Gas mileage gains

jbkelly0211

Well-Known Member
Joined
Jul 20, 2010
Location
Charlotte
Just wondering if anyone has had luck gaining a few extra MPG's, my 99 xj is only getting around 11 city/ 13hwy. I have 32" mt's with a k&n filter and glasspack exhaust. I know regearing would do alot but I was wondering if any had results from bolt on's or any other tricks.
 
Are you going off the the miles shown on the odometer to figure your milage? If so you need to add about 10% to that to account for the amount the speedometer is off. Im getting an average of 16mpg in my 98 xj with 33's and stock gears.
 
if your speedo gearing is right, it sounds like you have something wrong- in my zj with 4.0, auto, 3.73's and 32's i get almost 18 hwy, 15 in town, it usually averages a little better than 16 combined. a xj should be a good bit lighter for better than that.
a cap/rotor/plugs, air filter, oil change(10w-30 conventional) and a can of seafoam go a long ways. the cheapest mpg's you can get is from a tire pressure gauge.
i did relocate the intake air temp sensor to the airbox, which was worth almost 1 mpg and it seems a bit snappier on throttle response.
 
you may also want to swap the glasspack for a better muffler, glasspacks tend to confuse ecu's or carbs and cost power & efficiency while just making a bunch of noise. try a thrush welded or flowmaster 40 or 50 series-they sound good on a 4.0 as long as you have a nice length of tailpipe behind them.
 
Are you going off the the miles shown on the odometer to figure your milage? If so you need to add about 10% to that to account for the amount the speedometer is off. Im getting an average of 16mpg in my 98 xj with 33's and stock gears.
This^. With the 32s you are driving more miles on the same number of rotations as your stock 28.5" tires. The difference would really be closer to 13% going from stockers to 32s. This would take your current figures of 11/13 to more like 12.5/14.7.

Since you mentioned you have a K&N, I would check your mass air flow sensor as well. K&Ns are notorious for over oiling and coating the MAF with oil, that coupled with their horrible filtering ability will let dust stick to that oil and foul the MAF, causing it to read incorrect. This can do all sorts of things from hurting fuel mileage and power, to even altering the transmission shift points. There is a special MAF safe cleaner made just for cleaning the delicate sensor.
 
^cherokees don't have a MAF sensor. When relocating the intake temp sensor the the airbox is it as easy as it sounds? Plug hole in intake manifold and drill a hole and put a nut on the inside of the airbox lid? Any downsides?
 
^cherokees don't have a MAF sensor. When relocating the intake temp sensor the the airbox is it as easy as it sounds? Plug hole in intake manifold and drill a hole and put a nut on the inside of the airbox lid? Any downsides?
I see, didn't know Heeps used speed density tuning. With that said, the K&N oil can also just as easily foul the IAT and MAP sensors as well, although not as common as fouling the MAFs.

Relocating the IAT is as easy as it sounds. If staying with the stock airbox, there likely wouldn't be any downsides. There is the potential for pinging or knock with low grade gasoline on warmer days, but the knock sensor should retard the ignition back, negating any gains of relocating the IAT in that situation and keeping things running smoothly.
 
the chrysler era 4.0's don't use a knock sensor, the compression is low enough where they rarely run into a spark knock issue, however; detonation from carbon & oil deposit in the combustion chamber on a high mileage motor does happen...
 
the chrysler era 4.0's don't use a knock sensor, the compression is low enough where they rarely run into a spark knock issue, however; detonation from carbon & oil deposit in the combustion chamber on a high mileage motor does happen...
Welllll then, relocate IAT, seafoam it, and keep an eye out for pinging when it is really hot out or under heavy load.


It's been 4 years since I've had a Jeep, and I only had it for 6months, so I'm a little rusty on my 4.0 knowledge. Just speaking to general EFI knowledge.
 
The best thing you could do is not be a Kelly. That has a profound impact on the weight of your right foot.
Not really sure whats wrong with yours. My TJ gets 13mpg everywhere, weighing over 5k pounds on 36's. The overdrive in the AW4 is better than the TJ, and the XJ is alot lighter and more aerodynamic too. What does your temp gage show when its warmed up? The hotter mine runs, the better the fuel mileage.
 
^So seeing how fast you can get to 45 from every stoplight doesnt help? Mine runs 210*, never changes. But one thing I am wondering, do all XJ's A/C clutches engage with the heat on? Or just mine.

Yesterday changed plugs back to Champion copper cores, new plug wires, dist cap, rotor, and took the TB off and cleaned every nook and crany, as well as relocated the IAT to air box. Hopefully this tank will go about 450 miles. I would be happy with that.

I also got magnets for my fuel line, seemed legit....

Just kidding.
 
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