1994 with poor fuel economy

[xjtrailrider]

Im tired of getting 12mpg(mixed driving) on a stock XJ, my trail rig does better turning 35's!

A little history;

94, AW4, NP231, all stock. 289,000 on the chassis and tranny

I just installed a newish motor, its a 96 block with only 90k on it. My last engine was burning 2 quarts a week. It only got 12mpg as well(mixed driving).

When I installed the new engine I replaced all of the sensors(CTS, MAT, O2, MAP, CPS, new distributor). I replaced the fuel injectors and the FPR. I also cleaned and lubed the three connectors on the ECU and the bulk head connector. I added NGK plugs and Accel cap/rotor. The exhaust is new, with a new cat. New fuel filter.

I cleaned the throttle body and the intake before I installed it.

It runs very well, strong acceleration, smooth idle, plenty of oil pressure.

NO CODES, NO CEL!

Any ideas? I covered about everything when I had the engine out of it but there has been no change. I know its not the motor. I was getting 16 mixed in my 96 XJ with this engine before my teenage son sent it to Jeep heaven. I have owned this XJ for 10 years and it has never gotten good mileage. I'm thinking it might be time to swap ECU's and see what happens.

This jeep drives so good, I'm not giving up on it

 

[xjtrailrider]

I'm not that good, I must have missed something!

Give me some ideas!

 

[Ambar1]

I am but an egg, but I'll refer you to this thread: http://www.jeepforum.com/forum/f11/my-goal-get-25-mpg-my-99-xj-30-x-9-5-r15-ats-1551782/

 

[xjtrailrider]

A couple of things I'm reading that I might look into;

I have a Bosch O2 not a NTK

My cat is one year old and it could be partially plugged since it was behind that oil furnace of a 4.0 that was in it.

My plugs are gapped at .035, I could probably stretch that out to .040 or .045. I have NGK V power plugs. Thats all I have ever run on my XJ's.

I run Rotella 15w-40 which might rob some power, I could drop to 10w-30 now that the weather is cooling.

I'm running 30x9.5-15's on TJ Canyons, that tire is a little taller than the stock 225x75-15's, which could throw off my mileage calculations.

 

[xjtrailrider]

Okay, I decided to revisit this thread.

I replaced the O2 sensor with a NTK and checked the cat, all is good. I'm still getting 11-12mpg mixed driving.

I have checked all grounds as well.

During all of this my MAP went out again so I put in a new one. My CPS took a dump and my coil. So all of that is now new again.

It seems to stay in open loop, even smells rich at idle. My next step is the ECU or :explosion

 

[four_shot]

Did you ever switch back to 10W30? internal losses could be caused buy running 15W40.
I know I noticed a difference when I switched from 5w50 to 5w40.

 

[xjtrailrider]

Did you ever switch back to 10W30? internal losses could be caused buy running 15W40.
I know I noticed a difference when I switched from 5w50 to 5w40.

Not yet but I am when it needs changing in a week or so. I still think its the ECU but how do I test OBD1 without the right scanner and when its not throwing any codes?

 

[AKghandi]

i have a renix with similar isues. i have 33s though so i get about 10 mpg.

i have suspected the ecu for a while. everything but the voltage to the o2 heater is correct.

 

[Johnnie Walker]

Not yet but I am when it needs changing in a week or so. I still think its the ECU but how do I test OBD1 without the right scanner and when its not throwing any codes?

Find someone local, or go to the JY and get an ECU cheap to swap in. Do not need a CEL to "show" a bad ECU. How's the trans shift? that and the t-case been serviced recently?

i have a renix with similar isues. i have 33s though so i get about 10 mpg.

i have suspected the ecu for a while. everything but the voltage to the o2 heater is correct.

NTK O2 sensor, and a tune-up.. Other than BAD exhaust leaks, O2 sensor been the main cause of my fuel consumption issues. Also, some tired engines are just that.
Also, if you are on stock gears with those 33's then that is definitely part of your issue, aside from a potential high mileage engine.
I despised driving my Renix XJ 4.0L/AW4/NP231 with stock 3.55 gears on 31's. But I still got 15+ MPG.

 

[AKghandi]

NTK O2 sensor, and a tune-up.. Other than BAD exhaust leaks, O2 sensor been the main cause of my fuel consumption issues. Also, some tired engines are just that.
Also, if you are on stock gears with those 33's then that is definitely part of your issue, aside from a potential high mileage engine.
I despised driving my Renix XJ 4.0L/AW4/NP231 with stock 3.55 gears on 31's. But I still got 15+ MPG.

Been there done that... http://www.naxja.org/forum/showthread.php?t=1123976

hijack/over

 

[xjtrailrider]

My engine is a low mileage 96 4.0 that I just swapped in. I freshened it up with new seals/gaskets/timing chain.

I also added all of this at the time of the swap;
New CTS
New MAT
New injectors
New ACCEL cap/rotor
New exhaust manifold
New FPR
NGK V power plugs gapped to .040
Cleaned the TB and intake in a ultrasonic washer at work(I'm a diesel tech)
There is probably more, I collected parts for a year.

Added since the swap due to failure;
New CPS
New MAP
New coil

I paid $50 for this XJ with a motor that was nothing more than a mobile oil furnace. It used 2 quarts a week and the drivers front door was no longer attached to the body LOL!

A little TLC and a fix to the door and I drove it for a year trouble free. I parked it last spring and started prepping the new motor to go in. I have slightly larger than stock 30x9.5-15 General AT2's but other than that its all stock even with sagged out rear springs.

I'm ready to roll it off a cliff but I swear its the best driving/handling/riding XJ I have had. And I have had a few! This thing goes around curves like a sports car and will roll down the highway with no shimmeys or shakes. Very comfortable to drive daily

If I could just squeeze out better mileage.

 

[Johnnie Walker]

Have you been able to put a Scanner on it to check sensor readings and such?
Closed loop and trans/TC lockup functioning properly? Ohm tested the CTS? E-fan works properly? May need to test swap another ECU, if all the sensors test good.

 

[techno1154]

This is a long shot but,....

Check the entire intake system again. It could be leaking air after the throttle body but not enough to cause the engine to speed up. Check the vent hose and on the top of the valve cover paying attention to the sixe of the venturi in the elbow that goes into the valve cover.

I do not know but those same spark plugs that worked good in the previous engine did not so good in my 1996 4.0L. I chucked them after a few thousand miles and re-installed the OEM Champions that were previously removed with much better results.

Yes, I know the injectors are new. Are they leaking? When was the last time you did an old fashion plug reading. Put a vacuum gage on the intake and take readings at idle, when you blip the throttle and at your normal drive speed.

Outside of these, I will have to go back into my old timer memory bank and maybe I could come up with some other things to try.

 

[xjtrailrider]

Thanks guys, I will concentrate on those as well. Plugs are cheap so I will swap them back out and its due for its first oil change on the newish motor so I will go to a 10w-30 from the 15w-40 thats in there.

The T/C locks up right around 45mph and holds down to 25mph as long as I light throttle it so that seems to be right

I forgot to mention in my list of things that I have done, it also has a brand new NTK O2