Filter performance as it gets old/dirty

[sky jumper]

this question is related to both air and fuel filters.

my basic understanding of filtering action is that when a filter is new it has big open pores that let more air/fuel through. as the filter gets old/dirty it lets less air/fuel through because the pores get clogged and are effectively smaller.

so this begs the question -- is it fair to say on old dirty filter actually removes more contamination that a fresh one? and a fresh filter will let more dirt through vs an old one?

if so, then is it fair to say one should not change air/fuel filters unless/until they become so clogged that air/fuel flow is compromised?

I know that oil filters are very different b/c they will bypass once the filter media gets too gunked up. but air/fuel filters have no bypass. so I'm wondering, should they just be left alone?

 

[PW01]

I'm certainly no expert, especially compared to the some of the knowledgeable folks on this board, but I'll give you my take on your question. in short, my answer is yes. Filters do become more efficient as they get dirty, the "holes" become smaller and catch finer debris, but this is a time/use sensitive scenario. There is probably a point where the filter is "dirty enough" to provide an ideal mix of filtering ability and flow, but once at that point the scale begins to tip toward reducing flow. That being said, I stick to scheduled replaced of 15,000 miles for air filters and 30,000 miles for fuel filters. I have no idea of where in the "scenario" I fall, but I've been following that schedule for years.

 

[sky jumper]

right. so I've seen posts on other boards where someone will change an old fuel filter, and brown dirty fuel will spill out the input side of the filter. inevitably they use that as evidence the filter was no longer filtering, and all that dirty fuel was plugging up their injectors.

my gut tells me otherwise. in fact, putting a new filter in without cleaning out the upstream fuel supply (dirty tank & fuel lines) will just let more crud get to the injectors, and the situation becomes worse (not better).

 

[SargeGTO]

Well...not to over simplify....but....
Filtering in its basic "concept" requires something to flow through it while trapping and holding other things. So it gets down to how big of "something" does it trap and how little of "something" does it allow through. Sure...when a boatload of "somethings" gets trapped it clogs up. Now oil filters get clogged up...they go into bypass and your engine doesn't die a nasty death but it also is living off of unfiltered oil. Lesser of two evils. Fuel filters clog up and your car doesn't run or runs really bad .....so you must accept some amount of "somethings" are gonna get through and into your fuel/oil system. In the fuel world you have additives put into the fuel at the tank farm that dissolves most of the "somethings"....and what gets through should blow right on through your injector/carb jets and burn up in the combustion chamber......now of course many of us utilize FP60/Lucas UCL/Techron blah blah blah to "help" this stuff get on through with out causing any harm.....In the oil world....of course there are additives in the oil to hold or suspend these "somethings" not allowing them to attach and congregate and keep suspended in the oil until they are filtered or drained out. In the air world....the trade off has always been...more filtration=less flow=less horsepower. Less filtration=more "somethings" in your engine= engine wear.
Bottom line to this story is you must adjust your filtration to meet your driving habits and vehicle purpose. You got a grocery getter...your going for maximum filtration for engine longevity and minimum horsepower or performance....you got a fire breathing quarter miler...you slack off of the filtration a few notches to increase flow of fuel/air/oil.....

 

[SargeGTO]

So bottom line is yes a clogged filter will filter better because nothing is getting through at all. Of course your sitting on the side of the road with a dead car.

 

[BOBISTHEOILGUY]

I agree, the longer filter is run, the more efficient it gets but to only a certain degree. like stated above, change it sooner and you wont have a problem. the fuel filter will cause your engine to run poorly as it gets clogged up and will kill your engine if run past its life. oil filters will go into bypass mode and not filter much at all. as for air filters, I change mine if the oil analysis shows I need it. it depends on how I drive and where I drive.

 

[GSDad]

I said this before in another thread. In 29 years in the A/C business, I've made a good living from folks who don't change their filters often enough. As long as the air can pass around the dirt to get through, it will . When it can't, the dirt gets pushed through. Using dirty filters in expensive equipment, is always the wrong thing to do. Try to find any equipment maker, wheather it be car engines, A/C units, or any other air breather, that recommends using dirty filters.
GSD