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Randal S

Randal S 8/25/2017

1997 Mercury Grand Marquis GS 8 Cyl 4.60L

Body & Interior

Average Economy guage shows 99.9 mpg

1997 Mercury Grand Marquis GS, extremely well maintained, One owner, 222233 miles, digital instrument cluster. No scanner error codes.
Sometimes after 2 or 3 fuel fill ups something odd happens to the average economy display on the digital dash panel. For the first 2 to 4 gallons of driving the average economy display will show 99.9 MPG. After 2 to 4 gallons have been used the display returns to normal operation. No other functions on the dash seem to be effected. All the following work as they should; fuel gauge, distance to empty and instant economy readings all seem accurate based on previous observatations. In short, only this one anomaly is seen. I usually keep the display selector set on average economy. But sometimes after a fill-up I will notice the display increasing at a rate of about .1 MPH every 1 or 2 seconds to unrealistic numbers. If I reset the display it shows 99.9 until 2 to 4 gallons have been burned. Any thoughts?

2 Answers


Jimm

Jimm 8/25/2017

The fuel economy gauge makes a calculation by counting the number and duration of pulses made by the fuel injectors as they squirt gasoline into the combustion chambers of the engine. The onboard computer system divides the distance the car travels by this estimated fuel consumption.

The MPG reading 99.9 is the fuel trim circut going bad. Most likely cause is the soldered connection to the digital dash.

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Jimm

Randal S 8/25/2017

Ok, but how would filling the fuel tank effect the trim circuit or a solder point on the dash? As I said, I was not sure exactly what the PCM was reading to make the calculation. I thought possibly injector PW, O2 sensor and perhaps input from WSS or transmission were all involved. Anyway, I appreciate your input. You have made it clear that I will need to monitor this further to eliminate that possibility of the concurrence with fill ups being a coincidence.

Jimm

Jimm 8/25/2017

Not that the filling of the fuel tank is a factor, but more to the point the incorrect readout of the 99.9 MPG indicates a fuel trim circuit, and therefore the questionable solder condition.

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