I've had my Dak for about 3 weeks and have made several trips and burned a few tanks of gas. It's an 06 Quad Cab 4.7 w/3.55 gears, OEM 255/65/16 tiresand a topper & 30k miles. So far I've been getting from 15.5mpg for a tank mostly around town to 18.5mpg for mostly on the road (hand calculated) I've reset the console calculator at each fillup and so far, it's consistently been optimistic by 1.5 MPG (indicating 17-20mpg).I did some searching and read a comment that they're often off (2mpg hi/low). Is that the case or are they mostly just off to the high side? There's nothing in the Owner's Manual or the FSM for calibration or even an explanation of how they work. I expect that, at minimum,itcould use averageinjector duty cycle, engine RPM and vehicle speed over some elapsed timescan frequency to generate a cumulative MPG over the distance from last reset. For something like this to work they'd have to program in flow rate tables for the injectors to convert duty cycle to mass flow. If the injector flow rates, tachometer or speedometer were off they would all introduce error but the speedometer error would also affect the hand calculation so that's not likely. RPM doesn't sound like something that would get out of calibration easily so I would expect the injectors to be the most likely source oferror in the calculation. If the injectors are off it would tend to cause a rich or lean condition but a correction would be trimmed in by the O2 sensor input in cruise mode wouldn't it? I checked my tailpipe and it's got a little black but it doesn't smell rich at idle. My last vehicle was a Ford Focus and the tailpipe was still white-glove clean at100k miles so I don't know how significant a little carbon is to the equation. Anyone study this and/or figure it out?

 

Just my 2 cents,. I have noticed the overhead to be off about a mile up or down, normally up (probably from me coasting with the 6 speed). I also have noticed my trucks tail pipe is black, but its winter here and the temp tonight should be around 0.

I wonder how many peoples tail pipes look like soot? My last ford was also clean compared to my current vehicle.

 

Last summer I had the resonators removed from my exhaust and had a new tailpipe welded in. It's just aluminized pipe, but the end of it (maybe like 2 inches) looks like a woodburning chimney it's so black. I guess there's a lot of unburned carbon coming from my engine.

Hopefully when I get my duals installed, I won't have the same problem with the stainless tips. []

Any suggestions from you gurus would be appreciated.

 

Any black soot in your tailpipe is fuel you paid for but didn't getthe work out of! [:@]

 

The overhead console trip computer does use injector duty cycle
but it also assumes that the fuel pressure those injectors see
is exactly the middle of the psi specification.

When the vehicle is new
the fuel pump tolerances are tight
and the fuel filter not very restrictive
then you fuel psi getting to the injectors
will be a bit on the high side of the long term average
and your overhead trip computer will be fooled
into thinking each injector pulse is putting out LESS fuel
than the higher pressure is presently pushing through them.

This could be corrected for with a fuel pressure sensor that the PCM computer could read
but that would cost extra $ to a Chrysler that is already 'operationally bankrupt'
according to Chrysler CEO Bob Nardelli.

So it is understandable that the Chrysler engineers
set the trip computer calibration
to what they think the 'average' psi will be at....say half the vehicle life
or about 100,000 miles.

 

Hank, thats a great explanation, thank you!

On my 2004, it is usually off by 1 or 2 mpg (sometimes high, sometimes low) but on my 2006 it has been dead on (within .5 everytime I have checked)

 

Thanks for the feedbackfolks. I don't know who'll be considered the geekiest on this thread. Me for asking the question or HANKL for answering! [sm=type.gif]

 

i just wanted to know how to reset the distance to empty. everytime i fill up it reads 342, but i get between 380 and 420 on a tank of gas. so the first 100 miles or so it does not move. i tried to disconnect the battery before a fill up and that did not work. but all the others will reset

 

Ya, I get the same rough mileage, as you do, and mine reads 313 at the max, and will stay there for a good hundred miles.....

 

i have dual stainless and it still burnes the same way your discribing...... prob just Dodge