Stock XJ... What to do?

i have a 1999 stock XJ, with the 4.0. i want to improve power and also fuel economy, what are some things i should do first?

1. intake
2. cat-back
3. header
4. hi-flow cat

...but to tell you the truth, power and fuel economy are usually inversely related...

djstew42 said:

i have a 1999 stock XJ, with the 4.0. i want to improve power and also fuel economy, what are some things i should do first?

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First, welcome to . Second I would start with a good tune up then run some Seafoam through it you can pick it up at your local auto parts store. Make sure you follow the directions on the bottle. I normally run a bottle of Lucas Fuel system treatment through mine every few tanks. then of course follow with a drop-in K&n filter and a freer flowing exhaust.

lift it and put some 35's on. Then take the 35's off and be amazed at how fast your rig is.

djstew42 said:

i have a 1999 stock XJ, with the 4.0. i want to improve power and also fuel economy, what are some things i should do first?

Click to expand...

FlexedXJ pretty well summed it up.
Questions:
What kind of mileage do you get now?
The consensus around here is the well tuned, mild XJ gets between 17 and 21 mpg. There are a few(very few) people(nutballs?) who claim 25 mpg. Nobody seems to be able to explain why their rigs get so much better mileage. Anyway, shoot for about 20mpg highway.
How many miles on the Jeep? If it's approaching 100K, and getting substandard mileage, it probably has a sensor problem(like the O2 sensors are going bad)
Do not lift/add bigger tires if you want good mileage. 235-75s or 30x9.50s are about the limit, then you start seeing a drop in mileage. You'll also start to notice the rig seems down on power unless you re-gear the axles.
A**kicker: the faster you drive, the worse the mileage is.( mainly due to the extremely pleasing, stylish, BRICK shape. )

As for power, more power is going to equal less mileage(if you're using it). \

theNAST1EST said:

1. intake
2. cat-back
3. header
4. hi-flow cat,...

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A little extreme, I think. The '99 already has a pretty good intake. A good header and exhaust will help a little, but won't really "pay" for themselves. I'd say keep the stock stuff unless/until it's ready for a replacement, then look for an upgrade. The stock cat on the 4.0 is a pretty good one(for our power numbers, anyway). Again, if it doesn't need replaced, I'd leave it.

If you want to go to the effort for power, I'd go in this order:

Exhaust header
Cat back system or new custom pipe
If you're going that far, a new head pipe(stock pipe has a kink in it right below the manifold. The kink will come back if you stuff the FL tire all the way up. )
One of the air box/air tube replacement kits
Add a 62mm throttle body,
The 00/01 intake is a little better then the '99,(I wouldn't bother)

'nother a**kicker: the 4.0 is pretty balanced as far as breathing goes. So adding any single piece of the above mentioned equipment won't make a huge difference. If you're doing it piece by piece, start with the exhaust.

That's about as far as you go without digging into the long block and/or fooling with the computer. It's a lot of money to spend for a little gain. The 4.0's not a V-8, and you can't make it go like one without a lot of coin. :dunno:

welcome to the club!

(P.S. Running the sea-foam through the engine is fun just to watch the wagon turn into a fog machine Do not do this in your garage!)

tbburg said:

As for power, more power is going to equal less mileage(if you're using it).

.

Exhaust header
Cat back system or new custom pipe
If you're going that far, a new head pipe(stock pipe has a kink in it right below the manifold. The kink will come back if you stuff the FL tire all the way up. )
One of the air box/air tube replacement kits
Add a 62mm throttle body,
The 00/01 intake is a little better then the '99,(I wouldn't bother)

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:twak:...is there an echo in here?





:laugh:

just to add to the echo

if you go for a header do the downpipe too. new header without new downpipe is still restricted flow... so have the money for both if you want to do it right.

money wise I'd say;

tune up
K&N filter or make a cone intake setup (don't buy kit, can be homemade for much less $$)
bore your own throttle body (find the write-ups, dremel and your done)
complete exhuast, if you can save up, do it all at once.



the 99 intake is the same as the 00+ (I swapped one into my 97, 98 and older is square runners, 99 and newer is rounded runners)
other than that, drive slow, lower it a bit, new plugs, wires, normal tune up gear and you should be good.