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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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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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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.
It learns your driving habits.
 

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So it will actually reduce your mileage to obtain better emissions ratings?
 

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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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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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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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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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Thank you Mongo for Confiming my thoughts...still need the chip for sustained improvements to milage
 

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Thank you Mongo for Confiming my thoughts...still need the chip for sustained improvements to milage
Ummm, NOT necessarily.

If its a matter of changing your spark plugs, new spark plugs will provide better mileage than old ones, the PCM will adapt to that and pulling the battery cable to reset the PCM will speed up that adapting. New Spark Plugs give better mileage, you don't need a chip to sustain the benefits of new spark plugs.

Look at it this way:

The PCM has a baseline tune programmed in it. As you drive, the PCM adapts and taylors that tune for what it reads from the sensors in the motor. Every motor is little bit different and operates under different conditions, that tayloring of the tune can pay off big in a better running and performing motor.

That tune, whether its baseline or the taylored/adapted tune, is programmed to be that compromised point between performance/economy/emissions.

In most cases, the baseline tune is NOT as good as the taylored/adapted tune. BUT the taylored/adapted tune is a compromise between performance/economy/emissions. So in some rare cases, reseting the PCM to the Baseline, might change that tune toward more performance or economy at the expense of emissions, until it adapts back.

That is what the Chip or reprogramming does, it changes the balance between economy/performance/emissions that the motor will tune itself to.

It might also change the "Safety Factor" built into the PCM. The manufacturers are deathly afraid of huge warranty costs, so the engine are tuned with a lot of "Safety Margin" built into them. They have to desing to the lowest common denominator, and know darn well they'll lose in court against some women that destroyed her motor, claiming how would I know driving the car while the motor made lots PopCorn Popping and Knocking Noises would destroy the motor?

So, the chipping can make improvements all around by doing things like making more aggressive ignition mappings, etc. Leaner WOT fuel maps, etc... Then its up to the drive to recognize the symptoms of the problems of the very aggressive tuning and correct it before it does damage.
 
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