Once again, I have more questions, mainly to do with the engine, the 4.0.
I'm not getting the mileage I hear other people getting with the L6. I've repaired the header (stock), run a bottle of injector cleaner with a tank of 91, replaced the spark plugs cap and rotor (not wires) and replaced the fuel filter, but only getting bout 110 out of half a tank.
I've replaced the O2 sensor, but the diagnostics show the mixture going totally berserk, from lean to center to rich and back.
It also seems like the idle RPMs are really low, about 500-600. It gets real shaky around there.
Anybody got some tips or ideas?
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Have you checked your exhaust for leaks or cracks? That seems to affect your mileage a bit. Also, if you are running larger tires, that will throw it off. A gearing swap will help that. The idling issue could be related too. Couldn't tell you how to fix it but I'm sure there's a way. I would think that the Mixture issue could be dorking it up too.
Just to be clear, what are you hearing is the regular mileage for the l6?
And what are you getting?
Also, what year is your xj?
Hope this helps.

 

I've been under it recently while it was running and I didn't hear anything out of the ordinary.
Me and my dad welded a crack in the header right after we first bought it, but that's the only leak I've had so far.
Everything is stock all around, tires, gearing, brakes, diffs, the whole thing, stock.
It could also be that I got a bad O2 sensor, I got it for $15 off of Amazon so I wouldn't be surprised if it's a p.o.s.
I'm hearing people getting 16-17 (~300 miles per tank) while I'm getting 11-12 (~200mpt).
I have a 94 sport, 4-speed Auto, Dana 30 in front, Chrysler 8.25 in rear.
Thanks for the prompt reply.
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test your tps , your aic, map sensor, coolant temp sensor too. if you have the Haynes fix-it book, that will show you how to test all that.

 

If you had a crack that you repaired in the exhaust manifold there could be a possibility that your cat is clogged. With the extra air going into the exhaust before the O2 sensor it could cause it to run rich in turn adding extra fuel to the mix then to cat having to collect more gas then normal causing it to clog. I would get a new set of O2 sensors from NTK/NGK and put those in. Nothing else seams to work well in the jeeps. I know there is a way to check for exhaust restrictions using a vacuum gauge on the intake manifold.

 

Will it have passed emissions if a clogged cat was the case? And where can I get a O2 sensor that won't fail me but won't be a butt load of money, if there is such a thing?

 

When I did my 2 O2 sensors i got them both for about 80 dollars from amazon. there is also rock auto, they have good prices as well. look to spend about 40-60 dollars each for the O2 sensors. What year is you Cherokee anyway?
Just saw your post about the year. In 94 you only have 1 O2 sensor.
Depending out how your state checks emissions it could have passed with a cat that is going bad.

 

That's how the O2 works. Fluctuates back and forth.

 

Have you actually checked your mileage the correct way; divide miles travelled by gallons used? If you're estimating (when you said you got 110 miles per 1/2 tank) you could be way off. My gas gauge is not linear, when it gets down into the red and the yellow low fuel light comes on I still have almost 6 gallons left.
I have a 89 with AW4 and 3:55 diff's, stock, 20-22.5 highway mpg.

 

This.

This twice, thrice and more.

If your gas gauge is any part of your "calculations", you are doing it wrong.

There is only ONE way to know your mileage. Record the miles traveled, divide by the gallons pumped. From the reading on the pump. Not estimated. Not tanks. Measured gallons pumped. Nothing else.

Do this tank after tank after tank. If you have an Android phone, get Acar. It's an amazing app.

If you are talking about tanks or half tanks or quarter tanks or fill-ups, you have no clue what your actual mileage is.

Worse, you think you know. You don't.

You may be chasing ghosts.

That was $15 wasted.

NTK/NGK. ONLY. At NAPA it's about half of what the dealer charges.

Not Bosch. NTK/NGK.

I don't know if you call that a boatload of money, but anything else is a waste of money.

 

Thanks for the info. It helps allot.

 

Okay, thanks