Sudden drop in MPG

Jmazza0

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May 27, 2020
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San Diego
Good evening everyone, I've recently noticed a drastic change in my Jeeps gas mileage from around 15mpg to now 9-10mpg. I'm pretty good at keeping up with the maintenance (no check engine lights), and I know my lift & 35" tires don't help but I've had those modifications on for the past 4 years I've owned my Jeep. I checked all my oil levels and they are just over full and still clean. I cleaned my air filter, and even tried adding some Lucas to my fuel to see if that'd help but doesn't seem to of had any effect.

From my recent research I'm seeing there is a chance it could be from a cracked Exhaust Manifold fooling the O2 sensor making it run rich. I've done some visual inspections and haven't found anything that looks like a crack, although it's hard because of how much old mud has been caked up on the pipes. it looks like I have aftermarket exhaust manifold from what I could see but I'm not sure when it was installed as my previous owner had to have done it.

Also my jeep does have a bit of a ticking sound while cold, not sure if that's also a sign of a crack?

As for my methods of testing fuel mileage, it's about 60 miles to the mountains from my house so I'll usually fill up before I go wheeling and reset the odometer once full. Then I will hit the last gas station before my turn off for the trails and do a quick calculation of the miles driven. I'll do the same thing for the ride back as most of the drive is on the interstate (I try to keep it under 65mph which is about 2400rpm).

What are your guys's thoughts for the sudden drop in mpg? Thanks for your time!
 

Goatman

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Cats get clogged? New fuel?mileage drop is basically only a few choices. Less air in, blocked air out, excess fuel, some new load? (Tires too soft, hauling extra weight, brakes dragging, etc)
 

Thunderhead

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Good evening everyone, I've recently noticed a drastic change in my Jeeps gas mileage from around 15mpg to now 9-10mpg. I'm pretty good at keeping up with the maintenance (no check engine lights), and I know my lift & 35" tires don't help but I've had those modifications on for the past 4 years I've owned my Jeep. I checked all my oil levels and they are just over full and still clean. I cleaned my air filter, and even tried adding some Lucas to my fuel to see if that'd help but doesn't seem to of had any effect.

From my recent research I'm seeing there is a chance it could be from a cracked Exhaust Manifold fooling the O2 sensor making it run rich. I've done some visual inspections and haven't found anything that looks like a crack, although it's hard because of how much old mud has been caked up on the pipes. it looks like I have aftermarket exhaust manifold from what I could see but I'm not sure when it was installed as my previous owner had to have done it.

Also my jeep does have a bit of a ticking sound while cold, not sure if that's also a sign of a crack?

As for my methods of testing fuel mileage, it's about 60 miles to the mountains from my house so I'll usually fill up before I go wheeling and reset the odometer once full. Then I will hit the last gas station before my turn off for the trails and do a quick calculation of the miles driven. I'll do the same thing for the ride back as most of the drive is on the interstate (I try to keep it under 65mph which is about 2400rpm).

What are your guys's thoughts for the sudden drop in mpg? Thanks for your time!

Have you gone wheeling and aired down? Did you air back up ?
 

RMETeeJay

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CO, USA
Agree with @Goatman and I would also check the pre-cat O2 sensor, check for vacuum leaks, and also check that your coolant temperature is staying around 210 once warmed up.
 
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Jmazza0

Jmazza0

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May 27, 2020
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Cats get clogged? New fuel?mileage drop is basically only a few choices. Less air in, blocked air out, excess fuel, some new load? (Tires too soft, hauling extra weight, brakes dragging, etc)

How could I check if my catalytic converter is clogged?
 
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Jmazza0

Jmazza0

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Agree with @Goatman and I would also check the pre-cat O2 sensor, check for vacuum leaks, and also check that your coolant temperature is staying around 210 once warmed up.

If it was an O2 sensor wouldn't I get a check engine light? I haven't checked all my vacuum lines yet so I'll give those a look and listen. My coolant temp has been fine and even while wheeling it doesn't overheat at all.
 

Jerry Bransford

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A sudden loss of mpg is typically caused by a bad upstream O2 sensor and no, that won't always cause a CEL. The computer can't always tell when the O2 sensor is giving bad data to it so a CEL can't always be expected. While newer TJs have two upstream O2 sensors, yours only has one so it's not going to be all that expensive.

If you decide to replace the upstream sensor, go strictly with the NTK 23151 like at https://www.rockauto.com/en/catalog...77,exhaust+&+emission,oxygen+(o2)+sensor,5132

The NTK is what the factory installed and other sensors like from Bosch will cause problems. If you went for the Mopar on that webpage it'd be an NTK in a Mopar box.

Or get it from Amazon at https://www.amazon.com/dp/B001PNCKV8/?tag=wranglerorg-20
 
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Jmazza0

Jmazza0

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A sudden loss of mpg is typically caused by a bad upstream O2 sensor and no, that won't always cause a CEL. The computer can't always tell when the O2 sensor is giving bad data to it so a CEL can't always be expected. While newer TJs have two upstream O2 sensors, yours only has one so it's not going to be all that expensive.

If you decide to replace the upstream sensor, go strictly with the NTK 23151 like at https://www.rockauto.com/en/catalog...77,exhaust+&+emission,oxygen+(o2)+sensor,5132

The NTK is what the factory installed and other sensors like from Bosch will cause problems. If you went for the Mopar on that webpage it'd be an NTK in a Mopar box.

Or get it from Amazon at https://www.amazon.com/dp/B001PNCKV8/?tag=wranglerorg-20

Awesome, good to know! Thank you for the information and helpful links. I'll get one on order and hopefully that's the problem
 

Jerry Bransford

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Usually but his 99 only has one upstream O2 sensor and its sole downstream sensor only monitors the catalytic converter and doesn't affect the engine. In this case I'd replace just the sole upstream sensor.
 

Goatman

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Check the cat with a borescope down the O2 sensor hole. This depends on exhaust layout though.
Also, psi is high. Check for even tread wear, but most here are running 24-28 psi on the street.
 
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Jmazza0

Jmazza0

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San Diego
Usually but his 99 only has one upstream O2 sensor and its sole downstream sensor only monitors the catalytic converter and doesn't affect the engine. In this case I'd replace just the sole upstream sensor.

Just got the new O2 sensor in today and did the replacement. Lucky the old one wasn't seized in and took about 5 seconds to replace. I also disconnected my battery to reset the electronics and when I reconnected everything I had no CEL so I'll see this Sunday on one of my longer trips if this fixes the issue. Thank you for the advice
 
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Jmazza0

Jmazza0

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Here was my old O2 sensor. I don't think it looks that old

20200605_132827.jpg
 

Lauren Polez

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Aug 24, 2021
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Saint Louis, MO, USA
Just got the new O2 sensor in today and did the replacement. Lucky the old one wasn't seized in and took about 5 seconds to replace. I also disconnected my battery to reset the electronics and when I reconnected everything I had no CEL so I'll see this Sunday on one of my longer trips if this fixes the issue. Thank you for the advice
Did this wind up fixing your problem? Had a sudden drop in my mpg of about 5 mpg... Trying to find the cause, no lights...