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Discussion Starter #1
Was in heavy traffic for about 1 hour on I 5 goin south through Tacoma WA today for about 10 to15 miles or so and the reading would go from 0 standing still to 99 As I crawled along one or two mph, then down too 25, up down and even when I got out of the slow traffic it was still bounceing up and down. When I got too my destination I cleared all the readings and it would still do it as I moved the truck to park it. It run good other wise and drove fine, it started right up three or four times after I had it parked. This is a 360 mile trip over and the same going back so hope it runs fine when we head back Saturday. Do you think I should see if I have it checked Friday before I head back to Eastern Washington :(
 

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It's working normally. You'll notice it goes to 1-5mpg when accelerating and it'll hit 99 when you coast or take your foot off the gas. I am assuming it's looking at vacuum pressure in the manifold and then making a calculation from there. Every car I have seen with a mpg display works like this. Nothing to worry about.
Eric
 

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I was going to say it is working just fine. It is supposed to be a near real time current MPG, it will change rapidly based on your input on the gas, your speed and the terrian.
 

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Yup, that's what I get too. Does it read a reasonable number when you are driving along at a steady speed?
 

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Yup, that's what I get too. Does it read a reasonable number when you are driving along at a steady speed?
It did the same thing driving in normal city traffic after I got off the interstate to where it is parked now which was about 15 miles. I intend to go for a short drive on back roads and see if it works like it should. Scared the **** out of me in that 4 lane number to bumper crap as I'm used too those back country roads. :eek: :eek: :)
 

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Yup, that's what I get too. Does it read a reasonable number when you are driving along at a steady speed?
It's working normally. You'll notice it goes to 1-5mpg when accelerating and it'll hit 99 when you coast or take your foot off the gas. I am assuming it's looking at vacuum pressure in the manifold and then making a calculation from there. Every car I have seen with a mpg display works like this. Nothing to worry about.
Eric
. My F150 never did this fast fluctuations in any kind of traffic and drove it 37000 miles before I traded it in on my ECO! That is why I was so surprised, or worried.. I checked both trip logs and they read the same 24.8 mpg on the tank of fuel.
 

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I have found in heavy traffic I actually gain MPG overall. Just let your foot off brake and it will get to 10mph or so on its own showing 99mpg and brake again when needed. First car I have ever owned to gain MPG in stop and go
 

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It just depends on if the reading is instantaneous or if it is averaging out your mpg over a certain period of time. If you think about it, coasting doesn't use any gas so it makes sense that it would read 99 if you are coasting.

I'd experiment with a bit and if you still feel like you are getting poor readings, then I would take it in to have someone look at it.
 

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Discussion Starter #9
The reading will go to 35, or 68,,down to 5, up to 99 then Just keep fluctuating up and down but not stopping at a number ,just flashing them as they go up and down with us crawling along with the motor at idle. This is all at speeds that hardly register on the speeddomiter. Very strange.
 

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its gameified on purpose, keeps people constantly vigilant. Its like riding a bicycle with AVG speed ratings, you're always trying to move the needle...(or not)

 

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Discussion Starter #11
Took the truck out for a 15 mile drive on two lane side roads. Seemed okay today but think I will take in next week and have them take look and see if there were any codes. There has not been any CEL, will post what I find out. ��
 

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1st no manifold vacuum on a diesel.
2nd, MPG is calculated using a number of factors (Probably forgot a few) including speed, engine load, throttle position, engine temp, etc. Don't reset for awhile and it should average settle down. On the big Rams 2500/3500 diesels we refer to these as lie-o-meters for a reason. The only mpg numbers we believe are hand calculated. Anything from the display is BS. They might get close but that's about it. :)
 

