Horrible Gas Mileage

[DR7]

Hi Everyone. I have a huge issue & need help huge with my TJ. I live in California & have to deal with smoging my Jeep! I have a 2004 i6, with about a lil over 135,000 miles! I have done recent oil change, o2 sensors, new spark plugs, & fixed the 2 elbows (EVAP) goin into the intake manifold. (Both coming from the purge valve/solenoid. My question is im literally watching my gas gauge decrease so fast like 150 to 160 miles per tank! I used to average 280 or so per tank. I have a stock 19 gallon tank. Everything on my TJ is as was. But cant seem to figure where this sudden drop like drastic drop of fuel economy! I love my TJ & am so bumbed because driving it has been so limited. Did I mention i live in California lol $3.50/gal

 

[AMS417]

Hi Everyone. I have a huge issue & need help huge with my TJ. I live in California & have to deal with smoging my Jeep! I have a 2004 i6, with about a lil over 135,000 miles! I have done recent oil change, o2 sensors, new spark plugs, & fixed the 2 elbows (EVAP) goin into the intake manifold. (Both coming from the purge valve/solenoid. My question is im literally watching my gas gauge decrease so fast like 150 to 160 miles per tank! I used to average 280 or so per tank. I have a stock 19 gallon tank. Everything on my TJ is as was. But cant seem to figure where this sudden drop like drastic drop of fuel economy! I love my TJ & am so bumbed because driving it has been so limited. Did I mention i live in California lol $3.50/gal

Have the catalytic converters tested. A reputable muffler shop should be able to do a back pressure test. What plugs did you install?

 

[Jerry Bransford]

I'm betting one of the two upstream O2 sensors has gone bad. Like spark plugs, O2 sensors do eventually go bad and need to be replaced. It's the upstream O2 sensors that tell the PCM if the fuel mixture is correct, too lean, or too rich. When they go bad they can report to the PCM the fuel mixture is too lean and the PCM then sets the fuel mixture to full-rich trying to correct the air-fuel mixture even though it's not really too lean. Erroneously setting the fuel mixture to full-rich will wreck the mpg and performance of the engine.

If you replace the upstream O2 sensors make sure to only install NTK or NGK which is the OE brand. Definitely don't let the parts store talk you into using Bosch O2 sensors, they don't work right in our Jeeps.

 

[Chris]

Yep, the very first thing that comes to my mind is potentially clogged / failing cats.

 

[DR7]

Have the catalytic converters tested. A reputable muffler shop should be able to do a back pressure test. What plugs did you install?

Their brand new. Spark Plugs Autolites Iridium XP 985as was suggested from another wheeler. The New o2 sensors are NGK as suggested as well.

 

[DR7]

I'm betting one of the two upstream O2 sensors has gone bad. Like spark plugs, O2 sensors do eventually go bad and need to be replaced. It's the upstream O2 sensors that tell the PCM if the fuel mixture is correct, too lean, or too rich. When they go bad they can report to the PCM the fuel mixture is too lean and the PCM then sets the fuel mixture to full-rich trying to correct the air-fuel mixture even though it's not really too lean. Erroneously setting the fuel mixture to full-rich will wreck the mpg and performance of the engine.

If you replace the upstream O2 sensors make sure to only install NTK or NGK which is the OE brand. Definitely don't let the parts store talk you into using Bosch O2 sensors, they don't work right in our Jeeps.

Ive replaced all 4 O2 sensors with NGK plus drained & cleared the PCM

 

[DR7]

My air box is stock with New WIX air cleaner as well

 

[DR7]

Tomorrow if I have time after a side job. Im goint to test the sending unit from the gas tank as I had heard that the TJ’s gas gauges go out a lot. Not sure if thats true but im tryin everything I can. I have no codes either!

 

[Jerry Bransford]

Ive replaced all 4 O2 sensors with NGK plus drained & cleared the PCM

Is the temperature gauge indicating the correct engine temperature? If not replace the coolant temperature sender. If the temperature gauge is indicating the correct engine temperature I'd next replace the IAT (intake air temperature) sender. The PCM uses both of those sensors to determine when things are warm enough to switch from open-loop to closed-loop operation. If the PCM thinks the engine is still cold it will keep the engine in its open-loop operation which keeps the air-fuel mixture set to full rich.

