Why does the lie-o-meter lie? - Ford Powerstroke Diesel Forum
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Old 03-12-2014, 07:54 AM
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Why does the lie-o-meter lie?

ok, I just don't get it. It's great when I tell the wife to look at it to see how much better my MPG has increased since my mods, but I know better and what she doesn't know won't kill me ... Where does the computer pickup it's info and why wouldn't this be corrected during a program update? Last week I reprogramed with my new SCT and the lie-o-metter is telling me 22.5 MPG WOW!!!! Then I go filler-up and calulate it at 15.9 That is a very large spread.
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Old 03-12-2014, 09:03 AM
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there are several obvious factors- forward speed vs. engine speed ect.. there are some tucked away..

I know for a fact that the Fuel Rail Pressure Sensor (FRP) plays a heavy role.. I lost mine some years back while traveling a long run, and clocked 40+MPG in the next 300 miles or so.. and it was actually about 19ish iirc..

the FRP is singular with no back-up or redundancy capability.. the pressure in the rails determines how much fuel slips past the piezometric injectors and a known duty cycle/ pulse width, otherwise known as 'how long the injector gate is open (piezometric injectors shrink a crystal under power to allow fuel past as opposed to HEUI injectors used on the 7.3 and 6.0 which generate their own pressure via a tiny piston)... if you lose the FRP there is no other way for the computer to know how much fuel is being used with a certainty... if you lose the FRP the PCM dictates a weird unannounced 'limp' mode that disallows hard accelerations, and cruise control is gone.

THE way to make more power is to add fuel... the means to 'tune' a rig for more power is to use it's available functions to send more fuel... one of the ways that is accomplished is by altering the fuel pressure to engine speed ratio, ALONG WITH altering the pulse width of the injectors to allow more fuel past... when you tell a computer to forget its previous translation of (out of left field not accurate numbers, but for discussion) 24kpsi for 2400RPM and (x) throttle position and tell it to provide 27kpsi to that same 2400rpm and throttle position, whallah- more fuel.. more power.. but the files/tables on the computer concerning economy are still equating 24kpsi to 2400rpm and throttle position haven't been altered- and there is the disparity you see on your dash...

it also happens with rigs that aren't tuned.. those can be attributed to two things in my opinion.. one: unhealthy FRP operating at the top or bottom of it's parameter instead of in the middle (where they are calculated to work), and also due to regeneration function- the regen burns fuel unaccounted for in engine cycles and 'regular' circumstances, and it is a lot.

the FRP I KNOW beyond doubt plays a huge role.. the rest is conjecture, but the only way it makes sense to me.
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Old 03-12-2014, 09:33 AM
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Be careful of what you ask because you'll get way more info than I can comprehend.
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Old 03-12-2014, 10:12 AM
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My lie O meter has been pretty steady on telling the truth on average I get 15-16 hwy and hand calculated and a stead 15.2 city and it hasn't been off yet on that
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Old 03-12-2014, 10:48 AM
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And I thought it was just a paddle wheel that needed calibrating.....
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Old 03-12-2014, 04:37 PM
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Quote:
Originally Posted by dnaber View Post
ok, I just don't get it. It's great when I tell the wife to look at it to see how much better my MPG has increased since my mods, but I know better and what she doesn't know won't kill me ... Where does the computer pickup it's info and why wouldn't this be corrected during a program update? Last week I reprogramed with my new SCT and the lie-o-metter is telling me 22.5 MPG WOW!!!! Then I go filler-up and calulate it at 15.9 That is a very large spread.
Momma said lie-o-meters are ornry because they have all those teeth and no toothbrush.


See the thread recently in the General section about this topic and tire size. Several of us took our best guess.
Dave I. likes this.
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Old 03-12-2014, 06:32 PM
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If you look at the h&s MCC fuel tables it converts a commanded fuel mass value into a desired pulse width value based on fuel rail pressure. On the tables for higher power (wild, hot) the same commanded fuel mass values for a certain frp have a much higher pulse width value. So on the hotter tunes the computer thinks it is using something like 80 grams of fuel but it is actually using much more. At least this is what I guessed because it seems like the higher the tune the more the lie o meter is off
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Old 03-12-2014, 06:57 PM
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Originally Posted by ckrueg View Post
If you look at the h&s MCC fuel tables it converts a commanded fuel mass value into a desired pulse width value based on fuel rail pressure. On the tables for higher power (wild, hot) the same commanded fuel mass values for a certain frp have a much higher pulse width value. So on the hotter tunes the computer thinks it is using something like 80 grams of fuel but it is actually using much more. At least this is what I guessed because it seems like the higher the tune the more the lie o meter is off
I have never had the privelage of looking at SCT tables, but 1 constant for my truck has been that since I switched from SCT to a MM, my lie-o-meter is much closer. When I ran an early RCD tune (585), my lie-o-meter would actually read 30-35 MPG.
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Old 03-13-2014, 10:33 AM
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Mama says that happiness is from magic rays of sunshine that come down when you’re feeling blue.
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Old 03-13-2014, 12:41 PM
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Originally Posted by Capt dong View Post
Mama says that happiness is from magic rays of sunshine that come down when you’re feeling blue.
Thank you. I was beginning to think no one was going to catch that.
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