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04 Mercedes C230, 08 Rondo LX V6
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359 Posts
Discussion Starter #1
Same tires & pressures, same brands of gas, no check engine light. But getting consistent 20-21 mpg on hwy towing light boat trailer vs. 24-25 mpg last summer. 61K miles on my 08 V6. Suburban driving w/o trailer also giving 15-20% poorer mpg. All figures summer driving with lots of AC use. Any ideas?
 

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2010 Subaru Impreza 2.5i Premium 5-door, *old* 2009 Kia Rondo LX 4cyl 2.4L
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492 Posts
Same here, been trying to figure it out myself. Been getting 17-18mpg 100% city (25-26mpg hwy to Santa Fe and back) on my 4cyl 61K miles. Last year, 22mpg city and 29-31mpg highway. Same tires as last year, run consistent 33-35psi. Almost always fill up at the local QT by my house. Replaced plugs earlier this year. Have a new engine air filter on the way this week (~9K miles on the Wix filter currently installed).
 

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04 Mercedes C230, 08 Rondo LX V6
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359 Posts
Discussion Starter #3
Did the new plugs make any difference? My understanding is that dirty air filters have very little if any effect on mpg as the ECU will adjust fuel injection based on measured air flow rate.
 

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04 Mercedes C230, 08 Rondo LX V6
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359 Posts
Discussion Starter #4
This site

Poor Fuel Economy

lists a bunch of issues that can reduce mpg w/o triggering check engine light

I think I'm gonna take a copy of it to a local well-respected independent shop (who incidentally quoted less than half the dealer labor to replace plugs) and ask them look for any of the quoted OBD II codes.
 

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04 Mercedes C230, 08 Rondo LX V6
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359 Posts
Discussion Starter #5
OK, I have an appt for Wed AM. Will post any interesting/useful findings.
 

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2008 Rondo LX - 2011 Forte EX
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103 Posts
fwiw, in my forte, i can achieve consistently more mpg when using mid range fuel on the highway, at least +4 mpg

its not as easy to confirm in my 08 V6 Rondo, other than it runs stronger, noticed most when using cruise.
 

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04 Mercedes C230, 08 Rondo LX V6
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359 Posts
Discussion Starter #8
Since there is no difference in energy content of different octane auto fuels, the only possible explanation is that the 89 octane lets the computer run the engine with more spark advance w/o knocking than what the 87 octane fuel permits. But it's hard to believe this could improve mpg by 15-20% in a relatively low mileage engine. Unless somehow the engine was pretty badly carboned-up and had to have the advance dialed WAY back to prevent knocking on 87.
 

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04 Mercedes C230, 08 Rondo LX V6
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359 Posts
Discussion Starter #9
According to OBD II info, everything is A-OK with the Rondo. Short & long term fuel trim O2 sensors working perfectly, etc, etc. Not even any pending codes. Maybe it's fuel formulation differences. Oh well.
 

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04 Mercedes C230, 08 Rondo LX V6
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359 Posts
Discussion Starter #10
Ran a couple of cans of fuel system cleaner. Then reset ECU by pulling 3 underhood fuses (10A ECU, 10A PCU, and 20A ECU-1) and leaving them out for >30 min. Yesterday 160 hiway miles at 70mph with AC running, today same miles returning with no AC. 27 mpg. At least for now I'm a happy camper.
 

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2010 Subaru Impreza 2.5i Premium 5-door, *old* 2009 Kia Rondo LX 4cyl 2.4L
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492 Posts
Ran a bottle of Lucas Fuel Treatment through my Rondo couple tanks ago. Before I was getting ~171 miles on 3/4 tank, last two tanks, been getting 210-220 miles on 3/4 tank. May have to pick up another bottle.

Gumout Regane is supposed to be pretty good, too. Heard good things on it from car buddies.
 

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2012 Optima SX Tech/Premium; 2008 Rondo EX V6 7-seater
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255 Posts
Ran a bottle of Lucas Fuel Treatment through my Rondo couple tanks ago. Before I was getting ~171 miles on 3/4 tank, last two tanks, been getting 210-220 miles on 3/4 tank. May have to pick up another bottle.

Gumout Regane is supposed to be pretty good, too. Heard good things on it from car buddies.
What is the mechanism claimed for increasing mileage? I just don't understand how this could work?

Let's assume it cleans out the fuel injectors. But if your injectors were clogged up, wouldn't that just prevent you from using the gas? Like you would squirt out more gas after using this product to make your fuel economy worse?
 
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