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post #1 of 17 Old 04-07-2015, 09:40 AM Thread Starter
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Help! Horrible 11mpg city driving (JKU 2007)

Folks,

like the title says, I am getting my pockets emptied by my JKU 07 with a horendous 11 mpg city driving. I'm running 31.5" tires and a 2" rancho lift, but other than that the jeep is bone stock.

additionnal info :
I've had engine pinging for a while on accelaration, I fill up with 91 octane. Spark plugs, wires, coil pack, PCV valve, EGR valve, are all new. light pinging is still there.

6 months ago my dashboard was showing 18 mpg, and nothing has really changed since then (the pinging situation was the same).

Anybody have an idea what could be causing this?
Cheers and thanks in advance for any help.
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post #2 of 17 Old 04-07-2015, 11:41 AM
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Did your rig throw any codes when the pinging began?

Have you manually calculated the mpg or are you strictly going off the dash readout?
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post #3 of 17 Old 04-07-2015, 12:00 PM
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My 07 had the same problem. The exhaust pipe looked sooty and mpg dropped from 16.5 to 14. I did plugs, air filter, cleaned the mass air flow sensor, throttle body, pcv valve, and ran two bottles of injector cleaner. All with no change in mpg. I read a thread on oil use and took another look at the pcv. I replaced it with a new one, and disconnected the battery overnight. Shazam. Back to 16.5 mpg. I can't explain what really fixed the problem, but still looking good two months later. BTW, in the past I noticed that after a five hundred mile round trip on the interstate, mpg was elevated for a couple of weeks.
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post #4 of 17 Old 04-07-2015, 12:03 PM
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I had a Toyota do the same thing, turned out to be a dirty mass airflow sensor. Worth checking out and you can check the condition of the air filter at the same time.
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post #5 of 17 Old 04-07-2015, 01:20 PM
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Just curious - why are you spending more on gas than you need to? The 3.8 (and 3.6) are rated for 87 octane...

Sure, it's not going to help the 11 mpg, but your pockets would be emptied slightly less. Edit: and it may be why your engine is pinging.

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Last edited by pizzaman; 04-08-2015 at 07:17 AM.
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post #6 of 17 Old 04-07-2015, 01:53 PM Thread Starter
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No codes.
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post #7 of 17 Old 04-07-2015, 01:54 PM Thread Starter
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Quote:
Originally Posted by LepreCon View Post
Did your rig throw any codes when the pinging began?
no codes.

Quote:
Have you manually calculated the mpg or are you strictly going off the dash readout?
started doing it today. Will see what it looks like after a full tank.
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post #8 of 17 Old 04-07-2015, 01:56 PM Thread Starter
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Quote:
Originally Posted by pizzaman View Post
Just curious - why are you spending more on gas than you need to? The 3.8 (and 3.6) are rated for 87 octane...

Sure, it's not going to help the 11 mpg, but your pockets would be emptied slightly less. Edit: and it may be why you're engine is pinging.
I started putting in 91 octane because of the pinging. it was the only thing that helped.
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post #9 of 17 Old 04-07-2015, 01:57 PM Thread Starter
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Quote:
Originally Posted by motorcycle matt View Post
I had a Toyota do the same thing, turned out to be a dirty mass airflow sensor. Worth checking out and you can check the condition of the air filter at the same time.
Air filter is brand new.
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post #10 of 17 Old 04-07-2015, 03:38 PM
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Pinging on a lower compression engine sounds like timing maybe ( chain), or a really dirty throttle body, injectors, to me. That's really bad mileage. Most of my friends with the 3.8 are in the 17 - 20 hwy. range.

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post #11 of 17 Old 04-09-2015, 02:26 AM
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Quote:
Originally Posted by pizzaman View Post
Just curious - why are you spending more on gas than you need to? The 3.8 (and 3.6) are rated for 87 octane...and it may be why your engine is pinging.
Actually, the higher octane resists pre-ignition. That's one reason why in the muscle car era, higher octane was required for higher compression engines. The lower octane fuel would pre-ignite and cause knock. That said, I agree that he shouldn't be having to run high octane in his 3.8.

