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  #1  
Old August 30th, 2019, 09:29
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paulhead paulhead is offline
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Bad gas mileage 10-11 MPG

I know a common problem but wouldn't mind some input. 1992 4.0 AW4, 4.88s, 35s. Fuel lines and rails were redone before sold to me so it could pass smog which it did easily. I've had jeeps before and I know it shouldn't be this bad. One thing I've noticed is it's shifting weird likes it's floating between gears once in a while. It was doing this really bad before but I replaced the TPS and it's normal now but still doing it very slightly.
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Old August 30th, 2019, 09:41
RCP Phx RCP Phx is offline
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Re: Bad gas mileage 10-11 MPG

What have you done to check the motor(comp check, vacumm, spark plugs, coil, wires, and timing)? I'm getting 16mpg with a high hp stroker!
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Old August 30th, 2019, 10:07
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Re: Bad gas mileage 10-11 MPG

No codes, plugs and wires are fairly new, I guess I could check for vacuum leaks with starter fluid.
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Old September 4th, 2019, 10:58
Scrubber3 Scrubber3 is offline
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Bad gas mileage 10-11 MPG

Torque converter?
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Old September 4th, 2019, 11:25
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Re: Bad gas mileage 10-11 MPG

Check the basics, tune-up, comp, vac leaks, manifold leaks, and then check the O2 sensors. Nearly all XJ's have or will have a cracked exhaust manifold.
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Old September 4th, 2019, 11:34
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Re: Bad gas mileage 10-11 MPG

hows the power feel?
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Old September 4th, 2019, 14:11
Black1990jeep Black1990jeep is offline
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Re: Bad gas mileage 10-11 MPG

dragging brakes? does she roll easily in neutral?
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Old September 4th, 2019, 15:33
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Re: Bad gas mileage 10-11 MPG

Quote:
Originally Posted by Scrubber3 View Post
Torque converter?
Interesting, not engaging 100%?
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Originally Posted by IslanderOffRoad View Post
hows the power feel?
Good power
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Originally Posted by Black1990jeep View Post
dragging brakes? does she roll easily in neutral?
yeah, thought about brake drag too, not noticing it.
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Old September 5th, 2019, 05:55
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Re: Bad gas mileage 10-11 MPG

Ever pulled the distributor cap and looked inside?

Don't use starting fluid to check for vacuum leaks. It can travel whereas carb cleaner and brake cleaner stay a liquid much longer.
A common issue is loose intake manifold bolts. Snug them up as needed.
The vacuum line from the intake manifold to MAP sensor is critical. any slight leak and the fuel mixture goes richer.
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Old September 5th, 2019, 12:06
EvaB99XJ EvaB99XJ is offline
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Re: Bad gas mileage 10-11 MPG

Regarding the transmission still shifting funky after the TPS swap - has the transmission fluid and filter been changed recently?
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  #11  
Old September 5th, 2019, 13:29
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Re: Bad gas mileage 10-11 MPG

Run a can of BG44k through the fuel to clean out the carbon and the injectors. Old clogged injectors is one of the most common things for crappy gas mileage.
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Old September 5th, 2019, 15:06
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Re: Bad gas mileage 10-11 MPG

torque converter clutch not engaging will do it for sure, since without the clutch you are relying on hydraulics alone. if you feel like the trans is shifting weird I would probably start there.

check for fuel leaks / damp fuel lines (no kidding)

use the key trick to pull codes and see if anything there that didn't trip the CEL

are you running a 180 or 165 thermostat? the computer tries to get the engine temp to 195 by running a rich mixture so that the cat converter works efficiently, if temp stays low the computer will forever be dumping unnecessary fuel. O2 sensor can do it too but not usually that aggressive
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Old September 5th, 2019, 23:23
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Re: Bad gas mileage 10-11 MPG

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Originally Posted by ehall View Post
torque converter clutch not engaging will do it for sure, since without the clutch you are relying on hydraulics alone. if you feel like the trans is shifting weird I would probably start there.

check for fuel leaks / damp fuel lines (no kidding)

use the key trick to pull codes and see if anything there that didn't trip the CEL

are you running a 180 or 165 thermostat? the computer tries to get the engine temp to 195 by running a rich mixture so that the cat converter works efficiently, if temp stays low the computer will forever be dumping unnecessary fuel. O2 sensor can do it too but not usually that aggressive
No codes, pretty sure I'm running a 195 thermostat, it's running at operating temp.
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  #14  
Old September 13th, 2019, 12:47
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Re: Bad gas mileage 10-11 MPG

Ive had my intake man fail to seal multiple times. I would start by using a propane torch off. and searching the engine bay.
or you could use smoke.
whatever floats your boat.
if its a vacuum leak it will show.
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Old September 13th, 2019, 12:47
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Re: Bad gas mileage 10-11 MPG

otherwise fuel pressure test.

then check spark.
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