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#4707138 - 03/26/18 06:17 AM Air Filter and Cars That Run Rich
Speak2Mountain Offline


Registered: 04/07/17
Posts: 343
Loc: Mississippi
THEORETICALLY Speaking: Would a higher flow air filter (K&N/S&B) do more to prevent running rich than a lower flow/higher capture filter?

Theoretically speaking.....

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#4707143 - 03/26/18 06:23 AM Re: Air Filter and Cars That Run Rich [Re: Speak2Mountain]
Brybo86 Offline


Registered: 10/08/11
Posts: 2927
Loc: Chicago, IL
Sure in an old carbed engine.

In a modern car I would be looking at something else as the ecu controls all the fuel.

Mb dirty throttle plate?
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#4707197 - 03/26/18 07:43 AM Re: Air Filter and Cars That Run Rich [Re: Speak2Mountain]
MichiganMadMan Offline


Registered: 07/18/11
Posts: 250
Loc: SE Michigan
As previously stated: With a carburetor air filter restriction matters.

With a modern fuel injected car, it won't run rich. I bought a Chevrolet Aveo5 recently that had been neglected by all previous owners. The screws that held the lid of the airbox on were rusted solid indicating nobody had been in there in a long while. When I tried to hulk them out, the metal insert tore out of the lower half rather than releasing the screw.
The air filter was...horrific. First I poured the sand and gravel out of the airbox, then I plucked the grass and leaves out. The filter was nearly completely blocked (I was trying to figure out why it had 25 hp, instead of the usual 35..).
When I installed a new airbox with a new filter, I about did a burnout.

Fuel economy was unchanged, the O2 sensor was switching before and continued to do so after. Computers are really good at adjusting for variances in environment.

A note: Cars running speed-density fuel injection are more prone to "get lost" with their fueling than cars running a MAF (where they measure the actual air going into the engine).
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#4707198 - 03/26/18 07:44 AM Re: Air Filter and Cars That Run Rich [Re: Speak2Mountain]
dbias Offline


Registered: 12/24/14
Posts: 124
Loc: USA
MAF sensor should be metering air and the ecu sends fuel appropriately. Looking at short term and long term fuel trims is more useful in identifying rich and lean conditions.
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#4707259 - 03/26/18 08:43 AM Re: Air Filter and Cars That Run Rich [Re: Speak2Mountain]
ARCOgraphite Offline


Registered: 05/17/09
Posts: 11909
Loc: N.H, U.S.A.
Dirty filter will throttle the engine. My car runs rich - black tail pipe - I blame it on MAF MAP or even low octane fuel - it runs cleaner if I add 30% 93 octane and the car has MUCh more low end grunt.

So look to a too tight K sensor if everything else is OK.

Give the MAF a CRC Cleaner shower to start - make sure airbox is tight and no rodents chewed any vacuum control lines.

also if you drive too easy all the time the chamber and cat will be crusty.


Edited by ARCOgraphite (03/26/18 08:44 AM)
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#4707272 - 03/26/18 08:54 AM Re: Air Filter and Cars That Run Rich [Re: Speak2Mountain]
artificialist Offline


Registered: 09/23/07
Posts: 8598
Loc: Florida
Not only does it cause carb engines to run rich, it can also cause a non-computerized diesel engine to make more black smoke.
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#4707335 - 03/26/18 10:01 AM Re: Air Filter and Cars That Run Rich [Re: Speak2Mountain]
Olas Offline


Registered: 12/11/13
Posts: 3929
Loc: Manchester, England
It's entirely dependent on control systems.

Anything with maf/map/lambda sensors should maintain afr regardless of pressure drop across the filter.

Anything open loop injection ie mechanical-pump diesels or also some of the earliest injected petrol engines will get richer as the filter gets obstructed.

Carbureted engines will maintain afr up to a point because the plugged filter slows the airflow across the carburettor pulling less fuel, but after that given point it just becomes a bolt-on choke.


Edited by Olas (03/26/18 10:03 AM)
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#4707340 - 03/26/18 10:04 AM Re: Air Filter and Cars That Run Rich [Re: Speak2Mountain]
CT8 Offline


Registered: 10/09/14
Posts: 10746
Loc: Idaho
Originally Posted By: Speak2Mountain
THEORETICALLY Speaking: Would a higher flow air filter (K&N/S&B) do more to prevent running rich than a lower flow/higher capture filter?

Theoretically speaking.....

it depends on why the engine runs rich.
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#4707354 - 03/26/18 10:13 AM Re: Air Filter and Cars That Run Rich [Re: MichiganMadMan]
Ducked Offline


Registered: 10/25/12
Posts: 4239
Loc: Taiwan
Originally Posted By: Speak2Mountain
THEORETICALLY Speaking: Would a higher flow air filter (K&N/S&B) do more to prevent running rich than a lower flow/higher capture filter?

Theoretically speaking.....


