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#540358 - 12/28/02 11:24 PM Can a clogged fuel filter cause bad gas mileage?
Patman Offline



Registered: 05/27/02
Posts: 19281
Loc: Oakville, Ontario
For the last year or so the gas mileage on my wife's 2000 Civic has been getting progressively worse. She is doing the same type of driving, does not idle it all that much anymore, but yet her MPG this last tank was 20.5, compared to 25-26 at this time last year in similar weather. It's actually been warmer lately too, but yet the last few weeks the MPG has gone steadily down each tank.

I'm going to be getting her trans fluid changed tomorrow and figure I'll get the fuel filter done too while I'm there. The car has 40k on it and neither of those two things have been done yet.

The thing is, the car runs great, doesn't feel down on power, and when I changed the plugs last week they looked fine, it didn't look like the car was running rich at all.

I'm stumped as to how a car can get bad gas mileage without running rich. I even did most of the driving in the car over the last week and while I did do all city driving, I definitely didn't idle it too much and it's definitely getting terrible mileage. [I dont know]

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#540359 - 12/29/02 12:13 AM Re: Can a clogged fuel filter cause bad gas mileage?
dragboat Offline


Registered: 06/04/02
Posts: 1933
Loc: Oklahoma
Whats the VI at 40C and 100C of your new oil compared to the M1 you had in it?Even the cranking VI ?

Maybe the gas ? Always in the equation

I would really, really have the valves adjusted on that car [Smile] We read and hear about Hondas like you have burning the exhaust valves,,at these miles it is a good time to prevent the head from being removed later on.

[ December 29, 2002, 04:15 PM: Message edited by: dragboat ]

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#540360 - 12/29/02 02:36 AM Re: Can a clogged fuel filter cause bad gas mileage?
satterfi Offline


Registered: 06/20/02
Posts: 903
Loc: CA
I suppose if the pressure drop across the filter was large enough it cause the injectors to give a bad spray pattern. Could be dirty injectors too.

Fuel filters should last longer than 40k miles. 60k is the replacement interval on my 88 Civic.

Dirty air filters and low tire pressure are more common causes of poor mpg.

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#540361 - 12/29/02 04:15 AM Re: Can a clogged fuel filter cause bad gas mileage?
Patman Offline



Registered: 05/27/02
Posts: 19281
Loc: Oakville, Ontario
The air filter isn't dirty, I pulled it out a few weeks ago and it still looks like new, and only had a few thousand on it anyways. Tire pressure is up to 35psi, I just checked it recently since I had all new tires installed.

It can't be the new oil, as she was getting poor MPG for quite a while now, even while using the same 5w30 Mobil 1 this car has always run.

Who knows, maybe the valve adjustment does have something to do with it, but if it was far enough out of whack that it would hinder gas mileage, wouldn't it also hurt the power output? If anything the car is running stronger than ever lately. It doesn't feel any slower than it ever did.

I always use the same gas, 87 octane Esso, and so it's not like I changed brands or anything. It's been a steady decline in MPG over the last year, not just since the cold weather started or anything. Even in the warmer months the MPG has been lower than it was in last year's colder months.

I doubt the car has dirty injectors, as I've run injector cleaner in it once every year and even just ran a full bottle of Neutra in there about a month or two ago.

I guess I'll start out with that fuel filter change tomorrow and go from there. The car is still under the powertrain and emissions warranty, so if it's something like a bad o2 sensor they'll have to replace it. Although I doubt it's that, since it's not setting off any codes. When my o2 sensors went in my Firebird this summer the car ran like crap and set off the check engine light too.

[ December 29, 2002, 08:16 PM: Message edited by: Patman ]

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#540362 - 12/30/02 08:04 AM Re: Can a clogged fuel filter cause bad gas mileage?
Patman Offline



Registered: 05/27/02
Posts: 19281
Loc: Oakville, Ontario
I went to two different fast lube places and neither would do the fuel filter.

I wonder if it's possible that the thing is even close to being clogged? When I had a friend change my fuel filter on my old 98 Formula with about 25k or so on it at the time, we cut open the filter later and it looked perfect inside, not even close to clogged up by any means. As a matter of fact we really couldn't see anything inside it. I do realize that the human eye can't see particles below about 40 microns, but I would think a whole bunch of them together would be visible.

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#540363 - 12/30/02 12:27 AM Re: Can a clogged fuel filter cause bad gas mileage?
TheLoneRanger Offline


Registered: 05/27/02
Posts: 1456
Loc: Southeast United States
Patman, I doubt it's the fuel filter, unless you've gotten some contaminated fuel in the past. My first guess would have been an O2 sensor, but you're right, it would trigger a code. Since the bad mpg has been gradual, what about a dirty MAF sensor? The MAF controls the air/fuel ratio, and a dirty one will not trigger a code.

They are quick and easy to clean, with either alcohol or brake cleaner.....

