Drop in MPG

Just got a complete 30,000 mile tune up a dealer for 2006 subaru forester. Immediately noticed a 3-4 drop in MPG averaged over 3 full tanks of gas. Dealer says car was reset to defaults and has to learn my driving habits. What kind of milarkey is this?

Three tanks is a long time to adjust. I suspect something else. What all was done during the “turn-up.” There is no standard turn up on modern cars.

Why did you have the dealer do this work. Dealers are no better or worse than independents, but almost always cost more.

thanks for the reply. Was a “standard” dealer recommendation. Included all fluid changes, tire rotation, check all belts, air and oil filter, plug check or change or whatever else they could think of to add to the cost. I kind of bit into it. I usually just change oil, filters, every spring and fall. I think something was readjusted wrong. They have 30,000, 60,000, 90,000 interval rec., etc. Seems like I should have gotten a mileage per gallon increase, instead of a decrease.

The maintenance services are not dealer recommedations; they’re factory recommendations and the owners manual specifies what should or should not be done. However, I do disagree with some factory recommendations since there is a tendency to put things off in the interest of making their cars appear to be more maintenance free.

Spark plugs should not be left in for 100k miles, fuel filters should be changed every 25-30k miles, auto trans fluid should be changed every 30k miles, etc.

Since 3-4 MPG is a huge drop I would suggest dropping by a local AutoZone, Advance Auto, Checkers, etc and have them pull the codes. If the car is really suffering this badly a code or two should be set. They will do this free.
The only thing I can think of off the top of my head would be if a vacuum line was inadvertently knocked loose(which the ECM will compensate for by going richer), a MAF sensor connector is loose, or something like that.

Thank you very much! You are correct. This was a factory rec maintenance. I said dealer rec. perhaps wrong choice of wording. They did all the little things on the maintenance rec. I rememember change in fuel filter, spark plugs, etc.

At least you gave me a few points to think about. When I called the service department, all I got was kind of a duh, I dunno. It’s winter, etc. I informed him that I’ve driven it through two full winters and (summer with AC on all the time) and the MPG never changed until he did the 30,000 mile maintenance service. If after the next tank, it still sucks gas, I’ll take it back with your suggestions.

Please explain to me about codes. I’m not a mechanic except as a kid when auto’s did not have computers and you could repair the things in your backyard. My life went elsewhere and not into car mechanics.

Unless the dealer had some reason to scan the computer and reset it, the computer should not have been reset to default. And even if it was, it learns quickly.

You haven’t started using ethanol-blended gasoline, have you? Every time I get a tank of E-10 (which I avoid if possible) the mileage on my car drops 3-4 mpg. EVERY TIME. The mileage returns to normal as soon as the E-10 is gone.

No E-10. Thanks

When talking with a dealer service dept. never put much faith into anything a service writer says. The vast majority are mechanically illiterate and so they have a very bad tendency to babble nonsense rather than just say “I don’t know”, followed by actually finding out the answer to what was asked.

The engine controls pretty much take care of themselves and there are no adjustments to speak of.

To simplify the computer code thing it basically goes like this. The air/fuel mix is controlled within certain parameters by the ECM, or engine computer. If your engine starts getting outside of those parameters the ECM cannot control it and the Check Engine Light will come on and/or codes relating to running rich (too much fuel for the amount of air being consumed) will be set in the computer.

So if your car’s mileage has really dropped that much and there is a legitimate problem there should be a code stored in the ECM memory.
The reason I mentioned a possible vacuum line being knocked loose, etc. is that this means additional air will be sucked into the engine through that disconnected hose. The ECM will see this and try to compensate for it by richening the air/fuel mixture, which means lowered fuel economy.
However, an engine often has a rough idle or slightly rough idle if the vacuum leak is tied into the intake manifold. A leak in the air induction system between the throttle plates and air filter may not cause a rough idle but could cause problems that you may not physically notice.

Hope that helps to explain it anyway. As I mentioned, I would get AutoZone or one of the others in your area to pull the codes. It’s free and only takes a couple of minutes. If you get any codes post the results back here for further discussion.

Are you sure about that? A lot of states and even some local governments these days are requiring ethanol blends during the winter, and it could be possible that your locality didn’t require it the last two winters, but does now.

I saw no notice, but will check it out. thanks

it funny I read another person had cel on and dealer said they did nothing to engine but this person said their mpg dropped also. you can find it previous pages from here. I wonder if there is tsb from factory and when they get their hands on a car they change something to prevent a major problem the factory might have to repair free. hummmmmmm

another person a few pages before this one some one had the same problem. I wonder if the dealer has a TSB and does something to prevent a major problem they might have fix free.