Theoretically speaking, no. Neither air filter is designed to "prevent running rich" .

Even a carbed engine is built and tuned to deliver the correct mixture with whatever restriction the OEM filter applies, so IF there was an effect at all, the higher flow filter would run lean, rather than the OEM filter running rich.

Practically speaking, even on a carbed engine, there is negligable effect until the filter is heavily clagged, at least according to the most readily available study.

https://www.fueleconomy.gov/feg/pdfs/Air_Filter_Effects_02_26_2009.pdf

Originally Posted By: MichiganMadMan
As previously stated: With a carburetor air filter restriction matters.


As previously stated, no it doesn't.

At least not according to the above study. (I think they only tested one carbed car).

Severe air filter clogging matters, although to get more than a 2% drop in fuel economy they had to add so many shop towels that drivability was severely impacted and the filter parially collapsed.

Designed air filter restriction has negligable effect.

Recently I rigged a water manometer as a restriction gauge and decoker. The filter has been on the car since I bought it (about 7 years IIRC) in dusty Taiwan and the restriction was not measurable. I had to add paper towel and toilet paper to the filter to get the gauge/decoker to work.


Edited by Ducked (03/26/18 10:35 AM)

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#4707400 - 03/26/18 11:02 AM Re: Air Filter and Cars That Run Rich [Re: Brybo86]
WyrTwister Offline


Registered: 01/13/13
Posts: 1374
Loc: Texas
Originally Posted By: Brybo86
Sure in an old carbed engine.

In a modern car I would be looking at something else as the ecu controls all the fuel.

Mb dirty throttle plate?


Or a messed up sensor , like may be a coolant temp sensor . For example .
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#4707412 - 03/26/18 11:11 AM Re: Air Filter and Cars That Run Rich [Re: Speak2Mountain]
i_hate_autofraud Offline


Registered: 05/19/16
Posts: 702
Loc: Canada


A high flow air filter, K&N, etc that needs filter oil pretreat can be a problem on any
late model engine with a MAF sensor, trace amounts of oil migrate into the MAF and dirt sticks to
the MAF element inside, then sets codes, this takes a while, maybe a year or so.

The codes set last time were related to fuel injection related, not directly pointed at a MAF
problem. The easier thing was to remove and clean the (GM) MAF with a spray and dry it out with
a warm hair dryer. If the MAF is powered up with ANY cleaner inside, it can blow the hot-wire element
since they are super thin, when cleaner finally flashes off, the wire can pop. I cleared the codes
and they never came back.

If you clean and oil your re-usable K&N and add a bit extra oil it may filter better, but use it
on a car with a carb not a MAF sensor! Don't get me wrong I like the K&N and the cleaning kit, it works well!
Just not worth the hassle when it risks the MAF sensor!

I noticed the duct behind the K&N filter had signs of dirt that got thru. I washed and oiled the K&N
filter and put it back in the box to sell off with the cleaner kit. I'm back to a good paper pleated filter!

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#4707505 - 03/26/18 01:04 PM Re: Air Filter and Cars That Run Rich [Re: Speak2Mountain]
LeakySeals Offline


Registered: 02/18/11
Posts: 6541
Loc: Hudson, NH
Originally Posted By: Speak2Mountain
THEORETICALLY Speaking: Would a higher flow air filter (K&N/S&B) do more to prevent running rich than a lower flow/higher capture filter?

Theoretically speaking.....

Theoretically speaking the MAF would sense more air flow with the K&N. Report that to the ECU which would add fuel to keep the mixture correct. So if anything it would be richer. Maybe.
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#4707530 - 03/26/18 01:41 PM Re: Air Filter and Cars That Run Rich [Re: Speak2Mountain]
circuitsmith Offline


Registered: 12/17/09
Posts: 611
Loc: Wash, DC
The more recent carburetors float bowl and air jets are vented to downstream of the air filter, so filter restriction is largely compensated for.
This is evidenced in the fueleconomy.gov paper linked above.


Edited by circuitsmith (03/26/18 01:41 PM)
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#4707652 - 03/26/18 04:02 PM Re: Air Filter and Cars That Run Rich [Re: Speak2Mountain]
Silk Online   content


Registered: 07/26/03
Posts: 4466
Loc: New Zealand
More recent as in a long, long time ago. Atmosphere vented carbs will run rich with a blocked filter by the late '60's, early '70's float chambers were vented to the airhorn, and won't go rich with a restricted air filter.
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#4709416 - 03/28/18 10:16 AM Re: Air Filter and Cars That Run Rich [Re: Speak2Mountain]
i_hate_autofraud Offline


Registered: 05/19/16
Posts: 702
Loc: Canada


K&N "may" allow more air / power, but you'd have to redline the engine to see the improvement either on the track or
dyno.

City / hiway driving at legal speeds at part throttle would see no difference - except the "confirmation bias"
of an owner who puts in a K&N and then fools themselves into believing there's more power at 2,000 rpms!

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