has anbody had any issues on this working intermitently? I've looked in my books and all over and can't find anything that says how to repair or replace. I think it's nothing more than tightening a ground because it will sometime work and when it does hiting a bump in the road will cause it to stop. Any ideas of location to fix this annoying issue.

 

i switched mine to average and it's been fine... instant never seemed to work right in my 89... but average has been fairly accurate... :cheers:

 

I like to leave mine on average. The instant changes too frequently to be much use. Does anyone know what time frame or mileage the average is based on, i.e. is it the last 100 miles, 2 weeks, or just total mileage and gas use?

 

i believe it is an average from the time you reset it based on fuel usage...because when ya first reset it the initial value will stay for awhile... such as if i hit reset in town and i'm running the lights it will hover around 11 MPG...if i reset it on the interstate and reset it it can stay around 35 mph .. so it has to be a fuel usage/milage base... :cheers:

 

The average and instant is measured at the injector pump. The more pulses the pump produces, the fuel readout is adjusted. Average is measured by the average reading of the instant fuel reading it adjusts constantly. Average never resets until you reset it maually on the dash.

fyi Range is calculated by the amount of fuel in tank, and the average fuel economy. Drive around town and play with these buttons, its really cool...paying attention to the road is overrated!

 

I never believe what the gauge says.
Do you?
tony
:flag

 

Not after I ran out of gas with one bar still showing for fuel and 45 miles of range.

 

Where is the "injector pump"? Is this on the fuel rail also known as the fuel pressure regulator? I turned my avg/instant to the off position. The avg usually reads 99 while the instant read 0, I'm assuming these are the system defults.

 

This is not true. Instant Miles Per Gallon is calculated by the computer by a alogrithm using the injector pulse width and vehicle speed as the basis for information.

The fuel pump provides fuel at at pressure ABOVE the regulated pressure required by the injectors. The Fuel Pressure Regulator keeps fuel available to the injectors at a regulated pressure. Unused fuel is returned to the tank by a separate line. Thus using the Pump cycles as a source of measurement would be highly inaccurate because it is always pumping more fuel than the engine needs!

Using the Injector pulse width can provide fairly accurate means of measuring fuel flow because it is measuring at a constant PSI the actual amount of fuel going into the engine. The longer the injector is open the more fuel is passed. Injector pulse width varies accorcing to the engine load.

There are variables that affect accuracy because of what PSI the Fuel Pressure Regulator is set at (using an Adjustable Fuel Pressure Regulator without changing the program on the computer chip will mean an inaccurate reading), how accurate the O2 sensor is, what the manifold air temp is, etc, etc, etc.

Those people customizing chips for modified engines will often change the computer program parameters to get a more accurate IMPG reading.

The Average Miles Per Gallon is just a simple calculation by the computer of the amount of fuel remaining in the tank vs. the miles traveled since the last fillup or another way of saying it would be the amount of fuel used since the last fillup vs. miles traveled. Any accuracy is dependent upon the fuel level sensor which sometimes have questionable measurments.

An aside, from the description of the problem (intermittents) the diagnosis of a loose or faulty ground is probably the problem here.

Hope this is understandable and helps. Good luck.

 

Where would be the location of the ground wire or wires you would check?

 

run on average also...

 

I run on average. The longer I drive on a tank of fuel, the less often the average changes.

Greg

 

I find it a little entertaining to put it on instant mileage while going down a big hill. I've never got it to go over 99 mpg(which I guess is how high it will read) though.

Most of the time I've got my guage set to either oil temperature or coolant temperature.

Randall

 

Is this possible on my 90 or only on later C4's?

 

Check out the other thread "what do you leave your gauge set on?" I believe someone posted there that there was not a digital readout on a 90-91.

Randall