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My 01 xj is getting 10mpg... Help?

Have you tried going back to standard OE NGK copper plugs? I have never found these old iron inlines to like fancy plugs. Even the coil on plug ignitions still seem to like the copper plugs best in my experience. I know plugs won't make a huge difference here but hey you never know. Also I put in a set of the 784 Chrysler injectors in my cousin's 2000 and he saw a significant boost in mileage with those and the new NGK v-power copper plugs. His speedo is still off but he would reset his tripometer every time he filled up and he said he can go 40-60 miles further on a tank than before. And we just used some junkyard injectors. A gamble for sure but I have had good luck with junkyard injectors and have put in about 5 different sets now. Some say the 784 injectors flow too much and that math and sensor data prove that but as far as I have seen they have proven well in the real world so agree to disagree. I wish we could figure out his exact mileage but he has been procrastinating on buying a new speedo gear.
 
i can usually make it to 270 miles when my light comes on, and my last 10 fill ups have avg'd 15.5-16.0 gallons.

1997 auto 197k
3" lift
245/75/16 MTR
4:10
 
mounted my roofrack much closer to the top and gained 2MPG on my last tank. its funny how the little things add up

1990 XJ 4.5" lift, AW4, stock gearing on 32's, Ford injectors, shell premium gas 16MPG highway :moon:
 
theres a guy somewhere online on one of the XJ forums that bores XJ throttle bodies for like 100$. I highly doubt a bored TB will add to MPG, probably stay the same or go down even. after i went from 52-60mm on my renix the off the line was nicer, but i didnt notice any gains in MPG.

check for clean grounds, exhaust/intake manifold leaks, vacuum leaks, test the TPS, CPS, also winter gas sucks, and that probably isnt helping. verify that all o2 sensors are functioning properly. just because some are new doesnt mean much, and the post cat sensors could still be bad as well, which I doubt would kill MPG. consider sea foam in the intake and in the gas tank. how new is the air filter, etc?
are you 100% the TC is locking up when you are cruising ? Determine if the cat is good as well, that could easily do it.

running too cold, staying in closed loop?
 
blondejoncherokee said:
theres a guy somewhere online on one of the XJ forums that bores XJ throttle bodies for like 100$. I highly doubt a bored TB will add to MPG, probably stay the same or go down even. after i went from 52-60mm on my renix the off the line was nicer, but i didnt notice any gains in MPG.

check for clean grounds, exhaust/intake manifold leaks, vacuum leaks, test the TPS, CPS, also winter gas sucks, and that probably isnt helping. verify that all o2 sensors are functioning properly. just because some are new doesnt mean much, and the post cat sensors could still be bad as well, which I doubt would kill MPG. consider sea foam in the intake and in the gas tank. how new is the air filter, etc?
are you 100% the TC is locking up when you are cruising ? Determine if the cat is good as well, that could easily do it.

running too cold, staying in closed loop?
Click to expand...

Positive. I have a rail shifter with TC lock up I use off road, so I know what it feels like.

No post cat sensors on an 01. Up streams could very well be bad. I should just swap them for mopar units.

And yes, my engine runs rather cold, never hits 210 at normal operating speeds, or just cruising.
 
How's the alignment? After a wheeling trip my mileage was TERRIBLE. I was getting 10MPG. It wasn't until I noticed the tires were wearing on the outside horribly that I checked the alignment. Immediately went from 10 to 15mpg. Had a bad wheel bearing, not sure if that helped replacing it or not.

My mileage started to dip down around August for some reason and my alternator was whining a lot (whistling). I had to jump start it a few times. Turns out my battery dropped a cell and was overloading the alternator to the point that the engine felt it!

The other things I'd look at:
- Are you running Ethanol gas?
- Serious electrical drains?
- Alignment
- % Ethanol gas?

Might be worth swapping the injectors?
 
I took my front wheels off today. Couldn't even spin the wheels to spin the ring and pinion. It was seized, and it was causing the front locker to disengage.

I decided to check the pinion... It was completely unengaged from the ring gear... I think the gear set is toast. I can hear it skipping over the ring as I turn the pinion.

Great. Front axle is toast. I swapped in some outer shafts only, so now it at least rolls and the wheels aren't locked up.

Man... did not want to have to rebuild my axle that was rebuilt a year ago.
 
Damn, that sucks. Now at least you know what was causing the vibration you were feeling. Good news is it isn't the transfer case. Have you taken the from cover off to see what the damage is?
 
The sad thing is that the front output on the tcase is still shot too.

I'll take the cover off today.

This sucks. I just got a new tcase to rebuild, and I've been saving to fix that, now I have a blown front axle. I'm seriously getting tired of my jeep.
 
My friend at a shop up here, he does awesome work, but I think something just went wrong. When the pinion yoke worked loose it must have wrecked the setup.
 
Columbus, you just need to blow that whole thing up and start over. Take a video of the gears when you pop the cover open. I want to see them.
 
LOL for real!

Good news! Took it to my friend Matt's shop, he's the one who installed them.
The gears are fine. Turns out both pinion bearings, inner and outer were toast. He replaced both of them, changed the races and everything, and reassembled it. The pattern is perfect, and the axle is happy.
 
Switching back to the bad mileage topic.

Step 1 for me was installing a overhead consol so I can actually see what my fuel consumption is real time. I have now gone from around 11mpg to 13mpg, I managed this by adjusting how I drive. I am not driving like a old man either and I could improve on 13 if I really wanted to. I am just being easier on the gas, and manually shifting when it will help.

Step 2 is going to be new pre cat o2 sensors, "the ones before the cats." Then the pre cats are going bye bye so my vapor lock is gone and I no longer have intake manifold heaters... Yes I don't care if it is a federal law I am breaking and I know for a fact my jeep will still pass Oregon emmisions testing with my rear cat still there with flying colors.

I am going to do everything one step at a time so I can see the outcome and report back.
 
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