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Discussion Starter #14
1st no manifold vacuum on a diesel.
2nd, MPG is calculated using a number of factors (Probably forgot a few) including speed, engine load, throttle position, engine temp, etc. Don't reset for awhile and it should average settle down. On the big Rams 2500/3500 diesels we refer to these as lie-o-meters for a reason. The only mpg numbers we believe are hand calculated. Anything from the display is BS. They might get close but that's about it. :)
I know what you mean about lie o meters as that is what I felt about those figures on my other Rams or Fords.
Mine has settled down with no more fluctuations up or down, :) This took place in a 15 to 20 mile stretch of I 5 four lane highway of bumper too bumper traffic and was a major pain in you know where. But still plan to take it to the dealer as had a CEL which I have no idea what it was four. Ray
 

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That's the best way to go about it, i know way too many people that solely depend on the lie-o-meter.
best to get real world stats from other owners and even doing the math yourself.
then there are sites like fuelly to check
 

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I'm guessing the ECM counts the number and duration of injections and calculates overall fuel flow from that as well as estimated rail pressure. Once flow is known, MPG = Speed(miles/sec) / Flow(gallons/sec).
 

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I'm guessing the ECM counts the number and duration of injections and calculates overall fuel flow from that as well as estimated rail pressure. Once flow is known, MPG = Speed(miles/sec) / Flow(gallons/sec).
rpm, barometric pressure, ambient temperature, engine temp., vehicle speed, estimated fuel flow @ ?? RP (RP varies with with rpm/load, ECM can't calculate exact fuel flow it doesn't know what's going back to the tank/return line), boost pressure, transmission gear, TC locked/unlocked, throttle position. (missing a few I'm sure) as you can see with all these parameters the mpg display "might" vary just a touch. That is why the ONLY real MPG numbers are hand calced. distance traveled / gallons used. The less the display is reset the more consistent/stable "realistic" the display will become to calced mpg. When "tuners" come out the numbers that you will see reported will be all over the board. Tuners, depending on how they interact with the factory ECM and what tune level they are operated on, will give really wild extremes on the display.
 

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rpm, barometric pressure, ambient temperature, engine temp., vehicle speed, estimated fuel flow @ ?? RP (RP varies with with rpm/load, ECM can't calculate exact fuel flow it doesn't know what's going back to the tank/return line), boost pressure, transmission gear, TC locked/unlocked, throttle position. (missing a few I'm sure) as you can see with all these parameters the mpg display "might" vary just a touch. That is why the ONLY real MPG numbers are hand calced. distance traveled / gallons used. The less the display is reset the more consistent/stable "realistic" the display will become to calced mpg. When "tuners" come out the numbers that you will see reported will be all over the board. Tuners, depending on how they interact with the factory ECM and what tune level they are operated on, will give really wild extremes on the display.

How does barometric pressure have any bearing on MPG? Once fuel flow is known, it doesn't matter what temperature anything is, it's all about speed and fuel consumption.
 

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How does barometric pressure have any bearing on MPG? Once fuel flow is known, it doesn't matter what temperature anything is, it's all about speed and fuel consumption.
Barometric pressure is the measure of air density. ECM adjusts for that. for example: given the same weather conditions (NO tornadoes/hurricanes in the area) barometric pressure is lower in Denver (mile high city ring a bell/thinner air) than it is at sea level. System has to compensate for the amount of air ingested. Even with a turbo engine MAP (Manifold Absolute Pressure) sensor is still employed. More boost more fuel/less boost less fuel.
 

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Barometric pressure is the measure of air density. ECM adjusts for that. for example: given the same weather conditions (NO tornadoes/hurricanes in the area) barometric pressure is lower in Denver (mile high city ring a bell/thinner air) than it is at sea level. System has to compensate for the amount of air ingested. Even with a turbo engine MAP (Manifold Absolute Pressure) sensor is still employed. More boost more fuel/less boost less fuel.


I really doubt this truck has a barometric pressure sensor on it. The truck does of course adjust for changes in O2 density (lower ambient pressure). But the sensor for adjusting for O2 fluctuations is the O2 sensor, not a barometric sensor.
 
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