Mpg adjustment

[Rawze]

CM870's and CM871's have always had an issue where the reading in the dash is slightly higher than actual. Also, the higher your average fuel mileage in, the bigger the discrepancy.

Then for example (Assuming the program is not lying for one reason or another) ...
- getting 6.0 mpg ... usually reads about 5.0 - 5.2 mpg.
- getting 6.0 mpg ... usually reads about 6.1 - 6.3 mpg.
- getting 7.0 mpg ... usually reads about 6.2 - 6.4 mpg.
- getting 8.0 mpg ... usually reads about 8.4 - 8.7 mpg.
- getting 9.0 mpg ... usually reads about 9.6 - 10.0 mpg.
- getting 10.0 mpg ... usually reads about 10.8 - 11.3 mpg.

Also, NONE of the common rail engines have this issue such as the CM2250, CM2350, and newer ... It is mostly a CM870/871 ecm issue, and is well known.


ALL ENGINES ... If there is an issue in the program itself, it can also read incorrectly due to ...

- An incorrect program has been put in, and it does not match the internal engine hardware.

- A de-mandate program (delete) has been put in, and some idiot tweaked the injection process to fool the ecm into giving up more fuel than it can account for. This is very,very,very,very bad!.

- A de-mandate program (delete) has been put in, and some idiot has done a number of things that cause the ecm to mis-calculate the fuel flow. Again, this is very,very,very,very bad!.


So .. in you post ...

-- If your actual is 6.5 ...

-- and assuming that your program is not a bad delete any more (it got straightened out recently)...

... then on a CM871, it should be reading roughly about 6.6 - 6.8 or so.

This means that ...
If it is reading 7.5, and your actually getting 6.5, then you have an actual discrepancy that the engine is not accounting for. Most likely a fuel leak, or you are not accounting for fuel being used by an apu .. something.

Common places where fuel leaks are over-looked...
** When is the last time you pulled the Doser pipe away from the backj of the turbo and crank up the truck>?.. then got out and looked at the injctor tip to see if it is getting wet?. -= Many many truck owners have leaky Doser injectors and never know it because it is seeping fuel into the exhaust pipe.

** If you one of those fools who absolutely has to top off your fuel tanks to the brim.. then you are just costing yourself a mile to the gallon or more!. When you top off your fuel tanks to the top.. several gallons of fuel will bleed out through the bleeder tubes when it expands as it heats up. As well, as soon as you get to e hill.. the fuel settles at the back of the tanks where the bleeders are and seeps out going down the road.
= I.E..> NEVER top off your fuel to the brim!!.. Stick the nozzle as far into the tank as you can.. when it clicks.. STOP FUELING!.

** Many fuel leaks are hard to find. Some model KW's and Pete's have the fuel return lines going over a cross-member to the passenger tank .. where the line likes to rub thru.. and then drip fuel.. not onto the ground.. but right onto the drive-shaft.. where it gets slung away ... and you will never see this fuel leak!>

** APU units are also notorious for having fuel leaks that are hard to find too. Keep that in mind.

** Internal fuel leaks, such as leaky injectors and or leaky fueling or timing actuators can cause the fuel mileage in the dash to read much higher than your actually getting too!. An injector leak test should be done, as well as checking the live fuel flow at idle to see if the 'mg/stroke' if fuel going into the engine is reading unusually low while the truck is at idle with everything warmed up good.

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Just some thought on your post there.. I doubt there is anything wrong with the program since it has been straightened out.. so this points towards a fuel leak of some kind instead. Just make sure your fuel pressure sensor and your fuel temp sensors are reading correctly as well.