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You can bet the chip is affecting fuel mileage, they boost performance by using more fuel (usually premium) among other things..
The PCM interior plug is accessed by removing the passenger side kick panel. The PCM itself is bolted to the firewall below the glovebox. Here's a typical PCM upgrade install instruction: http://redhat3.admarkonline.com/~sup...install-16.pdf
Makes certain you disconnect the vehicle battery before screwing with any connectors. Failure to do so can damage electronic circuits.
Quite often when someome says they have put in a chip what they've really done is reprogrammed the PCM (Powertrain Control Module) with a tuner such as a Superchips.
If that's the case you probably won't be able to find out what they installed and to get rid of it you would have to either have the tuner that was used originally or take it to your dealer and have them reprogram the PCM back to stock.
I have the Diablo chip in my '01. You can see and feel the chip without taking the side kick panel off. I think you'll be able to see any "add-on" chip without too much trouble. If the computer has been reprogrammed, as Racerguy suggested, the best solution would be to contact the previous owner and find out what they did and if they'll return the program to stock(if that's what you want). Most likely would be cheaper than going to the dealer. If there is an added-on chip it takes all of about 3 seconds to pull it out, but make sure you disconnect the battery first as the computer will be a costly to replace.
Also, as far as gas milage goes I get slightly better milage with the chip and running premium. The cost of running premium is about $4 per tank more than regular(I usually fill up around 20 gallons) but with the 1/2 mile or so better milage I save about $2. So the actual cost of running the premium is about $2 extra per tank. Not much when the fill up costs almost $70. The slightly extra fun factor is worth the extra money for me, IMHO.
HI- the Ford dealer told me my '99 7.3 F250 has been chipped, that it is going bad, causing engine cutout and masking trouble codes. Can I "unchip" to stock by replacement? What about upgrading to new programming kits- maybe I need to unchip in order to reprogram? Thanks for any chip info. Mike.
If your chippedd you can see the chip on the end of the computer under the dash. just remove the chip & the truck will go back to stock. Like mentioned above unhook the battery first.