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Subaru Impreza 2009 new air filter caused drastic mileage drop

Howdy everyone. I’ve got a 2009 Subaru Impreza Outback Sport. I went in to a Jiffy-Lube type of place for an oil change, they showed me I had a dirty air filter and I had them change it. I went in averaging around 25 mpg (between high 24’s and mid 25’s), and after I left, I began averaging around 20 mpg, it currently reads 19.5. I went back to see if they had installed it incorrectly or left plastic packaging on it or something, but all seemed okay. Any thoughts on this?

My thoughts are as follows:

Have a competent mechanic check the installation of the air filter, in order to determine if the kids at Jiffy Lube managed to disconnect an electrical connector or if they did some other kind of damage to your car.
(Note: Nobody at Jiffy Lube is a mechanic, and as a result, few of them have any clue about what they are doing under the hood of your car.)

Resolve to never return to quick lube places for your maintenance.
In addition to pushing services that are frequently not necessary, they lack the expertise to do anything correctly–other than vacuuming your carpets & washing your windows.

An independent mechanic (and, in many case, the dealership) will charge the same amount of money that you are paying J-Lube to have untrained kids abuse your vehicle. As one of the veterans of this forum likes to say, Don’t go to a quick lube place for anything–not even for directions!.

We have had an unusually cold winter and a few reports of mileage drops, if there are no check engine lights, well go to FL and see if the gas mileage improves?

Barkydog, do you think the cold would account for a 5-6 mpg drop?

VDC, you’re right, I’m going to the dealer from now on. I usually do, but I was overdo, knew my schedule was tight (and the dealer by me requires a drop off) and I got a coupon in the mail. Dangitall…

Years ago, I worked on a car which had an aftermarket air filter installed

That filter was put together so shoddily, that the glue was actually causing the air filter to be restrictive. It was so bad, that the vacuum brake booster wasn’t getting sufficient vacuum

The fuel trims were out of whack, also

Putting in a factory air filter took care of all the problems

My point is that, every so often, a cheap aftermarket part actually creates a problem which didn’t exist before

Some of those filters (air and fuel) used by some fast lube facilities are not very good products. I call them “white box” filters although some have been provided in yellow boxes. These are usually sold dirt cheap to vendors and often at a price that is about 20% of what a brand name filter would cost. These are generally marked with an ID number and no brand names inked on the box.

A few dealers I worked for used to get flyers in the mail from those Chinese parts vendors advertising cheaper than dirt filters, plug wires, coils, belts, and even brake parts.

Got to feel confident having your life in the hands of a 4 dollar set of brake pads or shoes; cheaper still if buying in bulk… :slight_smile:


Also, check your tire pressures. They may be low. Cold air makes tire pressures drop.

It could also be a different problem that coincidentally happened around the same time. For example, a bad coolant temp sensor will cause the engine to run rich and reduce gas mileage.

If a stock air filter doesn’t do the trick, you can have a mechanic hook up an OBD scanner and check the computer’s fuel management status. If the computer is running “open loop” instead of “closed loop” when the engine is hot, that’s an indication that the coolant temp sensor is bad. It should be running in “closed loop” mode when hot. Open loop will result in excessive fuel use.

How dirty was the old air cleaner? Was it all caked up with gunk or just kind of dingy looking?

One thing that is important to fuel metering and gets exposed when changing the air filter - or can get contaminated by having a dirty one - is the Mass Airflow (MAF) Sensor. These are cheap and easy to clean and cleaning it is never a bad idea even if it doesn’t help anything with regards to the current question.

As for those cheapo parts . . . I hear that, in some cases, the box cost more to make than the part itself!

Of course, this wouldn’t be true with electronic items

“Don’t go to a quick lube place for anything–not even for directions!”

Hear…hear. Great advice @VDCdriver. For the life of me I just don’t understand how these places stay in business. We had a local quik lube place here that advertised (large sign in front of business) an oil change with filter for $12.95. I shuddered to think what kind of “oil and filter” was going in those vehicles.

Actually, I can’t take credit for that line, @missileman.
Joseph Meehan (who doesn’t do very much posting in this forum anymore) was the one who coined that saying, and I have always liked it.

@JosephMeehan–If you are still monitoring this thread, why do we hear from you so rarely nowadays?

Thanks @VDCdriver for the clarification. I knew I’d heard that phrase before and I liked it as well. I hope @JosephMeehan checks in real soon.

My car gets 26mpg hwy. drove 30miles yesterday at -6f and I got 23mpg on trip. I always fiddle with my trip computer and have seen 30mpg during mild summer days on same road. I drive 2-300 miles and reset my computer 2-3 times cuz I’m bored. I always seem to hit 26mpg. If I do 26.5 for 40 mile stretches I m happy. Just a game to play