Morten Hjorth-Johansen

I am still working an improving mileage, and my '00 (82k miles) is still performing worse than my old '96.

Here are the mods / replacements I have done:1. Changed injectors to 12-hole

2. Changed all 4 O2 censors (NTK)

3. Changed TB to 62 mm

4. Changed air filter to KN

5. Changed TPS (Mopar)

6. Changed MAP censor (Mopar)

7. Changed spark plugs NGK, 0.4 gap

8. Changed spark plug cables

9. Viper Coil Mod.

10. Cleaned IAC

11. Changed oil

12. Changed thermostat from 180 to 195

13. Changed alternator + cables

14. Changed fuel filter

15. Changed temperature senders

16. Scorpion 1.7 roller rockers
What have I missed??? Anything else to check???

fb97xj1

Buy a Prius.

Just kidding. Id rather drive the XJ. What is the mileage like as is, and how much more are you after?

I dont see grounds on your list. They must be clean. Always. Also, whats the fuel pressure been like? And how is the coolant, being a 2000? Tire pressures?

TrailerTrash

Have you reset the ECU since installing the 12 hole injectors?

RocketMouse

I've read that the injector "upgrade" doesn't really give you any better mileage... but rather more power... which = uses more gas, not save gas.
Maybe I'm wrong, but I've read that in a few places before.

Dave51

Where/who/what 12 hole injectors did you get?

Battle

There are so many other factors to mileage than engine performance. Tire size + gear ratio. Weight. Tire pressure. How you drive. We need more details. Also, how do you define "bad" mileage?

Also, why the 12-hole injectors and 1.7 roller rockers if you want to improve mileage?

ijeeep

12 hole injectors give a finer spray for a more thorough burn. The economy gains are modest but there. If everything else is up to snuff they also give easier starts, smother idle and improve throttle response. Improved throttle response is hard to resist so many end up using the loud pedal a little more frequently and just a little more vigorously than before which negates the economy gain.

ijeeep

Taking apart and cleaning every ground to shiney clean metal. Resistanc in the ground path between the sensors and PCM is like looking through dirty sunglasses. Neither brain is receiving a clear picture of what's going on.

Randy Bobani

Your work has been counterproductive if you are actually in search of better fuel economy. Those injectors, how many pounds per hour are they rated for? More than stock I'd bet. 62mm throttle body, more air more fuel. 1.7 ratio rocker arms, again, more air more fuel. So in order to restore fuel efficiency, you need to install the OEM throttle body and rocker arms. You also need to double check how much fuel your injectors are rated for.

The best way to restore the fuel efficiency of a 4.0L engine is to remove the head, clean the ports, resurface the valves and seats, and replace the springs. What really helps is bumping up compression. You can have the head resurfaced and then install it with a .030" head gasket from Cometic. That will get you over 9.3 : 1 compression.

00t444e

What milage are you getting?

Sig220

Not meaning to be a smart a**, but the biggest detriment to gas mileage in a XJ is a headwind. I make some regular 250 mile trips in my XJ and my mileage varies from 17.5 to 22.0 mpg. Using the same speeds (cruise), same route, same fuel fill ups. The only variable was the headwinds.
Face it, we are driving a vehicle with the aerodynamics of a brick. Other then that, to increase fuel mileage you must increase the efficiency of your drivetrain. You can make it flow fuel better, but you had better increase the exhaust flow or it reflects negatively on your efforts. You can change the
gear ration of your axles, but it kills your low speed performance. It is all a trade off but still the headwind will be there or if you are lucky behind you! So my practice is just to maintain my stock drivetrain (change oil, tranny fluid, sparkplugs, etc) and hope for tail winds!
JMHO, YMMV

RocketMouse

^^^ this... we need data....

and above... agreed about the injectors... Just going off of what I had come across. And yes... better throttle response could easily make most get a little more heavy on the skinny pedal.

Griffyorcono

I hate having to be that guy and I am not calling anyone out specifically. But some of you guys are really truly talk out of your *** lol. If you want better gas mileage you need to go with what has been tested to work. If anyone on here give you some gibberish about oh throwing magic pixy dust in my tank or removing my thermostat helped gain mileage don't listen to them, you'll be chasing your tail.

RocketMouse

i.e..... 1999 XJ 4.0L, AW4, 110,XXX miles, (drivetrain stock), 235/75R/15 on stock rims, 2" lift
Typical driving - city & hwy averg speeds 30-70, most time around 45-50mph (are you getting into OD or not as well)
Driving style, relaxed (not agressive) no jackrabbit starts (not hypermiling, but not agressive)
Tire pressure, type of tires.
Alignment, do brakes drag?... etc... details of things that can affect the overall operation.
Current emissions if you live where they are required to be checked.
Fuel pressure at the rail, idle and under acceleration, temps , oil, coolant, etc. all can be factors too.
The more info like this you give all of us to work with the better we can help you with your question/issue.
I keep mine well maintained and I average 14-19mpg depending on the kind of traffic/roads I'm on through most of that tank of fuel. And I'm at sea-level.

Dave51

Exhaust back pressure (i.e. clogged cat)