Changed air filter. Now gas milage drops 20%?!

[JEDI]

2005 Mazda3i automatic, 85k miles

I change my air filter every 15k-20k miles.

I just changed it 2 weeks ago.

I get 31mpg in mix city/hwy. Now i get 25mpg since changing the air filter. i've filled up twice (10gallons each time), and both times i'm now getting 25mpg.

I usually get OEM at my mazda dealer while changing my oil.

This time i was at Advance Auto Parts and figured i'll get one while i'm here (Purolator):
http://shop.advanceautoparts.com/we...22880016-P_2_R%7CGRPFILTAMS_1076140593___

So is it the air filter? If not, what could be the cause?!

.

 

[3chordcharlie]

winter gas + snow tires + bad roads?

My average mileage is probably off 20-30% for the last two weeks.

 

[boardsportsrule]

I'd be very suprised if it had that large of an effect. I would ensure you installed it properly, etc. and consider other sources of change such as weather, knocking a sensor loose, etc. etc.

is this your first time changing the airfilter yourself?

 

[woodie1]

On cars with fuel injection the air filter has little effect on gas mileage in normal driving. The engine management system keeps the air/fuel ratio constant except for WOT. The colder weather, winter fuel blend, etc. is most likely your reason for reduced mpg.

 

[JEDI]

On cars with fuel injection the air filter has little effect on gas mileage in normal driving. The engine management system keeps the air/fuel ratio constant except for WOT. The colder weather, winter fuel blend, etc. is most likely your reason for reduced mpg.

What's WOT?

i've always changed the air filter myself. i buy the oem filter at the counter at the dealership while waiting for an oil change.

since owning the car (2005), mpg is usually at least ~30mpg no matter the temp/type of gas. it has NEVER been as low as 25mpg.

what kind of sensor could i have knocked loose while changing the air filter? and wouldnt there be a light on my dashboard?


edit:
if air filter doesnt matter on fuel milage, then why cant i just leave it dirty?

 

[JJ650]

WOT = wide open throttle

 

[JJ650]

What's WOT?

i've always changed the air filter myself. i buy the oem filter at the counter at the dealership while waiting for an oil change.

since owning the car (2005), mpg is usually at least ~30mpg no matter the temp/type of gas. it has NEVER been as low as 25mpg.

what kind of sensor could i have knocked loose while changing the air filter? and wouldnt there be a light on my dashboard?


edit:
if air filter doesnt matter on fuel milage, then why cant i just leave it dirty?

There are couple of schools of thought on this:

A dirty air filter will change your mileage. A dirty filter is starving the engine for oxygen at a given rpm. It is throwing the "normal" combustion off. The fuel curves are distorted and the computer has to compensate. The lack of fuel burnt (due to lack of 02) fouls emissions controlling systems (o2 sensors, etc), which in turn forces the computer to change fuel rates and ignition timing...baisically retarding power. This is to compensate for the lack of air. Reduced power is the biggest issue.

Also, a dirty filter is not up to the task of filtering properly, allowing more dirt/dust particles to get thru. Over time and with enough particulates in the engine, it will cause damage. For example: Cylinder walls can become scored and you will lose compression (loss of power, bad mileage, or worse depending on how much gets by).

With most newer cars the fist paragraph really isn't 100% true since the computer adjust for these conditions. With older carburated power plants, a clogged filter significantly changes power/mileage.


The other idea is that an engine is one big air pump. The less restriction, the less energy used to pull air thru. A clogged filter is a restriction point and thuse requires more energy to pull the same amount of air.

 

[compman25]

There are couple of schools of thought on this:

A dirty air filter will change your mileage. A dirty filter is starving the engine for oxygen at a given rpm. It is throwing the "normal" combustion off. The fuel curves are distorted and the computer has to compensate. The lack of fuel burnt (due to lack of 02) fouls emissions controlling systems (o2 sensors, etc), which in turn forces the computer to change fuel rates and ignition timing...baisically retarding power. This is to compensate for the lack of air. Reduced power is the biggest issue.

