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I have a 2003 Civic EX with 193k miles that [I]was[I] getting over 40 mpg. I have completed every maintenance task on time since we got the vehicle with 50k miles on it. I use synthetic oil and wait till around 7k miles before I do oil/filter change. I just replaced the air filter, the spark plugs have 40k miles on them, and my tranny got the bearings, cluth and fluid replaced in the last 20k miles.

Anyway, all of the sudden, I went from getting 400-450 miles per tank down to 350. This is a heavy commuter vehicle the drives 500 miles a week on the exact same roads in the exaxt same conditions. I was thinking it was the fuel filter maybe, but then realized this vehicle doesnt have one (thanks to your forum help =).

I cant think of anything that would cause this drop....I run seafoam through the crankcase and fuel occasionally. I also use fuel injector cleaner every other month or so. I make my blends strong with 1 bottle to 1/2-3/4 of a tank of gas.

Thanks in advance!

Alex

 
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I was thinking maybe MAF sensor, IAC, PCV valve??? An exhaust component maybe?

I filled it up today only got 31.738 MPG......this is the lowest I have ever seen in that car. I know you all might think I am crazy, but that car has always easily maintained low 40 mpg...

 
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Have a quick read over this, the first thing I would replace if my mileage began to drop would be the 02 sensor. I'd also suggest going to get the codes read, sometimes a code will trigger in the ecu without a cel.

https://www.civicforums.com/forums/1...e-up-mods.html

 
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Should I replace both O2 sensors?

I have a scanner at home and will check tonight. You are my hero....I will complete all the tasks on the link and update later! Thanks again

 
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EGR valve was disgusting.....should I clean or replace?

 
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bump

 
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hey guys..i have a civic 2001 lx coupe with 126k and i only get bout 22-25 mpg..can some one help me please

 
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i use the honda synthetic blend oil and i changed both of the o2 sensors and still the same ...

 
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? I have gotten 32 or so since I got mine. Only had it a couple years though. Coming from Chevy and Ford V8s I will not complain about ~32. I would love to have more, but not complaining.

With all that said, the blends of fuel today suck suck suck. Our 5.3 trailblazer we had got 17-18 when we got it in 04. We sold it a couple years ago because it had dropped to 12. It ran fine and I couldn't find reason for it, so we sold it..

My 02 4.6 F150 gets around 13 now, town or highway and got 18 to 19 on the highway a year ago when I bought it...

The only thing I can account for it is the governments restrictions of the fuel.

 
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I think I found the problem...the EGR valve was disgusting. I have driven 100 miles on my new tank and am at 7/8 a tank still. I have gotten 40-45mpg constistantly for the last couple of years.

My mpg has actually been INCREASING over time. I know noone will believe me, but I actually got 50 mpg once. I refueled at 500 miles because my gas light hadnt come on yet, and it only took 10 gallons.


NOTE: I have a 2010 Civic EX that will only get 31-33 mpg....even though the manufacturer rating is 32-36 highway.

 
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replace the egr valve. they can usually never be cleaned enough to reuse. the carbon is cooked on and short of scraping it off, it wont get clean like a new one. even without an engine light, there are many parts that can work "well enough" even tho they are lowering the efficiency of the engine. oxygen sensors are another example. they should be changed every couple years but most people go 10 yrs or until the engine light comes on.

 
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Thanks gearbox, I purchased both 02 sensors, a new EGR valve, and a valve canister for $290 shipped from rockauto.com =( Expensive bill, but I am sure it will pay off. The vapor canister for my make and model is like $60 for whatever reason; a lot more than the $20 one in your thread. EXCELLENT write up though, I will post back after parts are installed and efficiency improves (hopefully!)

 
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the one i mentioned is the evap air filter, not that expensive. the vapor/charcoal canister is more expensive but shouldnt need replacing unless the engine light comes on.

 
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Uh oh....I looked for an evap air filter but could only find vapor canister under multiple websites. I saw the picture and everything, I even found the exact same one that was in your picture. But the website said it would not fit in my '03. Do you have a link? Should I return what I bought?

 
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hmm well if its that light piece i have in the pics, thats just an air filter. it doesnt do anything except filter very small amounts of outside air coming into the evap system. i doubt it would ever go bad to the point of causing a problem, but every 100k i figure is a good time to change it so it doesnt stay clogged. the charcoal canister is something different and more expensive, it has activated carbon to absorb fuel vapors. also the charcoal canister can trigger an engine light if it goes bad.

 
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OK that makes sense. But do you have a link to where 'said' filter can be purchased? I purchased the following: thinking it was what you were talking about

http://www.rockauto.com/catalog/moreinfo.php?pk=1002643

 
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what is the name of it? that looks like the charcoal canister and not the air filter.

 
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i think #10 is the one you posted which is the charcoal canister. my air filter is #12 with the two tubes going into it and much smaller. you can hold it in your hand.

http://www.hondaautomotiveparts.com/...L+PIPE+%281%29

 
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STANDARD MOTOR PRODUCTS Part # CP3075 More Info {#17011S5AA30} Intermotor

You are exactly right....I purchased #12 =/ Hopefully I can return it!

 
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well you could still use it, just wont solve any of your problem. but im confused because the link you just posted was for #10, yet you say you bought #12?

 
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Typo....meant to say I purchased #10.

 
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oh okay. yeah you could still use what you got already. never hurts to change it.

 
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I am going to install all the parts today. I'll let you know how work out! I am going to order the #12 air filter and install next weekend.

 
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Cool. Let us know the results.

Something that is often overlooked is TIRE PRESSURE. Make sure your tires aren't underinflated. They lose about 1psi a month.

My 305k mile EM2 Civic was generally getting about 35mpg AVERAGE which is lower than it has been, so I did the following about 5k miles ago:
* Oil/Filter
* MTF Fluid change
* Spark Plugs
* Both OEM Honda O2 Sensors
* Cleaned the throttle body
* Cleaned the IACV
* Inflated tires a little over recommended pressure (38psi)...fine as long as you don't exceed the maximum pressure stated on the sidewall (44psi in my cars).

It went up to about 39mpg AVERAGE now. I am like you, in that about 98% of my driving is hiways and interstates, so I get better mileage than the guys traveling around the city.

 
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I have completed the oil/filter change, MTF fluid change, secondary O2 sensor, throttle body, IAC, and tire pressure within the last couple hundred miles. My spark plugs only have about 50k on them so I am gonna wait till about 60k to change them out.

I also replaced the charcoal canister and EGR valve. I am at 38 mpg now, so it is a lot better. I just got my primary O2 sensor in the mail today and will install this weekend. The original part they sent me was the wrong part in the right box =/ !!!

I'll update again later =P

 
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Note that it took about 2 tanks of gas until my MPG went up when I replaced the O2 sensors.