Disconnecting bty for better MPG? - Jeep Commander Forums: Jeep Commander Forum
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post #1 of 11 (permalink) Old 04-20-2011, 11:03 AM Thread Starter
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Disconnecting bty for better MPG?

So Monday of last week I put in new plugs and air filter thinking that MPG would change a bit and better then the 13MPG I was getting, I drove until last Friday and no changes at all. While packing up for a weekend trip last friday I remembered reading that disconnecting the bty would reset the computer and by doing so MPG would change. So while packing I disconnected the negative for 20 minutes or so, hooked it up again and off I went. I seen a change, HWY over 16 MPG and city now over 14 almost 15MPG depending on my foot. I'm not sure disconnecting is what made the change but if so it was worth the minute it took to disconnect and connect.
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post #2 of 11 (permalink) Old 04-20-2011, 11:08 AM
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ok ill do that

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post #3 of 11 (permalink) Old 04-20-2011, 11:38 AM
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When I replaced my plugs, I had the battery disconnected during the switch. I put the new plugs in and reconnected the battery. Three days later I took the family on vacation. Traveling at interstate speeds, my mileage went from approx. 17.5 mpg to 20+. This did not last long however. I have read that the computer "relearns" it's emissions readings. After a month or so the XK settled back to the 16.5 to 17.5 highway mpg.

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post #4 of 11 (permalink) Old 04-20-2011, 12:24 PM
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When I replaced my plugs, I had the battery disconnected during the switch. I put the new plugs in and reconnected the battery. Three days later I took the family on vacation. Traveling at interstate speeds, my mileage went from approx. 17.5 mpg to 20+. This did not last long however. I have read that the computer "relearns" it's emissions readings. After a month or so the XK settled back to the 16.5 to 17.5 highway mpg.
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post #5 of 11 (permalink) Old 04-21-2011, 06:39 AM
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So it will actually reduce your mileage to obtain better emissions ratings?

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post #6 of 11 (permalink) Old 04-21-2011, 08:22 AM Thread Starter
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So it will actually reduce your mileage to obtain better emissions ratings?
My understanding is that it adjust to your driving habit and so getting worse mileage would be due to a heavy foot or style of driving is what i was told. I am doing the same trip this weekend to the cabin and fishing so the roads will be the same. I will see by monday if there as been any changes and post the results.
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post #7 of 11 (permalink) Old 04-21-2011, 02:05 PM
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We have not taken the XK on a long highway trip since that vacation. The driving done since the plug change settled back to more combo driving. I still only get about 17 on highway trips if i reset the dash gauge and check it over 60 miles or so.

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post #8 of 11 (permalink) Old 04-21-2011, 08:09 PM
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I mentioned this in another thread but on a trip from Indianapolis to Atlanta and back, I averaged 18.8 mpg. This included about 5% driving in Atlanta itself. I went about 1-2 mph over the speed limit or about 72 most of the way, 67 in a 65. I thought that was good for a lifted jeep and having 26 boxes of swag for a trade show. It makes me wonder what it would have been if I had done a reset.

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post #9 of 11 (permalink) Old 04-25-2011, 01:17 PM
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Quote:
Originally Posted by indymtb View Post
So it will actually reduce your mileage to obtain better emissions ratings?
The engine controller tries to maintain 14.7:1 AF ratio for all operations except warm-up and Wide Open Throttle (WOT).

Compared to old carbeauted motors, this is a big improvement and you got better mileage while getting cleaner exhaust.

But, with todays technology, you could run motors much leaner and get much better mileage, but that would increase the emissions, so we get poorer mileage to keep cleaner exhaust.

The EGR, by diluting the incoming oxygen with some exhuast gas, will make the motor run leaner in the sense it uses less fuel for the same amount of power going out, but still keep the same apparent AF ratio of 14.7:1, since there is less oxygen. That is why disabling an EGR can result in poorer gas mileage.

BUT, what you folks are seeing is; pulling power from the PCM erases the volatile memory, it does the same as putting the PCM in quick learn mode.

The PCM is adaptive, it does more than adjust to driving style, it adjusts (tunes itself) for production variances in the motor, wear changes, ambient conditions, fuel, etc, etc....

As your old spark plugs were erroding away, the PCM was adjusting itself to compensate. You swap in new plugs, there may be some gain in retuning the motor for the new plugs, and the PCM will do it itself. BUT, pulling the power to the PCM and clearing all the old correction factors the PCM learned and stored to work off, its like putting it in quick learn mode and it adjustments the PCM makes have a greater effect faster.

If there has been no change to the motor, likely the PCM has learned all the best settings, and clearing the volatile memory would have the opposite effect, it would make the motor perform worse and get worse mileage until it has tuned itself/adjusted back to the correction factors prevously stored in it.

If people are getting improvements clearing volatile memory in vehicles that have had NO changes to the motor (and I have seen posts for other vehicles with people claiming to see improvements) its a fluke, or more likely a temporary condition where the motor is likely making more pollutions and with that better mileage/performance and will quickly tune its way back to what its setting where before.


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Last edited by Mongo; 04-25-2011 at 01:20 PM.
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post #10 of 11 (permalink) Old 04-27-2011, 09:24 PM
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Thank you Mongo for Confiming my thoughts...still need the chip for sustained improvements to milage

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