MPG Issue

I have a '99 XJ 4.0L, auto, NP231.

This past summer I replaced several thing under my hood. I got Bosch Platinum +4 spark plugs, new spark plug wires, distributor cap & rotor, a Red Top Optima, a Rusty's Airtube with 6x9" K&N cone filter, and a Flowmaster 40 series exhaust.

My MPG didn't seem to improve much, if at all!!!

Would doing either of these three things help make a difference? :
1) Replace the o2 sensors (no clue where they are or how to do it)
2) Replacing the cat with a high-flow catalytic converter?
3) Leaving the key set to "ON", unplugging the battery over night, thus "resetting" my computer?

Thanks in advance!!

McFly said:

Bosch Platinum +4 spark plugs

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Dump them and go back to the OEM plugs - previously Champions, but I hear this has been superseded in favour of NGKs. I had nothing but trouble in my '01 with these, including poor MPG figures - though my cars love them. *Shrug*

1) Replace the o2 sensors (no clue where they are or how to do it)

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Follow the downpipe from the exhaust manifold towards the back of the XJ. At some point, you'll see at least one (there may be more on yours) thing that looks sort of like the top of a spark plug screwed into it with some wires running out of the top back up into the engine bay. This is your O2 sensor. Unplug (or cut; there may or may not be a plug-in connector to it) the wires and use a wrench to unscrew it.

3) Leaving the key set to "ON", unplugging the battery over night, thus "resetting" my computer?

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Just remove the negative lead from the battery's terminal; no need for the key to be left in or the positive terminal to be removed. Overnight might be overkill; my 2000 seems to do the reset in about an hour (as did my '01; YMMV). FWIW, do the reset just after filling up the tank - if you got a tank of bad gas, the reset won't do you much good once you start driving the XJ again. Typically, it seems to take about a tankful for the computer to relearn how to make things go.

i have that exact sme setup and also have seen little to no mpg gain...but it sounds a lot cooler

reseting the computer wont do you any good. The only time to do that is
right when you make some repair/change. If you made some change months ago the computer has 'learned' on its own by now.

From my time of doin stuff like that all i've found is ways to waste money. All i do is a k&n up frount and a cat back and call it good. milage gains will never be big but helps in the low end power if you are luck 1 mpg gain 2 on a good day but if your are looking for really big gains unless ya do gears it will not happen.

i got the best gas milage when my xj was completely stock.

89xj said:

i got the best gas milage when my xj was completely stock.

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OEM goes to great efforts to maximize MPG. There are huge penalties/incentives from the government to meet mpg targets (CAFE). Any aftermarket modification to improve mpg will be incremental at best and the cost for mpg improvement will generally NOT be offset by reduced fuel costs unless modification is extreme and the vehicle is driven many miles.

If there were cheep and easy ways to substantially increase the mpg of a vehicle, the OEM will implement them as original equipment on "stock" vehicles.

I would say you need to look at the cross over pipe in the exhaust. I've heard its restrictive in the stock form. Any system is only as good as its weakest or most restrictive point. Your other mods will help, but until you balance them, then they don't/won't work to thier fullest potential. I have one car that many years back I put on headers. A friend said I'd see some improvement, but not a lot, until I did the rest of the exhaust. That is what I found to be. It was a noticable difference with a true daul exhaust. At least seat of the pants feel. But that was almost 20 years ago. Still, have the car though.
Tom

gjxj said:

reseting the computer wont do you any good. The only time to do that is
right when you make some repair/change.

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Yes, but the computer is also constantly adjusting timing and air/fuel mixture based on current driving conditions. If you get a bad tank of gas (in my experience, anyway), resetting the computer after MPG figures have plummeted is a good way to get it back to where it should be.