Quote:
Originally posted by Porsche944
Oops! I forgot to mention that the MPG gage uses a signal that is proportional to injector duty cycle. The longer the injectors are open, the lower the indicated MPG.
|
More than that.
It receives a signal from the FI brain to tell it the injector duty cycle (from which it can compute the precise amount of fuel delivered assuming a stock fuel pressure regulator) and it also receives a signal from a hall-effect sensor in the speedo (on the early cars) or from the speedo sender (later cars). At speeds above 5mph (I think this is the threshold speed, I'm not sure) the mpg guage reads miles per gallon. Below the threshold speed, it reads in gallons per hour (there is a grey scale that shows gph markings).
The usual cause for a mpg guage malfunction is corrosion on the terminals to it from the speed sensor causing it to read in gallons per hour all the time or the typical guage grounding issue that can cause the same symptom; no reading from the speedo so no trigger to change to reading mpg and not gph.