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Maybe you don't know all about pre-combustion. "Back in the day" when cars had hand throttles, and spark advance levers on the steering column, you found out that if you ran with the spark retarded, the engine traded output power for heat at the normal RPM range. If you ran with the spark too advanced under low rpm load, you ran the chance of melting your pistons. In the 50's we got centrifugally and vacuum advanced distributors to reduce human error. But advance curved were tailored to specific octane ratings. The higher the octane the slower the fuel burned, the slower the flame front moved and therefore the more advance the ignition could be. You would dare run your GTO on low octane fuel. Of course it only cost $0.35 per gallon.
The auto boys in the 80's decided that with the multitude of fuel grades, one way to insure that engines could run on whatever fuel used, was to install a knock sensor. If the fuel grade was too low and started the flame front too early in the cylinder, it could be reduced by starting the spark later (retarding). This is because spark timing is now done electronically not mechanically. However, there is a price to be paid. With lower octane fuel, the injectors still inject the same amount of fuel, but the ignition starts too late to burn it all,(knock sensor retards the spark) millage and power suffer and the engine runs hotter. Still this is better than having the flame front run into the top of the piston before it reaches top dead center. That generates tremendous heat in the cylinder which melts pits into the piston and eventually through the piston.
Long story short, with knock sensors, using lower grade fuel means reduced power and millage and everything runs hotter especially the cats.
I doubt that this has anything to do with the fuel light.
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