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Old 10-14-2011, 11:32 AM   #1
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Where did 2 MPG go?

OK: I moved my aux lights from the windshield to the front bumper (they cast a far better beam lower---anyway). I disconnected the wires from the neg terminal, made the necessary changes, installed the new bumper brackets from Qudratec, mounted the lights, re ran the wiring, reattached to the neg terminal, tested the lights and PRESTO!!! great lighting!!! WHOOPIE!!!

Ok,yesterday I get in and my dash display tells me "gASCAP"....hummmmm...so I checked and sure enough the cap was loose?????

How?? I have zip for a clue but I did do a fill up so it might have been me.......BUT........I drive 150 miles on pure highway, and I usually see 21.4 to a high of 22.1 MPG on my display. I get 19.2!

NOW....all I see is 19.2 MPG to a high of 19.4MPG ALL the time! Is this drop weird or was I getting a false reading prior to this and now am I seeing the actual use?

21 was GREAT.....19 ain't too shabby but......Where did the 2 MPG go?

Same gas 87 Shell.

THIS is strange!...or is it?

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Old 10-14-2011, 11:40 AM   #2
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gas theives or you should calculate your milage by hand. fill the tank drive it to less than half, or less than 1/4 and then fill all the way back up. MPG = miles/gallons... if your only using the computer on the dash you might actually be getting less...

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Old 10-14-2011, 11:42 AM   #3
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The lights are suckin the gas right out ovit


When is the switch over to "winter gas" if there is even such a thing ?
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Old 10-14-2011, 11:42 AM   #4
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I've been having troubles with my gas cap. It used to *click* twice, and seat securely. Then, I had the gascap light thingy come on. I checked it, it was loose. And the filler neck seems a bit looser.
I took it to the stealership here in SoCal, they said there was a recall for a computer update... to fix the gascap light warning? And that the cap was within tolerance (they checked the 2 other 11's on the lot) and the neck, well, there was nothing they could tighten. When they drove it around... I checked it... and sure enough, the cap was tight again (although they said they did nothing). It didn't *spin* click, click, click... in place anymore, but seated tightly after 2 clicks.
2 days later the light came back on. I took the cap off, and replaced it - no more light. But.... yesterday, the cap is now loose again. (No light yet).
Stranger things have happened, but not recently. Oh, and I GAINED 2 mpg (yeah, right).
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Old 10-14-2011, 11:44 AM   #5
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gas stations may have already started using the winter blend, this will kill your mpg,,, and yeah, they use winter blend in gasoline.
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Old 10-14-2011, 11:50 AM   #6
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I was about to say probably winter blend.
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Old 10-14-2011, 11:56 AM   #7
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Honestly this is the first time I have had an onboard display and I go from the MPG to Temp-Compass......so maybe I never "saw" a difference prior to this......winter blend huh? Not too sure about that one.......

The gas cap is really cheezy.......I'll keep an eye on this too....The drop (overnight) is kinda of strange. Just saying.
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Old 10-14-2011, 12:13 PM   #8
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Quote:
Originally Posted by cu in oz
Honestly this is the first time I have had an onboard display and I go from the MPG to Temp-Compass......so maybe I never "saw" a difference prior to this......winter blend huh? Not too sure about that one.......

The gas cap is really cheezy.......I'll keep an eye on this too....The drop (overnight) is kinda of strange. Just saying.
Gas gauge is notall that accurate. Can't really go by the gauge.

Winter blend has more ethanol I believe.

One thing I do notice is vehicles seem to run better with WB. could be me or the fact that the temp is cooler but dunno.
And o ya---MPG's take a small hit.