 

[DR7]

Is the temperature gauge indicating the correct engine temperature? If so I'd next replace the IAT (intake air temperature) sender. The PCM uses both of those sensors to determine when things are warm enough to switch from open-loop to closed-loop operation. If the PCM thinks the engine is still cold it will keep the engine in its open-loop operation which keeps the air-fuel mixture to full rich.

Temps been great & normal. I have a flex a light electric fan to remove the load from the crankshaft driven stock fan. But i so truly appreciate your help & response & I will change it as soon as I can. Do you recommend any? I usually try to stay Mopar/OEM

 

[DR7]

Temps been great & normal. I have a flex a light electric fan to remove the load from the crankshaft driven stock fan. But i so truly appreciate your help & response & I will change it as soon as I can. Do you recommend any? I usually try to stay Mopar/OEM

Is the temperature gauge indicating the correct engine temperature? If not replace the coolant temperature sender. If the temperature gauge is indicating the correct engine temperature I'd next replace the IAT (intake air temperature) sender. The PCM uses both of those sensors to determine when things are warm enough to switch from open-loop to closed-loop operation. If the PCM thinks the engine is still cold it will keep the engine in its open-loop operation which keeps the air-fuel mixture set to full rich.

Also, i should of mentioned its the flex a lite electric fan with 2 stage aluminum radiator. (For best flow). Plus has an electric dial to set temp, which i believe is 195

 

[Jerry Bransford]

Normal engine temperature the PCM is programmed around is 210. Trying to make the engine run cooler than the PCM is programmed around never works out.

 

[DR7]

Normal engine temperature the PCM is programmed around is 210. Trying to make the engine run cooler than the PCM is programmed around never works out.

10-4! Should I set the fan sensor to turn on at 210 or 215?

 

[DR7]

10-4! Should I set the fan sensor to turn on at 210 or 215?

i just matched the thermostat like the instructions stated.

 

[Jerry Bransford]

Whatever it takes to get the temperature gauge in the instrument cluster to read at or just a tad over 210 in normal driving conditions. Do try a new IAT sensor too.

 

[DR7]

Whatever it takes to get the temperature gauge in the instrument cluster to read at or just a tad over 210 in normal driving conditions. Do try a new IAT sensor too.

Absolutely will do! Again I truly thank you! I’ll keep you posted. I hope this may help someone else out there if they have the same issue!

 

[Viking Jeeper]

The thread title says "Horrible Gas Mileage" but you are referring to miles per tank. Those are two very different things. Have you computed gas mileage? Fill the tank up. Zero trip odometer. Drive until next fill up. Miles driven divided by gas purchased is gas milage. You say you used to get 280 miles per tank. Lets say you had 4 gallons remaining when you filled up as I hear that is when the low fuel light comes on. If you pump 15 gallons after driving 280 miles that is close to 19 MPG. Pretty high for a TJ.

 

[DR7]

I plan on doin this test today & for a few days to exact numbers as of now. Especially after testing & perhaps getting the IAT sensor as suggested from another wheeler. I’ll keep you posted.

 

[TJ2]

[QUOTE="DR7, post: 6505 Did I mention i live in California lol $3.50/gal
[/QUOTE]
WTF? We get $1.61 at Walmart Amarillo TX. And that's without showing a gun.

Mine spent 14+ MPG in town.
I have bought all my 4 NTK sensors and did not have the guts to do it yet
Bought it long time ago, I think was about $26 - $29 a piece I don't know why it is all different price.

I also bought the special socket wrench to remove it but every time I look at the rusts and limited space in there, I chicken-out to start the job.

 

[Jerry Bransford]

I also bought the special socket wrench to remove it but every time I look at the rusts and limited space in there, I chicken-out to start the job.

Those special O2 sockets with the slot milled into them for the O2 wiring are used to install a new O2 sensor, not remove the old one. If your existing O2 sensor is typical it is seized in the header and that slot cut into the side of the O2 socket can cause the socket to spread open and slip/spin on the sensor when enough pressure to remove the sensor is applied.

To remove the O2 sensor use a standard 7/8 deep socket or, depending on which one it is, a box-end wrench. Cut the wires off from the old O2 sensor so the standard deep 7/8" socket will fit over it. And because O2 sensors are so often seized, it may be required to get the engine good and hot to help get it to unscrew. It's can be a painful bitch to work in that area when the engine is hot so be careful.