Also, a dirty filter is not up to the task of filtering properly, allowing more dirt/dust particles to get thru. Over time and with enough particulates in the engine, it will cause damage. For example: Cylinder walls can become scored and you will lose compression (loss of power, bad mileage, or worse depending on how much gets by).

With most newer cars the fist paragraph really isn't 100% true since the computer adjust for these conditions. With older carburated power plants, a clogged filter significantly changes power/mileage.


The other idea is that an engine is one big air pump. The less restriction, the less energy used to pull air thru. A clogged filter is a restriction point and thuse requires more energy to pull the same amount of air.

Umm, you do realize the reason the dirty filter chokes the engine of oxygen is because it is filtering more, right? The more clogged it is the smaller the spaces for air and dirt to get through become. It is filtering more when it is dirty.

 

[SparkyJJO]

Umm, you do realize the reason the dirty filter chokes the engine of oxygen is because it is filtering more, right? The more clogged it is the smaller the spaces for air and dirt to get through become. It is filtering more when it is dirty.

Umm..... no.

Clogged is just that. Clogged. It is supposed to let air through, if it isn't doing that properly, then the engine isn't getting the air it needs. It isn't filtering better it is just plugged up and needs replaced.

 

[compman25]

Umm..... no.

Clogged is just that. Clogged. It is supposed to let air through, if it isn't doing that properly, then the engine isn't getting the air it needs. It isn't filtering better it is just plugged up and needs replaced.

Umm, yes. Think for a second. If it's clogged, what is it doing to all the pores in the medium? It's filling them up, making the available area for something to pass through smaller. Which means it is filtering better. I never said it was making it more efficient.

 

[SparkyJJO]

/facepalm

 

[Itchrelief]

/facepalm

What exactly are you arguing? He's saying that a dirty filter catches a greater % of particulates, which is true.

Are you trying to take the extreme case where the filter is so clogged that there is negligble air flow or something?

In the realm of real-world, commonly encountered examples, a used filter filters better, it just is a bit more restrictive. You change it when the restriction becomes excessive, but the new filter isn't going to be a better filter, just less of a restriction on the air intake.

 

[Itchrelief]

bold is my editing

Umm..... no.

Clogged is just that. Clogged. It is supposed to filter out particles while not being too restrictive to let air through, if it isn't doing that properly, then the engine isn't getting the air it needs. It isn't filtering better it is just plugged up and needs replaced.

I think this is the hangup here. We are working off of different understandings of what the filter is supposed to be doing

 

[StageLeft]

Prolly not that large an effect but I always, always get OEM filters. Always.

 

[brblx]

/facepalm

go ahead and turn that palm into a fist so you can knock some sense into yourself...

OP, i highly doubt the air filter killed your gas mileage. it's probably coincidental.

 

[randomlinh]

at the end of the day, it doesn't really matter because he's done this before. an aftermarket air filter isn't going to give you a 5mpg average difference.

double check your tires, make sure no leaks have occurred. Not leaving an extra body or two in the trunk, right? any changes in driving locations/habits? reseat the filter just in case?

alternatively, you can pull the battery and reset the ECU and see what happens.

 

[kevman]

its probably cuz you reposted that thing about the HP cameras, the repost gods have cursed you with a 20% MPG penalty.

 

[punjabiplaya]

I noticed a loss in mpg too presumably after the gas stations have filled up with the winter blend. the trip computer usually showed ~500 when I filled up, now filling up the same amount from the same station shows ~420.

 

[JEDI]

I noticed a loss in mpg too presumably after the gas stations have filled up with the winter blend. the trip computer usually showed ~500 when I filled up, now filling up the same amount from the same station shows ~420.

i thought winter blends come into effect in Nov?

 

[bruceb]

My car went from an average of 19.5 mpg down to 18.5 mpg after the last fill up. It has been very cold here in NJ and I also attribute part of the loss to the winter fuel blends. And of course, it takes longer for the motor to reach operating temps in the colder months. I only have a 20 mile drive to work.