Why does my MPG suck all of a sudden

So here is my story and it would be great if anybody could help me figure out what the problem is. First here is the setup my truck has. I have a 96 xj with a RE 5.5" and 33" pro comp x-terrains. Im using stock gears with a k&n air filter and a big old crack in my stock manifold. As for my exaust i ripped out the cat and put in a dynomax muffler (it sounds mean) The second 02 sensor is just disconected and the front is brand new. Thats how my truck is setup and here are my symtoms. First i got 112 miles to my last tank and thats with 85% highway. I also get a gunshot backfire and nice fireball when i downshift through the rpms and also if i rev the engine to 5k rpms and as it drops down to 2-4 there goes the fire ball. it sounds like it misses for a couple of seconds after that. The other thing is if i downshit during normal driving and i push the clutch and stop at a light the trucks idle drops to like 200-300 for about 10-15 seconds then shoots to 1000 then at 800 where it should be. I was hoping it was the 02 sensor but i changed it and it didnt help. this is my DD and 7 mpg just is too much. Any body have any advice?:flame: thanks guys.

nominated...

how about you fix your exhaust manifold, do a tune up, stop revving your engine to 5k rpms, re-gear back to where you should be, and probably get all new O2 sensors. Also might want to get a new IACV, TPS and MAP Sensor. Hell, maybe get your injectors flushed or replaced too.

x2 on the tune up...and clean your k&n. oh, and fix your manifold too. im doing mine this weekend.

the two biggest things that will help?

Fix the manifold and regear.

JNickel101 said:

nominated...

how about you fix your exhaust manifold, do a tune up, stop revving your engine to 5k rpms, re-gear back to where you should be, and probably get all new O2 sensors. Also might want to get a new IACV, TPS and MAP Sensor. Hell, maybe get your injectors flushed and replaced too.

Click to expand...

X2 Thats some funny chit.:dunce:

yeah, just fixing teh manifold will probably solve the backfire, but like everyone else said do a full tune up, clean the k&n etc...then clean the throttle body too, the k&n oil likes to collect and dry up around the butterfly and make it run like chit

and 33's on 3.07's wow :wierd:
once you get everything fixed you'll still be lucky to get double digit mileage

Don't let me stop you from replacing every sensor listed in the quest for fixing this problem, but I would first fix the manifold. Here's what I think is happening:

The crack is letting air in to the exhaust, which makes the 02 sensor think you're running lean, so you end up running very rich. Does it run better in open loop (Before fully warm)? If it does you can keep it there until you get the manifold fixed (Unplug the 02 sensor).

for sure manifold will help. make sure connections are good on injectors. clean injectors. see if you have any big dents on your catalytic convertor.

well like i said i have no cat and i did a tune up about 17k ago. WHILE WE ARE ON THE TOPIC what is the best way to fix my manifold i want a header but money is tight what can be done with this i would fix the stock if i save money but I didnt think that was an option. so i will pull the plugs and see what they look like but im gonna see about changing the sensors and other stuff youguys suggested. what would explain the bad idle after downshifting?

Also i found some gears cheap im wondering if i could do the job myself. how hard can it be right, i did my lift installed my sye and am now putting an engine and tranny in my buddys monte carlo anybody done the job b4 and i wilol do more reasearch but what gears would i go with with 33's

kenny schmitt said:

Also i found some gears cheap im wondering if i could do the job myself. how hard can it be right, i did my lift installed my sye and am now putting an engine and tranny in my buddys monte carlo anybody done the job b4 and i wilol do more reasearch but what gears would i go with with 33's

Click to expand...

just take the old gears out and put the new one in. you will be just fine.

OMG...this thread brings the suck on so many levels...

5.5" lift on an XJ and you wonder why you have bad mileage?
33" tires on stock 3.07s and you wonder why you have bad mileage?
Cracked exhaust manifold on a 4.0L and you wonder why you have bad mileage?
Gutted cat and no downstream O2 sensor and you wonder why you have bad mileage?
Driving it like it's from 'Fast and Furious' and you wonder why you have bad mileage?

Sheesh.

Ron White said:

You can't fix stupid.

Click to expand...

I concur.

...did I mention the regear?

Jim www.yuccaman.com

Yucca-Man said:

OMG...this thread brings the suck on so many levels...

5.5" lift on an XJ and you wonder why you have bad mileage?
33" tires on stock 3.07s and you wonder why you have bad mileage?
Cracked exhaust manifold on a 4.0L and you wonder why you have bad mileage?
Gutted cat and no downstream O2 sensor and you wonder why you have bad mileage?
Driving it like it's from 'Fast and Furious' and you wonder why you have bad mileage?

Sheesh.I concur.

...did I mention the regear?

Jim www.yuccaman.com

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hahaha so true, not to mention stock xj mileage aint so hot to start with

you wanna 'wheel get a jeep
you wanna have good mpg get an econobox

Get a new manifold!!!! any air leaks before the 02 sensor in front will seriously hurt mileage...you can try and weld it for a quick fix- but to get to it and weld it you almost have to have the manifold off..they have them aftermarket, just buy one!!!!
I've got to wonder why you pulled the cat-it really doesn't hurt anything if it isn't clogged or broken up...and if it was, then replacements are again available aftermarket...and while I'm not a tree-hugger, making a daily driver that was kind of clean pollute more is NOT cool...
Until you fix the manifold, throwing new sensors at it is a major waste...the manifold would be cheaper....

X3 and X4

im sure reving it up 5k rpms doesnt help much either. great way to toast the valve train on a poorly running engine.

I just bought a new manifold for mine off of ebay for $100 shipped to my house. It actually looks and feels likes its made out of heavier material. It comes bare metal so I sprayed with the 1200* paint from the parts store and baked it like the instructions said. My wife wasnt to happy cause it stunk up the house but oh well. My cherokee has 4 inches of lift with 33" MT Claws with the stock 3.55s but I keep mine up. I cant stand rigging my vehicle. It has a new motor, which only had 4,000 miles when I bought it 6 months ago. I replaced the leaky injectors, complete tune up, K&N cold air intake(Clean filter every oil change), TPS Sensor and IAC Valve, and the MAP sensor. I now have my manifold off cause it was cracked and I am going back with new manifold and O2 sensor. And while I had the manifold off I realized my motor mounts where broke so replaced them to. I get pretty good gas mileage considering the set-up.

You can get a junkyard exhaust manifold that isn't cracked, or weld yours up (or have it welded if you can't weld). That would be the cheapest way.

kenny schmitt said:

well like i said i have no cat and i did a tune up about 17k ago. WHILE WE ARE ON THE TOPIC what is the best way to fix my manifold i want a header but money is tight what can be done with this i would fix the stock if i save money but I didnt think that was an option. so i will pull the plugs and see what they look like but im gonna see about changing the sensors and other stuff youguys suggested. what would explain the bad idle after downshifting?

Click to expand...

put your effing cat back on. go to the junkyard and get another manifold, it won't cost much.

also the unplugged 02 sensor is probably contributing to your issues.