessively cold weather. (top part of the diagram) You can buy this mod on ebay in a fancy case for 10 bucks or so, or just pick up the resistor for 10-20c in your local electronics store.

For a more controllable system, using a variable resistor and (optionally) a switch wired through to the driving position is a better system. This needs more wire and somewhere to place the switch and control knob but gives the driver the ability to tune the system to get the best results. (lower part of diagram). Cost should not exceed 2 or 3 bucks for the components and wire.

If you plan to follow through on this and the following mods, a small case or panel that can be place on or under the dash is ideal. Get one that has room for 4 variable resistors and 4 switches on the front panel and space inside/behind for a small electronics board and you will be set.

I think this article is long enough so will save the other two mods for a future article. One half is simply a copy of this mod, but connected to the air intake temperature sensor instead. Only use this on it’s own if you drive gently as it not only reduces fuel by making the computer think that the incoming air is warmer than it really is, it also retards the ignition timing and makes the engine run slightly hot. The other mod is different in layout but makes the computer think the air is thinner and advances the timing to compensate.

As I’m splitting this article into 2 parts I will also cover the oxygen sensor mods in the next article. Just using these mods on their own will give a small saving, maybe 5 mpg, if you drive sensibly. In combination with a HHO generator they will allow you to make full use of the extra fuel created from water by giving you control over the cars computer.