Plummeting Fuel Economy

I have an '06 Subaru Forester (4 cyl, automatic) with about 97,000 miles on it. For the past 5 years I’ve had the car, it’s been getting roughly 26-28 mpg on average. Within the past 3-4 months, though, the fuel economy has dramatically decreased. Now, I’m lucky if I get 21 mpg. (I used to be able to get almost 350 miles per tank, and now if I make it to 275 I’m jumping for joy!)

I took my car to the shop for the 90K service, and they didn’t change the fuel filter as a part of that service. (They did replace the coolant, replace oil & filter, replace spark plugs, replace differential fluid front & rear, replace brake fluid, replace auto transaxle fluid, and then they did a bunch of inspections on belts, hoses, brakes, etc.)

I took the car back to the shop after this service so they could change the fuel filter, and they told me that because of the type of fuel filter I have, it should last the lifetime of the car. This sounded a little fishy to me, so is this true?

If the fuel filter isn’t the issue, what could be causing this dramatic drop in fuel economy, and what can I do to try to remedy this problem?

Many cars now have a “lifetime” fuel filter. It’s basically a big sock that surrounds the fuel pump in the tank.

Check tire pressure and wheel alignment.
Try cruising a few miles with minimal brake use, then coast down and stop, minimal brake use.
Get out and feel the wheels. If one wheel is hotter than the others you have a dragging brake.
Change the thermostat. Replace with an OEM (from Subaru). Good Idea to do this every 5 years anyway.
Clean the MAF (mass air flow) sensor.

The first thing to check for is a partially stuck open thermostat. The thermostat can stick where the coolant temperature may only get up to 130 degrees, and that feels pretty good coming out of the heater. But that’s not hot enough for the computer to go into the closed loop mode. So the computer thinks the engine is still cold and uses more fuel.

The next thing to check is coolant temperature sensor for the computer. Again, if this sensor is defective and is telling the computer that coolant temperature never gets above what’s required for it to go into the closed loop mode, the computer thinks the engine is cold and uses more fuel.


So-called lifetime filters are becoming common (garbage in my humble opinion) but a filter will not cause a loss of fuel mileage.

No Check Engine Light or codes present?

Another possible cause of poor fuel mileage is low tire pressure, even on one wheel only. Any service, minor or major, should include a tire pressure check but it’s also a minor thing that may often be brushed aside.
Just pointing this out in case it’s been overlooked.

A poorly functioning O2 scanner can cause low fuel mileage without setting a DTC. A good scan tool will show the voltage curve, but you must be able to interpret it. Check with circuitsmith; he’s an ace on O2 sensors.

Your car is rated for 20 MPG city and 25 MPG highway and 22 MPG overall. 21 MPG is about what it should be getting…

02 sensor would be my first choice of possible problems. Nearly every time I’ve had one go bad the fuel efficiency on my car will drop by 25-35%. If you do indeed need a new 02 sensor there are sellers on ebay who sell them much cheaper than most local auto parts stores.

Thanks for all the tips folks!

ok4450 - No idiot lights are flashing at me.

I have noticed that when I get into the mountains and am driving uphill for a long period of time (highway driving up a 7%-10% grade for about 15 miles) that the engine temp needle on my dash gets a higher than what it normally sits at. Usually the needle hovers a little above the first red line on the gauge, but while doing this mountain driving it tends to get up to the 2nd red line on the gauge. (There are only 3 red lines on the gauge, the 3rd being the highest temp the needle can indicate on the gauge.) Does this sound like it might be related to any of the things mentioned in the previous posts?

It certainly sounds like you have a cooling system problem, a stuck T-stat being a good possibility. If that’s true, the engine may be operating cooler (richer) than normal and yes, that could affect your mileage. And a stuck T-stat won’t trip a CEL.

Whether it turns out to be the T-stat or not, you need to get this cooling system problem diagnosed and corrected.

I made an appointment with my mechanic for early next week. They’re going to run some tests and try to get to the bottom of it. Thanks for the input everyone!

I will have to agree with Mechaniker…I didn’t see where you mentioned that your temp sensor is not reading where it normally does…so I’m not sure how or why we are talking about your cooling system.

THE and i mean THE most common ENGINE item that affects fuel economy is the O2 sensors…And this deterioration of fuel economy very often happens all without throwing an CEL…(check engine light)…

OTHER items would be tire pressure…a clogged AIR filter…etc…and YES and engine running too cool…but does your dash temp gauge not look like its in the normal position? No mention of that.

If coolant was changed I would make SURE the system is burped properly…and it probably IS…otherwise you would have other running issues at this time…but if it were me…I’d open my bleed nipple on the system to make sure.

If you want to determine how hot/cool your engine was running, just purchase an Infra Red thermometer…Bout $20 bucks at Harbor Freight…similar prices at Walmart etc… You can aim it at the cylinder heads, block, radiator etc…and see what temp your engine is ACTUALLY running at…then if those numbers are off (below 170 or so)…I would go to your engines thermostat and change it…Otherwise I wouldn’t suspect that your vehicle is running too cool AT ALL unless your dash gauge tells you otherwise.

IF it were running too cool…you would see a drop in mileage…but I don’t see any clues to this in your post so…I still don’t get it…