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Wild MPG instant reading

15975 Views 39 Replies 13 Participants Last post by  GDE
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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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: :)
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.
. 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.
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.
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. ��
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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