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By Jim Kerr

A reader sent me a note lamenting that his 2003 compact car with a 2.0-litre four-cylinder engine was getting only 14 litres per 100 kilometres (20 mpg). He compared this to an older carbureted 1971 Rover that used to get 7.0 litres per 100 km (40 mpg). He was ready to convert his compact car over to a carburetor and regular distributor in an attempt to improve his fuel economy, figuring it should double the fuel economy. He figured that at least the carbureted engine could be tuned, while his fuel injected compact car had everything programmed into the computer and could not be corrected.

So what does it take to tune a modern car?

While throwing out the fuel injection system and installing a carburetor may seem like a good solution when problems arise, it really isn’t. There are those who fondly remember the power of carbureted engines, but they also tend to forget the hard starting when cold, sticking or inoperative choke mechanisms, the need to adjust them every season, and the poor fuel economy during stop and go driving.

It is possible to achieve better fuel economy with a carburetor than fuel injection in some circumstances. Because of emission regulations and the computer programs required to meet those regulations, fuel injection systems have become the standard. Fuel injection systems have to operate the engine at a 14.7 to 1 air/fuel ratio; the catalytic converter does not work at ratios richer or leaner than that or tailpipe emissions will be high. With carbureted engines, it was difficult to maintain that 14.7 to 1 air fuel ratio, and that is the primary reason for the switch to fuel injection. A big benefit of the switch was better starting and driveability in all weather conditions.

While it is possible to tune the fuel injection computers for leaner air/fuel ratios to get better fuel economy, this increases emission output and is not an easy task. Repair shops are not allowed to modify emission systems, and that includes reprogramming computers for different air/fuel ratios, although it is not illegal for owners to do it as long as the vehicle still passes emissions testing in areas where it is performed. There are several “custom” programmers across the country that modify fuel injection computers or better performance and fuel economy but they typically only work with popular applications and most likely would not have the software to modify this compact car. Also, the programs and computers are supposed to be used on off-road or pre-emissions regulated vehicles, although we all know some of them are used in modern street driven vehicles.

So if you can’t change the program, how do you improve the fuel economy? Unlike tune-ups for old cars, modern vehicles are particularly sensitive to spark, vacuum leaks and engine temperature.

If a spark plug starts misfiring even a little due to a worn sparkplug, bad plug wire or poor electrical connections, it affects not only that one cylinder but all the others too. Unburned oxygen passes through the engine and into the exhaust system. The oxygen sensor measures this unburned oxygen and the computer “thinks” the engine is running lean. It reacts by injecting more fuel in all cylinders to bring the mixture back to specifications but in reality, the computer is being fooled. The engine isn’t running lean, it simply has a misfire.

An oxygen sensor contaminated with oil, antifreeze, silicon or carbon can also cause poor fuel economy. Technicians can watch the sensor readings with a scan tool or voltmeter and determine if the sensor is reacting properly. Replacing a poor sensor with a good one can increase fuel economy dramatically. I have seen it triple the fuel economy on really bad vehicles.

Vacuum leaks are monitored by air flow sensors and pressure sensors on the engine. When the computer sees low engine vacuum it is programmed to think the engine is under load and it adds fuel to compensate. A vacuum leak, even a small one, can create poor economy.

Ensuring the engine operates at the correct temperature and the thermostat is good will improve fuel economy. Cold engines require more fuel, so quick warm ups will help save fuel.

Cleaning throttle plates, replacing PCV valves and an air filters are still part of a modern tune-up, but when fuel economy drops, there are other things to look at before blaming the computer.

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