Ok so back on 12/11/11 I had some 784 injectors install in my 00 Jeep. I also installed some fresh NGK plugs and a new paper air filter. The plugs were properly gapped (0.35). I even reset the ECU as suggested in the how-to tutorial thread.

Before I did all this I was getting between 11.98-12.83mpg and I thought that was bad. But my last two tanks of fuel I got 10.61 and 10.65 mpg. I went from getting 205-215 miles out of a tank to 175-185 miles..

Other then the 784 injectors, paper filter and NGK v-Power plugs nothing has changed. Anyone have any idea what the problem could be? I am getting no CEL's. IMO it idles better, better throttle response and overall feels healthier but I am getting worse off mpg..

 

I'm not that mechanically inclined, but don't the 784's allow for more fuel flow, increasing acceleration and response? I would think that increase alone would lower the fuel mileage. Like I said, I don't know much about this, so I too am awaiting a response since I wanted to do the swap as well.

 

Your mpg's before the swap are pretty low in all honesty. In my experience with the injector swap, lower mpg's are a result of the driver. If you did everything correctly, making sure there are no leaks and reset the ECU, then you should see either no mpg gain (normal) or a slight mpg increase due to using clean injectors (occassional). Lower mpg's are nearly always due to a change in driving style - mainly in the area of acceleration. Don't go over 2200 rpm's when accelerating for a full tank and then see what you're mileage is. If it's still low, check your O2 sensors. They don't always throw a CEL code when they start going bad.

 

What kind of paper filter? When is the last time you have did a ignition tune up?

 

I thought the only recommended plugs for the 4.0 were the Champion Truck Plugs P/N 4412

 

Better performance=worse gas mileage.

 

It is winter time cranking it to let it warm up may be the problem

 

Agreed, install a cheap copper plug. Also consider your O2 sensor. What size tires are on your XJ? They look oversized in your picture. Also, is this city driving? If you are running in the city on 30-31" tires with larger injectors, 10-11 might not be too out of the ordinary.

 

I run e3's with all the upgrades
Injectors
Intake
Tb
Accel ignition system
Cai
5spd
31's
And in town I get 17
Highway 20
Winter
Town 15
Highway 17-18

 

The reason champions work so well and others don't is because you Cant run better plugs with a weak(old) or stock ignition system

 

Tires - 31x10.5x15
Lift - RC 3"
Gears - Stock

Most of the driving I do is city. I bought this Jeep around this time last year.
The only thing I did was:
- New Plugs
- Paper Filter
- Scheduled Oil Changes
- Coolant Flush
- RC Lift
- Wheels & Tires
- 784 Injectors
- Custom exhaust (2.25" piping) with Flowmaster muffler and a magnaflow cat right after the header.

I haven't touched the ignition or the o2 sensors. I figured if they were bad it'd toss a CEL, I guess not? It just baffles me because I have a friend with a 5.5" lift and 33's and he gets a steady 14-17mpg city.

 

Try a new o2 and get an Accel ignition tune up kit or msd for like 100$ its from the pick up coil to the wires, then pull your plugs re check the gap or make it bigger if you want ( a Accel or msd or screaming demon will allow between. 35 all th way to 65 for the screaming demon kit) and then see what happens

 

Also winter fuel gets less mpg so add 2 mpg to what ever you come up with to be on the safe side

 

Just a quick question i am new here and i did the injector swap also. is it normal that when i start her up it takes longer to turn over then when it had the stock injectors. i reset the ecu and was curious if anyone else encountered this?