Fuel filter to blame for bad mileage?

I own a 1993 Honda Civic VX, which I purchased new, and is still running great with 270,000 miles on it. I noticed over the last couple of years that the fuel economy had declined. I got 40/50mpg city/hwy when I bought the car, but it had declined to 30/35mpg.

After discovering that the fuel filter had never been changed in 270,000 miles, I replaced it myself for about $15. Voila! The original fuel economy is restored! Why is this? Doesn’t make intuitive sense to me. You would think if anything the mileage would be BETTER with a restricted fuel filter. Can anyone explain this to me?

Fortune Cookies, Anyone?

That is as easy to explain comparing it with the results from eating at a
Chinese Restaurant and actually being hungrier, just moments later, than you were before the meal!

The correct amount of fuel has to be mixed with the air entering the cylinder to get an efficient burn. If the mixture is lean because the fuel log pressure is drooping, there is too much air in the cylinder; the fuel heats the entire mixture to a lower temperature; less energy is extracted by mechanical work on the piston; proportionally more energy goes out the tail pipe. Usually the ECM will adapt the pulse width to allow for the low fuel pressure but the adaptive range may have been exceeded.

Thanks for posting your discovery. It provides a lesson.

This is a pre-OBDII car, so the engine computer didn’t complain (turn on the check engine light) about running too lean. Wasn’t the power loss noticeable?

Interesting post.

It might be more accurate to say that since weak pressure at the injectors means a spray not as fine (less active surface area, area in contact with oxygen, per volume of gas) rather than to describe it as a lean mix. Slower combustion due to larger droplets means less efficient use of the energy in the gas (a good strong flame front at the top of the stroke is most efficient), thus the energy in the fuel is not converted as effectively to mechanical motion. A “lean mix” usually, by virtue of the added oxygen, actually creates more heat, much like using a bellows on a fire.

I know, I’m opening up a technical arguement, but I sense an interesting discussion comng on.

I too thank the OP for the post. It does remind us that proper maintenance really does matter.

There are some interesting ideas on why, but here’s how I think about it - a fuel injection system is carefully designed and balanced to get the right amount of gas injected, assuming the gas is at the proper pressure. Reduce that pressure, and the system’s screwed up. Technical, I know…

In addition to all of the other good information, I just want to add that the OP can look forward to failure of his/her fuel pump, as a result of not changing the fuel filter for over 200k. Yes, I know that it is mind-boggling that the fuel pump did not fail already, but the fact remains that the OP put additional strain on this now-aged pump by failing to maintain the vehicle properly.

As the old saying says, “You can pay me now, or you can pay me later”. Now is usually cheaper in the long run.

I would need a pre-filter change, pressure value.

Compared to a after, fuel filter change pressure value…

To attribute the mileage change to the fuel filter change

Clogged fuel filters wiil produce much more dramatic symptons than a mileage reduction.

We need the numbers to come to a conclusion.

Why did you change the fuel filter? Did you do anything else…like, change spark plugs, air filter, etc. on the engine?
I’ll agree with the theory that the fuel would not be so finely atomised with lower fuel pressure; which would result in more misfires, also. EVERY engine misfires. The poorer the tune, spark, and fuel flow (into the engine), the more the number of misfires. The more the numbers, the worse the fuel mileage, and, vice versa.

Honda actually had no specified change interval for the fuel filter this vintage Civic. Mine was performed at 190k.

The middle curve on this graph shows fuel efficiency vs air/fuel ratio:


It comes from “The Internal Combustion Engine in Theory and Practice” by Charles Taylor, ISBN 0-262-70027-1.

Note that consumption is minimum at ~10% lean and increases at leaner conditions.

Cool, but it assumes all other factors as constants, including pressure, volume, and the conditions of the fuel. A clogged filter affects fuel pressure. I submit that this varies one of the assumed constants factored into this chart and thus renders it not representative of the question at hand.

Change the conditions of the fuel, change the active surface area per volume, and you change the curves.

I submit that the bottom end of the chart’s curve is showing that as the ratio of oxygen becomes insufficient for effective combustion, effective use of the energy in the fuel, then specific fuel use increases. It takes more fuel to do the work. As the curve passes the area where oxygen is sufficient for complete combustion of the fuel, then specific fuel use increases as the mix richens.

I like this thread. It’s challenging.

How low can the fuel pressure go before the car does not run?

Some F.I. systems need a minimun fuel pressure to even run (GM TBI, Multi Port)

All fuel systems need a minimum fuel pressure to run. I don’t know what that is, but at some point the pressure becomes too low for the injector to effectively mist the fuel, the fuel will dribble (like slowly squeezing the windex sprayer instead of pulling it quickly), and the air stream will be unable to carry the fuel to the cylinders in a sufficiently vaporized state to burn in a way that can effectively push the pistons down.

OK - thanks for all the replies. Here is some more information. I have maintained this car meticulously since I bought it new - I just never thought to change the fuel filter, even though the manual says to “check” it every 30,000 miles. I might add that I took it at regular intervals to the DEALER for the first 5 years, during which I would think they would have done this, but looking over my records of all the service performed - they “checked” it but never replaced it.

I replaced the plugs about 5,000 miles ago along with the air filter. Soon the timing belt is scheduled for replacement again (90,000 mile intervals). I plan to have the valves inspected/adjusted at the same time. I might add as an aside that this car gets better mileage than current hybrids. Why doesn’t Honda bring this “lean-burn” engine technology back?