mileage computation on an 86

[tdr1919]

Well. I've asked this before without getting a solid answer - my gas mileage indicator is whacky, goes all over the place. I have repaired my digital dash, (for a black out situation)re-soldered all the connections cleaned the connectors, and knock-on-wood, it is working fine, with exception to the mileage.
What inputs does the computer (dash) monitor to generate the gas mileage?

Thanks,
Tom

 

[vetteboy86]

Are you speaking of the mileage left on a tank (range) or the actual fuel economy??

 

[tdr1919]

Are you speaking of the mileage left on a tank (range) or the actual fuel economy??

fuel economy

 

[vetteboy86]

I'll have to try to look this one up. I know the instant fuel economy takes several things into consideration like load, speed, throttle position, etc. So I would imagine the the long term fuel economy would also use these things on a longer term. Something to think about is that the fuel economy you see on your dash is an average off all the driving you do. If you do not reset that every time you fill up, then what is displayed might not be what you get on your current tank of gas.

 

[tdr1919]

I'll have to try to look this one up. I know the instant fuel economy takes several things into consideration like load, speed, throttle position, etc. So I would imagine the the long term fuel economy would also use these things on a longer term. Something to think about is that the fuel economy you see on your dash is an average off all the driving you do. If you do not reset that every time you fill up, then what is displayed might not be what you get on your current tank of gas.

Vetteboy,
yeah, I've been looking at all the inputs to the dash, things like tach sensor, speed sensor, gas tank level all are steady on my display, BUT the shift indicator light is erratic the same as the fuel/mileage - so some input (or maybe the micro) that concerns both of these functions is bad, but I believe it is a connection because the last time it worked properly was when I change my spark plug wires, maybe there is something in this area that got knocked loose, my vacuum is good (and steady), mabe something off the distributor.
- Tom

 

[vetteboy86]

How far off is the fuel economy, or is it constantly changing? Also, no insult intended but are you looking at long range fuel economy or the instant fuel economy?? My initial thoughts would not lead me to the microcontroller controlling the display. There must be an put either on the shifter or the distributor to monitor the position of the shifter or the RPM's of the motor to indicate an upshift. I cannot be for certain as I do not no for sure.

 

[tdr1919]

How far off is the fuel economy, or is it constantly changing? Also, no insult intended but are you looking at long range fuel economy or the instant fuel economy?? My initial thoughts would not lead me to the microcontroller controlling the display. There must be an put either on the shifter or the distributor to monitor the position of the shifter or the RPM's of the motor to indicate an upshift. I cannot be for certain as I do not no for sure.

The instantaneous continually changes, showing 3mpg then 30 mpg the 5 etc.
the average (long range) will show a constant 4.5 mpg.

-Tom

 

[vetteboy86]

Yeah thats too erratic. What does the long range fuel economy do when you reset it? Also I am thinking that if it were a connection issue, the display would read correctly then incorrectly which may be what you describe. An instant fuel mileage of 30 is very possible when driving, then down to 4 may indicate when in fact the connection is compromised. I would watch it and see how they actually act. If the numbers it jumps between are close. The long range could fall victim to this as well, I would assume as I said before the variables are probably the same between the instant, and long range fuel economy.

 

[tdr1919]

Yeah thats too erratic. What does the long range fuel economy do when you reset it? Also I am thinking that if it were a connection issue, the display would read correctly then incorrectly which may be what you describe. An instant fuel mileage of 30 is very possible when driving, then down to 4 may indicate when in fact the connection is compromised. I would watch it and see how they actually act. If the numbers it jumps between are close. The long range could fall victim to this as well, I would assume as I said before the variables are probably the same between the instant, and long range fuel economy.

Vetteboy,
At one time it worked properly, giving me correct readings, on inst. it would read say 18, (quite steady) if I punched it or took off from a light it would go down to 5 mpg and gradually increase, to a fairly steady number, long range would usually be at 16 to 17 mpg.
- Tom