Is there any way to calibrate the fuel consumption display in the overhead console?

JamesB · Aug 25, 2016

Is there any way to calibrate the trip computer to correct the fuel economy display? Mine is consistently reading about 25% better economy than real world odometer/fuel fill calculation figures.

I've been doing a regular 650 km run (about 50km suburban/city driving, 100km rural roads, the rest highway) in my 2005 Durango Limited (320,000km, hemi, 5 speed, 3.92:1 axle ratio, stock except for a short flowmaster that the previous owner fitted instead of the rear muffler).

I drive it pretty gently on this run (cruise control most of the way running at posted limit plus around 5 km/hr), and I reset the display when I fill the tank about 50-100km into the run. The fuel economy display on the overhead console usually reads somewhere in the region of 12.5 litres/100km (~18.8 mpg) by the end of the run, and as as per usual, real world calculations indicate a fuel consumption in the 15.2 litres/100km (~15.5 mpg) to 16.6 litres/100km (~14.2 mpg) range (it depends on traffic and weather).

This large a discrepancy between the display and real world calculations tells me that there is something amiss in the way the display is doing the measurement/calculations.

Citadel Brian · Aug 25, 2016

The early ones were usually way off as you can see. My 07 Jeep GC with the Hemi is telling me I'm getting 15.8 and I'm only getting 14.2. I've learned to know what my total miles are for the rank and go off of that and not the computer.

JCC · Aug 25, 2016

I have never heard of calibrating the system, but I am sure there is an electronic sensor somewhere that maybe the issue. Anyone know what the sensor it uses to make the calculations? I am not aware of anything that measures actual flow of gas so thinking it probably just uses changes in fuel level? If that is the case the float off the fuel pump could be the issues. Does your gas gauge track accurately? Fuel pump been replaced?

I know in theory with everything being electronic the vehicle would know how much gas is being delivered to the engine simply by how hard you press the peddle along with calculations from other things like airflow sensor, but that type of calculation seems overly complex and prone to mistakes as minor parts age/wear.

Throttle body being fully electronic on the hemi makes it a suspect as well.

Not likely to be a free or easy fix though.

JamesB · Aug 25, 2016

Digging into the service manual, it does look like the fuel consumption figures are calculated by using the fuel level sensor.

From the service manual:

Fuel Level Data Support - The EMIC provides a current source for and receives a hard wired analog input from the fuel level sending unit located on the fuel pump module in the fuel tank. The EMIC uses this input to calculate the proper fuel gauge needle position and low fuel indicator operation. Based upon this input, the EMIC also calculates fuel level, average fuel economy and distance to empty data, then uses electronic messaging to transmit this data over the CAN data bus for use by other electronic modules in the vehicle.

In my Durango, the DTE is wildly optimistic (by 100-150km) with a full tank, but the figure approaches reality as the tank empties. The low fuel light consistently comes on at between 38 and 41km on the DTE display. According to the Service Manual, thr low fuel light should come on at around the 10% tank capacity point, so there's something amiss here as well.

From the service manual:

Each time the fuel level sense input to the cluster indicates the fuel tank is about ten percent full or less for ten consecutive seconds and the vehicle speed is zero, or for sixty consecutive seconds and the vehicle speed is greater than zero, the fuel gauge needle is moved to about the one-eighth graduation on the gauge scale, the low fuel indicator is illuminated

I will have to investigate this system a bit more. I suspect that I need to look at the fuel level sender.

JCC · Aug 25, 2016

Based on age of the vehicle if you do not know one way or the other probably close to a 50/50 split on if the fuel pump has been replaced. If it was replaced very likely it was not an OEM replacement so that may account for the inaccuracies. Could also be original equipment that the variable resistor that the float operates has just worn out so the voltage it is allowing through is not consistent with what it should be in any given position. Not sure this is a good enough reason to replace a fuel pump, but that is up to you.

JamesB · Aug 26, 2016

JCC said:
Based on age of the vehicle if you do not know one way or the other probably close to a 50/50 split on if the fuel pump has been replaced. If it was replaced very likely it was not an OEM replacement so that may account for the inaccuracies. Could also be original equipment that the variable resistor that the float operates has just worn out so the voltage it is allowing through is not consistent with what it should be in any given position. Not sure this is a good enough reason to replace a fuel pump, but that is up to you.

My gut feeling is that the pump/sender has been replaced, and somebody bent the float arm as it was being installed.

Did a run today after filling the tank as far as it would go and had covered around 150 km before the needle started dropping off F ... should have been at around 3/4 by that point. After nearly 300 km, the needle is down to 3/4 ... should be at around the 1/2 point.

I'll stick a volt meter on the feed from the sender. The resistor in the sender is fed from a constant current source, so the voltage at the output of the sender will tell me a lot about the resistance.

JamesB · Oct 14, 2016

Haven't had a chance to do a voltage test on the sender, but I am testing the sender's operation.

While the Durango has something like a 102 litre tank (based on 27 US gal capacity figure in the build sheet), I've never managed to get any more than about 83 litres into the tank when the DTE display is down to a single digit and the gauge is hard on the E peg.

My suspicion is that there's 20 litres (or so) still in the tank when the sender is telling the system it is empty. The low fuel light comes on when there's between 38 and 42 kilometers on the DTE display. It's also driven by the same sender, so it's as (in)accurate as the fuel gauge. Yesterday I did a 1100 km run and got a measured 13.1 litres/100km or 18mpg (which for a Hemi, AWD with 3.92:1 axle ratios, I'm pleased with) and the consumption display is showing 11.5 lit/100.

Time for a real world test of the fuel tank sender/gauge/etc. I am currently carrying a 20 litre fuel can in the back (can be difficult getting fuel late at night in this part of Nova Scotia), so I figured while I'm carrying a reserve fuel supply, why not run the tank to empty. If there was 20 litres still in the tank at DTE Zero, then I should get around 120km out of the vehicle before the tank is really empty. The driving I had to do today, plus tomorrow (Friday) and Saturday, involves involves a total of 120-130km of 80 km/hr posted back country roads and highways with very little traffic. Why not run the car to empty. The DTE display hit Zero km about 4 km from home at the end of last night's run. The drive to work today has increased the distance since the DTE Zero point to 46 km and it's still running. Tomorrow will be another 42 km run to work, and Saturday is a run of about 46km before I hit any real population or major roads. I expect to have run out of fuel before that point.

JamesB · Oct 14, 2016

Ran out of fuel on the way to work this morning, so that answers that. Got 64 km after the DTE said the car was empty and 102 km since the low fuel light came on, so that's about 10 litres "reserve" after the tank thinks it's empty. The low fuel light is coming on at about 16-17% fuel left in the tank instead of 10%. Still a mystery on the actual "usable" capacity of the tank.

Dumped the can of fuel in the truck, turned on the ignition, waited about 10 seconds for the fuel pump to prime the lines, and hit the starter. Fired up as if nothing had happened.

Is there any way to calibrate the fuel consumption display in the overhead console?

JamesB

Citadel Brian

JCC

JamesB

JCC

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