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post #12 of 17 Old 04-09-2015, 06:10 AM
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higher-octane fuel confers an advantage in some cars, but not others. It allows performance-oriented engines (specifically, those with higher compression ratios) to burn gasoline at higher pressures and higher temperatures. These conditions at the moment of combustion create better thermodynamic efficiency, so a greater percentage of the gasoline’s heat energy gets converted into motive power.

Octane rating is a measure of grace under pressure: how evenly a gasoline will burn under difficult conditions, like hard acceleration. Ideally, the vaporized gasoline inside an engine’s cylinder burns by the propagation of a wave of flame, ignited by the cylinder’s spark plug. This allows a smooth transfer of power to the engine’s crankshaft and the car’s wheels. But at higher pressures or temperatures, small pockets of gasoline vapor can prematurely explode, or self-ignite, creating a distinctive “knocking” sound, as well as potentially destructive shock waves.

Gasoline with a higher octane rating does not self-ignite easily, and burns more evenly than lower-octane fuel under harsh conditions, resisting detonation and knocking. Modern engines, with electronic sensors and controls, are very good at preventing detonation of lower-octane gas (this is why drivers no longer hear much knocking). But he carbon high-octane fuel is still specified when designers want to achieve better acceleration and power output, and when they are willing to accept a slightly bulkier and heavier engine with higher operating costs
in non high performance motors (our 3.8 is NOT this), in some scenarios , hi oct fuels can accelerate buildup within the exhaust and cats , wear/ shorten life of your plugs. I would try using 89 oct after following the entire cleaning process Drjay sggstd and bang on the cats w a BFH lightly & a little to see if you've gotten build up within them.
that almost seems like a non firing plug /5 cyl only mileage. I wonder if all the gas is being ignited , in fact.
there's a reason Chrysler states to use 87 and I agree the pings disappear w/ a slightly higher oct but 90-91 is too hot. just my 2cents.


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post #13 of 17 Old 04-09-2015, 06:37 AM
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Quote:
Originally Posted by Drjay View Post
I did plugs, air filter, cleaned the mass air flow sensor...
No MAF in the 3.8L JK. Do you mean the intake air temp sensor?

Carbon build-up in the combustion chamber can cause pinging, which will make the computer retard the timing to its fullest extent, which in turn will reduce performance and mpg's.

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post #14 of 17 Old 04-09-2015, 07:31 AM
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Inspect both exhaust manifolds for cracks. If one or both are cracked, air is drawn into the exhaust stream ahead of the oxygen sensors. They will then sense a lean mixture and cause the PCM to incorrectly increase fuel flow through the injectors.

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post #15 of 17 Old 04-09-2015, 07:28 PM Thread Starter
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thanks for the info guys, will look into this and update the thread once I know more. Cheers.
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post #16 of 17 Old 04-09-2015, 08:12 PM
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Quote:
Originally Posted by ronjenx View Post
No MAF in the 3.8L JK. Do you mean the intake air temp sensor?

Carbon build-up in the combustion chamber can cause pinging, which will make the computer retard the timing to its fullest extent, which in turn will reduce performance and mpg's.
You are absolutely right about the air temp sensor. I thought it looked different from what I was used to. Anyway, I cleaned it with MAF Sensor Cleaner.
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post #17 of 17 Old 04-10-2015, 11:58 AM
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Quote:
Originally Posted by ronjenx View Post
No MAF in the 3.8L JK. Do you mean the intake air temp sensor?

Carbon build-up in the combustion chamber can cause pinging, which will make the computer retard the timing to its fullest extent, which in turn will reduce performance and mpg's.
yep. my fat fingers ....srry


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