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  #31  
Old September 24th, 2019, 11:59
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Re: Bad gas mileage 10-11 MPG

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Originally Posted by 4x4JeePmaNthINg View Post
Could a bad temperature sensor be a possibility? Just spit balling.
secondary to the temp sensor attached to the temp gauge?
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  #32  
Old September 24th, 2019, 12:04
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Re: Bad gas mileage 10-11 MPG

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Originally Posted by paulhead View Post
secondary to the temp sensor attached to the temp gauge?
No, that exact one, on the thermostat housing.

Also do you know if your spark plug gap is correct?
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  #33  
Old September 24th, 2019, 12:10
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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
Pretty much ties into this, if its crapped out.
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  #34  
Old September 24th, 2019, 12:12
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Re: Bad gas mileage 10-11 MPG

Quote:
Originally Posted by 4x4JeePmaNthINg View Post
No, that exact one, on the thermostat housing.

Also do you know if your spark plug gap is correct?
Last year after buying the jeep it passed smog with flying colors because the previous owner spent a ton of money on it including the intake system so it could pass and she could sell it so I assumed all that would be in order. So I'll put that on the list with exhaust manifold and vacuum leaks. I sprayed with brake cleaner for vacuum leaks but think I may buy one of those $40 smoke kits on ebay.
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  #35  
Old September 24th, 2019, 15:12
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Re: Bad gas mileage 10-11 MPG

Quote:
Originally Posted by 4x4JeePmaNthINg View Post
Could a bad temperature sensor be a possibility? Just spit balling.
It would cause the computer to dump fuel, same as running a lower thermostat
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  #36  
Old September 24th, 2019, 15:29
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Re: Bad gas mileage 10-11 MPG

Temps reading normal
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  #37  
Old October 2nd, 2019, 17:08
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Re: Bad gas mileage 10-11 MPG

I don't know why I fight the easy stuff first. I assumed the plugs were good because it passed smog a year ago, my name is Dumass. I'll report a half tank later.
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  #38  
Old October 4th, 2019, 20:14
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Re: Bad gas mileage 10-11 MPG

I hate to tell you this.
but with the 35's and the lift. 10 mpg sounds right.
my stroker gets 8-12 city almost 20 highway. Im running 32's
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  #39  
Old October 5th, 2019, 07:40
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Re: Bad gas mileage 10-11 MPG

My last XJ had the same exact set up and was a Renix and I got better mileage so I have to disagree with you.
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Last edited by paulhead; October 5th, 2019 at 10:14.
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  #40  
Old October 5th, 2019, 10:03
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Re: Bad gas mileage 10-11 MPG

Is your speedometer/odometer reading correctly? Mine reads 10 mph high with 35 in tires and 4:56 gears.
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  #41  
Old October 5th, 2019, 10:13
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Re: Bad gas mileage 10-11 MPG

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Originally Posted by jcnoble View Post
Is your speedometer/odometer reading correctly? Mine reads 10 mph high with 35 in tires and 4:56 gears.
yeah its dead on
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  #42  
Old October 9th, 2019, 09:25
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Re: Bad gas mileage 10-11 MPG

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Originally Posted by cruiser54 View Post
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.
What would you be looking for inside the cap, let's say I had a similar issue with MPG.
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  #43  
Old October 9th, 2019, 11:08
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Re: Bad gas mileage 10-11 MPG

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Originally Posted by 90XJ_Seth View Post
What would you be looking for inside the cap, let's say I had a similar issue with MPG.
Black chareded contacts. Pretty sure it was my plugs, gonna top it off here soon.
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  #44  
Old October 11th, 2019, 17:12
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Re: Bad gas mileage 10-11 MPG

So after the plugs change 12.7mpg. Will report next tank full, and try to lighten my foot a little too.
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  #45  
Old October 15th, 2019, 14:43
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Re: Bad gas mileage 10-11 MPG

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Originally Posted by paulhead View Post
Temps reading normal
Your temp gauge on a 96 and earlier uses a separate sender from the ECU, so that doesn't really rule anything out, unfortunately.

87-90 4.0: 3 temp sensors
- 1 in the left radiator end cap to turn the electric fan on (HD cooling option only)
- 1 in left side of block low down for the ECU
- 1 in the back left side of the head on the top for the gauge

91-96 4.0: 2 temp sensors
- 1 in the thermostat housing for the ECU, which now also controls the fan
- 1 in the back left side of the head on the top for the gauge

97+ 4.0: 1 temp sensor. It's in the thermostat housing and the ECU controls the fan as well as sending the data to the instrument cluster

Honestly it sounds like you've gone through most stuff. If there is even a small crack in the exhaust manifold - very common at the 2 to 1 collector - it will suck in air in between exhaust pulses and this will cause the O2 sensor to read leaner than it should no matter how much fuel the ECU dumps in to compensate, because that air doesn't ever meet the extra fuel in any meaningful way until they both end up in the catalytic converter. I've seen an XJ get single digit fuel economy due to this problem and jump to 15-16 after it was fixed.
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