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Unread 03-15-2013, 11:38 PM   #16
Sparrows
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My fuel mileage just tanked. On my last fill up I only got about 150 miles out of the 15 gallons, so I'm only getting about 10mpg. My first few fill ups I was getting like 225 or so out of a tank. I think I blew the O2 sensor though. She throws an idiot light at highway speeds but not at "town" speeds (up to about 40mph.) I need to pull the code but I'm pretty sure its the O2 sensor.

I'll get it replaced this week when I get the $$$ and let you know if it changes anything after the next fill up. Oh and for specs I have a 4.0 HO, AW4, 33s, and 4.10 gears. I think I should be getting closer to 20mpg with the OD that I have...maybe I'll do a full tune up this week as well.

OP, can you check and see if it's throwing a code? I've learned that it can be giving codes without actually lighting up your check engine light. Just a thought.

Good luck.
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Unread 03-16-2013, 06:35 AM   #17
Jakes93yj
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Re: Gas mileage so low?

Yes it's a auto. When my grandpa had to drive it when his pt cruiser was getting fixed he somehow got 17. I never floor it anymore and take it easy around town.
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Unread 03-16-2013, 06:37 AM   #18
Jakes93yj
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Re: Gas mileage so low?

Quote:
Originally Posted by Sparrows View Post

OP, can you check and see if it's throwing a code? I've learned that it can be giving codes without actually lighting up your check engine light. Just a thought.

Good luck.
Cheers
How would I do this?
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Unread 03-16-2013, 08:02 AM   #19
pete1991YJ
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Its easy and free to get the codes, all you need is the key. This should be the first thing you do when you think there's something wrong. Here's a video that shows how:
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Unread 03-16-2013, 08:34 AM   #20
Jakes93yj
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According to that i got the code 12 33 55, and going by jeep horizons there is no 12, 33 is air conditioner relay clutch(my jeep has no ac) and 55 was the end code
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Unread 03-16-2013, 09:26 AM   #21
pete1991YJ
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Quote:
Originally Posted by Jakes93yj View Post
According to that i got the code 12 33 55, and going by jeep horizons there is no 12, 33 is air conditioner relay clutch(my jeep has no ac) and 55 was the end code
OK so there's nothing in the computer. That means it something that it can't directly detect. Next 2 things I would check is the fuel pressure at the rail, and the Thermostat. If the thermostat is working at 195 like it should, then how is the temp sensor in the water neck?

Here's why:

The fuel pressure should be obvious, if its too high then it'll squirt more than it should. Lot of times its a pinched return line at the tank, since its kinda tight up there... when the body mounts settle from age, it can pinch the lines.

As for the temperature stuff, the computer needs to see at least 180 deg F before it starts to read the O2 sensor. Otherwise it stays in "warm up, limp home" mode called "open loop". This is meant to keep from burning up the engine if somethings wrong, but you noticed it eats gas like crazy.

Pay attention to vacuum and exhaust leaks also. A leak at the header flange and cylinder head can fool the O2 sensor - I tighten mine down every year. Also beware of vacuum leaks. Especially on the fuel pressure regulator and MAP sensor. Those will kill the MPG's too.

Do some looking at it and let us know how it goes!
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Last edited by pete1991YJ; 03-16-2013 at 09:28 AM.. Reason: sp
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Unread 03-16-2013, 09:50 AM   #22
Jakes93yj
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So i should go and tighten my headers to see if they are leaking?
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Unread 03-16-2013, 10:19 AM   #23
pete1991YJ
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Quote:
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So i should go and tighten my headers to see if they are leaking?
Thats one thing you can do. I usually just give mine the once-over with a 9/16 socket and extension in the spring and fall every year. The bolts in back by the firewall are prone to loosen. It only takes like 5 minuts to checkup on it.
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Unread 03-18-2013, 01:41 PM   #24
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I may have missed this but if the jeep is all stock with the exception of the larger tires, how can he know his correct MPG without the proper spedo gear?
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Unread 03-18-2013, 01:47 PM   #25
tdewit
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I may have missed this but if the jeep is all stock with the exception of the larger tires, how can he know his correct MPG without the proper spedo gear?
Multiple ways. Use of a GPS is the easiest. Also, you can use a tire calculator to compensate for the different size. With my 31's on, my speedo is 10% off.
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Unread 03-18-2013, 01:50 PM   #26
Jakes93yj
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Re: Gas mileage so low?

Quote:
Originally Posted by pushead View Post
I may have missed this but if the jeep is all stock with the exception of the larger tires, how can he know his correct MPG without the proper spedo gear?
I have 30's if that helps. Speedos is dead on.
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Unread 03-18-2013, 02:12 PM   #27
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Quote:
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I have 30's if that helps. Speedos is dead on.
Are you sure about this, have you used a gps at different speeds to see if you are matching up?
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Unread 03-18-2013, 02:21 PM   #28
pete1991YJ
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Quote:
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Are you sure about this, have you used a gps at different speeds to see if you are matching up?
The gas mileage difference between 29's (stock) and 30's is within the margin of error for over/under inflation. A 5 PSI difference in the tires inflation could do that. So I wouldn't sweat the GPS.

Myself I break out the calculator and do the math off the trip meter/odometer.
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Unread 03-18-2013, 02:23 PM   #29
Jakes93yj
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Re: Gas mileage so low?

Quote:
Originally Posted by pete1991YJ View Post
The gas mileage difference between 29's (stock) and 30's is within the margin of error for over/under inflation. A 5 PSI difference in the tires inflation could do that. So I wouldn't sweat the GPS.

Myself I break out the calculator and do the math off the trip meter/odometer.
What should I run for best gas mileage, currently at 35psi all 4 corners, I take the amount of gas I put in and divide the trip meter by the gallons put in, its pretty accurate.
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Unread 03-18-2013, 03:31 PM   #30
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I have a 93 YJ, 4.0 automatic. When I bought it in 1996, it had 40,000 miles on it, 3.07 gears, and 235 75 15 tires. It would do an honest 20 MPG on the highway, 17 around town. I once drove from Maine to Pennsylvania without stopping for gas.

It does not get that mileage anymore, right now it gets right at 15 MPG with 210,000 miles on the clock, 3.73 gears and 33 inch TSLs.

The OPs milage seems low to me, it should do better than that.
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