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Old 09-04-2014, 10:28 PM   #1
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Losing MPG and Power

I have a 2008 Sahara Unlimited, still mostly stock. I'm 2nd owner. 65k

I was consistently getting 20-21.5 mpg. The last 1000 miles or so my mpg has dropped to 15.5 mpg. I'm using the same gas, driving the same roads, same time with top on and off, A/C, etc. Nothing has changed except mpg....and power.

I feel like I've lost some power too. Its hard to keep it going a consistent speed. Feels like I'm always either accelerating or coasting. At times the gas pedal feels like mush, making rpms actually drop a bit before suddenly jumping 1000 rpms with same pressure. Slightest incline instantly slows jeep forcing a downshift. Recently found turning OD off helps, but I'm afraid that will drop mpg into single digits.

What should I check? It wasn't like this the first 10k I had it. It so sluggish and lazy now, almost not fun to drive. Cleaned air filter to see if that improved it as well as ran seafoam thru a full tank. Always use 93 octane.

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Old 09-04-2014, 10:39 PM   #2
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Could be spark plugs

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Old 09-04-2014, 10:58 PM   #3
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Would plugs have that affect but without sputtering, back firing, rough starts, rough idle? It runs fine other than the mpg and power issues.
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Old 09-05-2014, 01:41 AM   #4
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Originally Posted by swampjoe View Post
Always use 93 octane.
Don't know much about engines and what could be causing this, but I do know that anything but regular is a waste of money, even on high performance vehicles, unless you are capable of coaxing the high performance out of them. Daughter drives BMW 328xi, wife X3. BMW recommends premium gasoline for both. I have never put anything but regular in them. Once filled up 328xi with premium just to see if I can feel a difference, and I honestly can't. Wasn't trying to burn rubber taking off at the stop light, but under typical driving conditions there is just no difference.
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Old 09-05-2014, 02:56 AM   #5
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If the decline has been progressive and you are still running on the original set of plugs that would be a very good starting point. As the electrodes wear out over time the gap gets bigger which leads to a bigger gap, which leads to weak spark, which in turn leads to incomplete combustion, which finally leads to lower performance and fuel economy. Eventually they foul and basically stop working.
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Old 09-05-2014, 10:55 AM   #6
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Originally Posted by GroovyGeek View Post
Don't know much about engines and what could be causing this, but I do know that anything but regular is a waste of money, even on high performance vehicles, unless you are capable of coaxing the high performance out of them. Daughter drives BMW 328xi, wife X3. BMW recommends premium gasoline for both. I have never put anything but regular in them. Once filled up 328xi with premium just to see if I can feel a difference, and I honestly can't. Wasn't trying to burn rubber taking off at the stop light, but under typical driving conditions there is just no difference.
Wow! If the manufacture calls for premium, then put in premium. High compression engines needs more octane in order to prevent detonation. Usually sounds like marbles in the engine. Which could lead to damaged pistons, piston rings and connecting rods. You may not notice anything right away, but down the road sometime end up finding you need an engine rebuild. Don't expect to run that engine for it's full life cycle.
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Old 09-10-2014, 07:50 AM   #7
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Did you ever replace spark plugs ? My jeep is the same year model roughly more miles. I also have seen loss in power and mpg.
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Old 09-10-2014, 08:30 AM   #8
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EGR Valve

It could be your EGR Valve. I threw a code (P0406) and ended up replacing mine. in the mean time until I replaced it, I noticed that mine had very similar characteristics.

while I havent noticed a change in MPG, I have noticed that its much more responsive... which in turn could be diminishing my MPG

A bad EGR valve will diminish your MPG
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Old 09-10-2014, 11:01 AM   #9
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I am having the same problem, kinda... I have 35's on 5.13 gears. Much better than stock, but I still down shift a lot, and lately I've been feeling more sluggish than when I first bought my gears! Should I do my spark plugs? I got my jeep used, so I'm not sure if they've been changed recently. It's a 2007 with 57000 miles. Should I look into changing them?
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Old 09-10-2014, 11:59 AM   #10
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Quote:
Originally Posted by swampjoe View Post
Always use 93 octane.

I would almost garuntee this is the problem.

Start putting 87 in it and see how it does for a few tanks. Here's why.

Higher octane fuels take more spark to ignite. If you put a more resistant fuel in a lower compression engine, it is going to have a hard time igniting the fuel.

Here's why:

Compression refers to the amount of pressure the piston puts on the fuel in the cylinder before it sparks and ignites it. It's similar to an air compressor.

The more compressed the fuel, the less spark is required to ignite it, and the more power/stroke the cylinder is going to produce. (Because the explosion will be stronger.). Think of gun powder. Lay it out in a line and light it. It will take a large flame and it will simply burn. Take that same powder and pack it, it will explode with more energy and require less flame. The harder your pack it, the less flame will be required and the more energetic the explosion will be. That's why a round of ammo can ignite with a such a small primer.

So now apply that to your engine. Your putting a fuel in that Inherently resists spark into an engine that's designed for a more volitile fuel. The plugs are colder, the compression is lower, etc.

I bet you 100.00 if you start running the fuel you are supposed to be, the problem will correct itself.
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Old 09-10-2014, 12:49 PM   #11
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Old 09-10-2014, 12:50 PM   #12
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We have a 2011 3.8 and though it is newer than yours the owners manual says "only" to use 87 octane.
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Old 09-10-2014, 12:56 PM   #13
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Originally Posted by FlyFishJeff View Post
I don't think the engine has a MAF sensor.
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Old 09-10-2014, 12:58 PM   #14
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I promise it's the higher octane use.

You probably didn't notice it at first because it kept mixing with the 87 that was already in the system. It probably took a few tanks to start noticing a symptom.
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Old 09-10-2014, 01:08 PM   #15
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Have you checked the air filter? If dirty, can cause the issues you describe.
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Old 09-10-2014, 01:30 PM   #16
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Originally Posted by Bent jeep View Post
I don't think the engine has a MAF sensor.
I just did a little research, and you are correct. I've only had the jeep for a few months, and I was thinking of fixes based on my experience with Hondas. Sorry I couldn't help!
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Old 09-10-2014, 02:30 PM   #17
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I just did a little research, and you are correct. I've only had the jeep for a few months, and I was thinking of fixes based on my experience with Hondas. Sorry I couldn't help!

Don't feel bad, I had to do the same thing when I was trying to find mine.
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Old 09-10-2014, 09:26 PM   #18
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o2 sensor?

I'm no mechanic but I had a Durango before my JKU and mileage took a dive. O2 sensor was sending a incorrect lean signal so it was throwing fuel at it to compensate.

Getting any color out of the exhaust, especially at start up. I could smell mine just sitting at stoplights it was running so rich.

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