air filter, emissions testing...

April 23, 2009 12:55 PM

logistics of taking air filter out for emissions test?

i heard that taking out the air filter would significantly in helping a car pass an emissions test. Makes sense to me, im just wondering if it is "safe" to drive for 30 - 50 mins without an air filter, as id like to warm up the car too. Whats the worst that could happen? its fuel injection if that matters. thanks! my car barely failed last time on the NO or NO3 count (or whatever nitrogen something they measure), also had not the best gas in there at the time. i would just like to double make sure im not going to have to go back for a 3rd time... any other simple tricks appreciated too. thanks!

posted by figTree to sports, hobbies, & recreation (10 answers total)

IANA professional mechanic, but I wouldn't run my car with the air filter out. Last time I changed the filter, there were a variety of small solid objects in the bottom of the air cleaner box (pebbles, twigs, pine needles, small insects, etc.) that had been drawn in through the air intake but stopped by the air filter. These are all things that I would not want to have come in contact with my Mass Airflow Sensor which costs $200 to replace or get into the intake manifold and do heaven knows what in the intake manifold.
I'd either spend the money to get the car up to passing smog, or barring that spend the money to buy a sticker.
posted by kid_dynamite at 1:04 PM on April 23, 2009 [1 favorite]

The simplest trick is to put in a brand spanking new air filter.

I'm not sure how your state works, but when I lived in GA the inspection tech would nose around under the hood to make sure you weren't cheating by disconnecting hoses, removing air filters, etc.
posted by torquemaniac at 1:09 PM on April 23, 2009

I think it would be unwise to drive without it. You don't say what kind of car you have so it's hard to give any real advice except to avoid hard acceleration during the drive, which will cause your car to drop out of closed-loop emissions control.
posted by fritley at 1:12 PM on April 23, 2009

Well, I know in my state (California), the air cleaner is a required component, so trying to pass a smog test without an air cleaner isn't even an option. That being said, it would be much better to solve the underlying problem.

NOx are caused by excess heat and pressure during combustion. At excess pressure-temperatures, the oxygen will combine with nitrogen, forming oxides of nitrogen (NOx; NO1, NO2, etc). Exhaust gas recirculation (EGR) problems are a leading cause of high NOx. EGR reduces cylinder temperature by displacing some incoming oxygen with recirculated (non-combustible; inert) exhaust gases. So a bad EGR valve or a damaged/blocked vacuum line leading to the EGR valve could be the real culprit here, and it where I'd start looking.

when they do a smog test, they test for a variety of gases. It would be helpful to know the make/model car you have, as well as the numbers for the other gases that were present during your last failed test. I'm not an expert, so I don't want to over-promise, but it's a lot easier to diagnose this stuff if you present a complete picture of the situation.
posted by mosk at 1:15 PM on April 23, 2009 [1 favorite]

Whats the worst that could happen?
Raw, unfiltered air entering your system? Right off the top of my head, I would say you'd end up killing your Mass Airflow Sensor (MAF). Depending on your car, that could cost you a couple hundred dollars.

I'm not sure why you think taking your filter out would improve your chances at passing inspection. The computer will sense the increased volume of air (via the soon-to-die MAF) and simply adjust the fuel delivery and timing. Heck, removing the filter might actually cause your car to run dirtier.

Put a new, clean filter in there.
posted by Thorzdad at 1:15 PM on April 23, 2009

Unless this is a relatively ancient, carbureted car, this strategy is all risk and no reward. It simply won't do anything but risk your engine.
posted by jon1270 at 1:24 PM on April 23, 2009

my car barely failed last time on the NO or NO3 count

if your car is failing on NO_x, get your EGR system inspected and the EGR valve itself cleaned out (or, if you're feeling spendy, replaced). the valve recirculates a little bit of exhaust gas back into the air intake. having a little extra warm CO₂ do some magic chemistry in the combustion chamber suppresses the production of NO_x.

if the valve gets clogged up with soot and exhaust junk, which they can and do, your NO_x emissions will go up. if you are at all handy with a wrench and know your way around under the hood, it might be worthwhile to pull the valve off and see if it's all gunked up. if so, clean it out with some of those little fuzzy pipe cleaner things and maybe some carb cleaner solvent stuff, and put it back in.

normally if your EGR fails in this way with a post-1996 model vehicle it'll light up the check engine light and produce an "insufficient EGR flow" code, but you may be on an edge where it's just restricted enough to fail on emissions, but not so restricted that the engine computer flags it as bad.

i can think of no reason why removing the air filter would have any effect on your NO_x output. probably a total waste of time.
posted by sergeant sandwich at 1:26 PM on April 23, 2009

The worse that could happen is your engine blows up from some FOD. That's pretty unlikely though and I've run many an engine without an air cleaner for short periods while messing with throttle bodies on both gas and lpg engines.

Running without an airfilter will lean out an engine ever so slightly but on a modern closed loop engine the coputer comensates for this almost immediately.

If you want to give it a try get one of those K&N filters and just don't oil it. It wont give you more power but it'll pass a visual inspection and prevent FOD while still flowing better than a paper filter with the disadvantage that more grit will get in your engine dirtying your oil and generally doing bad things to your top end.
posted by Mitheral at 1:27 PM on April 23, 2009

You could buy a high flow filter. But like others, I doubt this plan will help you in any way.
posted by Cool Papa Bell at 1:34 PM on April 23, 2009

i heard that taking out the air filter would significantly in helping a car pass an emissions test.

Whoever told you that was making it up, or repeating urban myth.

There is not really that much risk in running a car without an air filter. Yes, over time, your MAF sensor will become dirty and requiring cleaning, but it is not an instantaneous risk, nor is it irreversible.

It is, however, a complete waste of time.

You may need a new 02 sensor. You may need to check your vacuum system or your exhaust gas recirculation system for leaks. This is not something that can be bypassed, unless you want to get an illegal emissions test (not that hard in most places). You say you have been back twice already, but you have to DO something to make your car be better at emissions - this means fixing something, rather than trying to bypass things.
posted by Brockles at 7:33 PM on April 23, 2009