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#540364 - 12/30/02 01:30 AM Re: Can a clogged fuel filter cause bad gas mileage?
Patman Offline



Registered: 05/27/02
Posts: 19281
Loc: Oakville, Ontario
That's a good thing to check, thanks! I know a few of my f-body friends that had dirty MAFs and cleaning them made a big difference for them (although they never mentioned MPG, it was more of a horsepower issue in their case)

Why is it that these types of problems always happen in cold weather when I don't feel much like working on the car outside? [Smile]

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#540365 - 12/30/02 03:28 AM Re: Can a clogged fuel filter cause bad gas mileage?
digitaldrifter91 Offline


Registered: 09/14/02
Posts: 1902
Loc: cali
what kind of gas u running, i think the new civics require premium...its kinda early for the fuel filter..maybe it just needs a ecu reset due to the weather...isnt it still under warrenty? what kind of sparks u put in there?? that can affect ur mpg..but u should of reset ur ecu...

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#540366 - 12/30/02 04:02 AM Re: Can a clogged fuel filter cause bad gas mileage?
Patman Offline



Registered: 05/27/02
Posts: 19281
Loc: Oakville, Ontario
I run nothing but 87 octane in this car, it doesn't need premium.

I just put in fresh NGK plugs, the same as the OEM ones. I thought maybe the plugs were the problem at first, since my last plug change I put in a different part number NGK plug, ones specified for the 96-98 Civics I believe.

It is still under warranty, so if I can't solve the problem with simple stuff, I'll just take it to Honda. I just don't think they'll do much, they'll probably just say the bad MPG is our driving habits.

How do I reset the ECU in this car? In my Firebird I just pull the fuse for one minute and it resets it. Can I do the same with this Civic?

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#540367 - 12/30/02 06:36 AM Re: Can a clogged fuel filter cause bad gas mileage?
mdv Offline


Registered: 10/24/02
Posts: 515
Loc: MA
Several people have mentioned 02 sensors and throwing a code. The sensors don't always fail outright, but can degrade over time. At first they might not be bad enough to throw a code, but aren't operating as they were designed either. Something to consider.

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#540368 - 12/30/02 08:08 AM Re: Can a clogged fuel filter cause bad gas mileage?
digitaldrifter91 Offline


Registered: 09/14/02
Posts: 1902
Loc: cali
basically the ecu reset u described is the right one, but maybe if u want u can disconnect the battery..though if ur radio has a anti theft, which i think it does...u need the code..i say right now get honda to service it since its under warrenty..see what tehy say..and if tehy say its ur driving habbits just say that ur wife drives it like a grandma and doesnt go over 3k rpm..

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#540369 - 01/02/03 08:26 AM Re: Can a clogged fuel filter cause bad gas mileage?
Larry B. Offline


Registered: 10/13/02
Posts: 47
Loc: Buffalo N.Y.
I too have lost about 3 to 5 miles per gallon on my 3.1 litre cavalier .. . Are you sure the wife was getting better mileage last year at this exact time ? The refineries are now using a winter blend out your way and mine that reduces mileage by that percentage .. Larry B >>> Buffalo N.Y.

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#540370 - 01/02/03 08:47 AM Re: Can a clogged fuel filter cause bad gas mileage?
digitaldrifter91 Offline


Registered: 09/14/02
Posts: 1902
Loc: cali
im getting poorer mpg, i usually get like 300+miles to the tank, but now its around 285..usually after i change my oil it will be fine till it comes due for another change...i can get about 335 miles per tank sometimes..hehe thats good for a 91 honda accord

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#540371 - 01/02/03 01:05 PM Re: Can a clogged fuel filter cause bad gas mileage?
Patman Offline



Registered: 05/27/02
Posts: 19281
Loc: Oakville, Ontario
quote:
Originally posted by Larry B.:
I too have lost about 3 to 5 miles per gallon on my 3.1 litre cavalier .. . Are you sure the wife was getting better mileage last year at this exact time ? The refineries are now using a winter blend out your way and mine that reduces mileage by that percentage .. Larry B >>> Buffalo N.Y.

Larry, I keep detailed records of every tankful of gas in her car, and in Jan. last year she averaged 25.4 MPG. This Dec she has averaged 21.2. And she did way more idling last year since our son was only a few months old so she would warm up the car a lot more (now she just starts and goes) It was also colder last Jan than it was this Dec (we had a very warm Dec here)

I'm going to try changing her PCV valve this weekend, as it was the reason for someone else on here losing MPG. The strange thing is that her spark plugs looked fine, not like the car is running rich, and her oil analysis looks great, no sign that the engine is out of tune (in other words nitration is good) I still have not had the fuel filter changed, or checked the MAF yet.

[ January 03, 2003, 05:06 AM: Message edited by: Patman ]

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#540372 - 01/02/03 05:19 PM Re: Can a clogged fuel filter cause bad gas mileage?
BOBISTHEOILGUY Offline


Registered: 05/27/02
Posts: 1874
Loc: Ocala, Florida
Patman,

From what you are describing, it is not an uncommon problem. What most likely is happening is that the engine and filter is fine. The cat converters will tend to start clogging up reducing out going exhaust and reducing your mileage.This is due to unburned fuel from either lack of combustion in the cyl's which in your case appears not to be an issue, and another is where having run the wrong octane fuel will create more carbon build up caught into the cat.

Even though you're not feeling it, it can create slight back-pressure just enough to affect mileage but not performance by feel. Because of your good record keeping, the only way you noticed this issue is due to mileage otherwise, no problems would have come to your attention. This most likely would not be replaceable under warr as this would not be considered a problem by the dealer/ manufacture.

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