Degraded / Low fuel mileage, MPG GC 2009 -
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post #1 of 6 Old 09-20-2019, 01:09 AM Thread Starter
Pera Detlic
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2009 WK 
Join Date: Sep 2019
Location: Encinitas/San Diego
Posts: 2
Degraded / Low fuel mileage, MPG GC 2009

I have this GC 2009, 3.7L AWD, no mods/stock for last 7 years. To knock on the wood I haven't had ANY issues with it so far. Just regular maintenance. It has 70+kmiles. I mostly use it for commute to work that is not to long, towing the boat (19 feet, 1 axel) and to play n the desert-and just FYI, I am amazed how well this basic model goes over anything that clearance allows - sand, dirt, hils, rocks.
So, my usage of the Jeep is pretty repeatable. For years, my mpg was:
Freeway 18-24mpg
City/work commute: 15-18mpg
Boat towing: 13mpg

The MPG shown on the speedometer/console is very very accurate to what I calculate by old fashion mileage over what it took to fill up the tank. Within 0.2mpg.

In last 2-3 months I noticed that mpg for my work commute dropped to 13mpg. then gradually to 12mpg. then to 11 mpg. then to 10.5. Indeed, this was a true consumption. Verified by old fashion way. With every fill up.
Went on the freeway and got 15mpg.
No any noticeable symptoms, it seem to work normally.

So I went to investigate:
- Tires good, pressure good,aligned.
- brakes not binding, no stuck cylinder
- replaced air filter, spark plugs (NGK whatever number is for my model)
- cleaned MAF sensor using MAF cleaner.
- changed oil and filter
- Measured MAF voltage. 2.19V idle, 2.98V at 3000RPM. This must be one that sends PWM square signal. It is very slow to change voltage, like 30sec or more till it settles. Couldn't find details about it among 7201 pages of factory service manual :-(
- OBD: no codes
- vacuum just below 20
- idle load is 30% (is this normal?, My dodge ram 1500 van shows 6%, but Accord 2000 shows 20%). Again couldn't find what is nominal.
- O2 sensors do go up and down, they , also, seem to have some lag, or don't go up and down as often as some other vehicles I've seen. But this might be normal for newer cars and for JGC, not sure. When I do load test, step on it, wide open throttle, O2 voltage goes to rich. Once I lift off, it drops to lean for a while.

So I stopped here scratching my head. I can replace MAF. Or check/replace fuel filter. Maybe check fuel pump, But I would expect fuel delivery to cause noticeable issues. I can remove O2 sensors and clean them, little bit reluctant to do this, I think it s til has emission warranty.

Any ideas or suggestions? What to do next? Did I miss something?
I can't observe anything on it that looks out of the ordinary. Only thing I think I noticed that it is little bit less lazy after I replaced original spark-plugs. Old ones looked as good as 70k miles spark plugs can look. Just bigger then normal gap, little discoloration on the metal body (where socket wrench "grabs", from heat on cyl 2 and 4.


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post #2 of 6 Old 09-20-2019, 07:56 AM
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2005 WK 
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Location: Stamford
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Idle time and stop/go driving - not even complete stops - are killers on Jeep MPG. That is why several manufacturers make vehicles that now turn off their engines when stopped and then start the engine when the driver resumes driving. My suggestion, if you have the patience, is to drive the vehicle on the highway for a few hundred miles when there is no traffic at all and then check your MPG. Also, did you add anything at all to the vehicle that would create additional wind resistance? That also kills MPGs.
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post #3 of 6 Old 09-20-2019, 08:29 AM
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I doubt there would be any info on a MAF sensor in the manual.
If the MAP readings were out of spec high or low there would be a code, Also MAP and TPS correlation are compared to test the sensors.

With scanner I would look at - ECT, fuel trims, and o2 sensor readings- it is switching between (.4 and .7) or more like .1 and .8) ?

fuel mixes do change - summer blend winter blend, seasonal use of AC, the fishing trips- more often I hope to hear, another driver ? Correct air filter?

I would look at getting a new MAP - I am not sure how the inside would handle the cleaner.

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #4 of 6 Old 09-20-2019, 08:53 AM
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How does the exhaust smell? Most people will notice a rich smell from a bad O2 sensor before the MPG loss, but since you keep good records, you probably caught it early. Data from them may be incorrectly fluctuating, but not bad enough to throw or store a code, and the rich smell may be intermittent.

Swap the upstream and downstream O2 sensors and see if there's an improvement. If you can confirm a change swapping front to back, you can further determine which O2 sensor is bad by swapping left to right, but at 70k miles, you're better off just replacing both upstream sensors.
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post #5 of 6 Old 08-08-2020, 07:49 PM Thread Starter
Pera Detlic
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2009 WK 
Join Date: Sep 2019
Location: Encinitas/San Diego
Posts: 2
All fine now

I cleaned the MAP sensor. Checked everything listed in replies above. Nothing helped and nothing found. Then, a few weeks later, just for no reason, I noticed it took longer between fuel fill ups. Calculated consumption: It was 18mpg. And it is normal ever since. Looks like gremlins flu away, for now. Ever since, no issues again. Puzzled. Thanks everybody!
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post #6 of 6 Old 08-09-2020, 11:05 AM
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2007 WK 
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did you change locations where you bought fuel? different distributors have their fuel blended to there specs. my 07 hemi gets best mpg on shell mid grade, worst on chevron premium.
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2009 , fuel , mileage , mpg , wk

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