Just installed a CAI but also brought my tire pressure up to 35, they were at 32 and
I'm "showing" + 2 mpg. Assuming the tire pressure helped me a bit not sure on the intake
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Old 10-14-2011, 12:23 PM   #9
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Speaking of tire pressure why are the front tires several pounds lower than the rear?
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Old 10-14-2011, 12:33 PM   #10
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Originally Posted by Roadhawg
Speaking of tire pressure why are the front tires several pounds lower than the rear?
Pump em up
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Old 10-14-2011, 12:53 PM   #11
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Pump em up
*DUH* why didn't I think of that
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Old 10-14-2011, 01:37 PM   #12
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Unhooking the battery long enough to rewire lights could have given the PCM amnesia. It will take some time for it to relearn you driving habits, but once it does your mileage should come back.
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Old 10-14-2011, 01:39 PM   #13
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What does it learn and how does that affect MPG?
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Old 10-14-2011, 01:47 PM   #14
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What does it learn and how does that affect MPG?
It learns how much throttle you use to accelerate, how fast you push the pedal down, how much fuel it needs for a given set of operating conditions and how much timing it can use in those conditions. If the base fuel map is rich (which most are because they are set for sea level) it will learn to pull out fuel during almost all operating conditions, which means better mileage. If it can add more timing (if the JK can do that, I'm not sure what it's spark strategy is) it can also run more efficiently. This usually happens fairly quickly. Probably the big adjustments are done within the first tank of gas and the fine tuning before the second tank is gone. He should be close right now.
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Old 10-14-2011, 01:53 PM   #15
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Couldn't those conditions vary greatly? Say you're normally a laid back easy with the gas driver, but now you have an emergency and you drive like a bat out of hell? Why would it be beneficial for it to learn a specific way as opposed to reacting to the varible input?
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Old 10-14-2011, 02:21 PM   #16
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Yes, they do vary a lot, but that's just how they do it. In practice you probably can't even feel the difference even if you were trying to. Every vehicle sold in the U.S. since 1996, and most computerized cars before that, have short and long term fuel trims. The long term is resposible for what are basically permanent adjustments for elevation, driving habits and other things that are usually consistent. The short term makes adjustments for what's happening right now. If the change in short term trim is so drastic that it can't accomodate the whole change it makes an adjustment to the long term trim and then starts over with the shrot term. Most of the time all of this happens seamlessly and you would never know the difference, even with a drastic change of driving habits. It just takes a little while for it to establish that base line.

For the emergency scenario, you couldn't even tell the difference. At most the learned parameters will change the overall output by only a few hp, nothing drastic.
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Old 10-14-2011, 02:28 PM   #17
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Very interesting stuff. That's the first time I've heard it explained where it made sense. Thank you.
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Old 10-14-2011, 03:16 PM   #18
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Clueless myself but I was disagreed with when I mentioned this whole ECU "relearning" thing in a past thread.

Thanks for the clarification Oilwell
Great job
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Old 10-14-2011, 06:39 PM   #19
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Since joining this site I have learned a ton. I'm not an Off-Roader,But I LOVE the JK, its just a great ride, but honestly one of the best things is comming here and reading all of the threads and the answers to questions posed.

This learning expearience is pure fun for me and after driving for 50 years, I'm finding a new form of pure automotive joy.

I would like to publicly thank each and every one here......this is one of the reasons the internet and the information exchanges it allows is so incredable. Thank you......one and all. Thanks a lot. Jim Terrio.

(BTW there are some really great folks here.....one can tell from reading the posts and responces that many mnay here are first class. Nice.....its very nice....it renews faith in the goodness of folks.)
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Old 10-14-2011, 07:46 PM   #20
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Originally Posted by cu in oz
Since joining this site I have learned a ton. I'm not an Off-Roader,But I LOVE the JK, its just a great ride, but honestly one of the best things is comming here and reading all of the threads and the answers to questions posed.

This learning expearience is pure fun for me and after driving for 50 years, I'm finding a new form of pure automotive joy.

I would like to publicly thank each and every one here......this is one of the reasons the internet and the information exchanges it allows is so incredable. Thank you......one and all. Thanks a lot. Jim Terrio.

(BTW there are some really great folks here.....one can tell from reading the posts and responces that many mnay here are first class. Nice.....its very nice....it renews faith in the goodness of folks.)

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