Sudden & Consistent Drop in MPG

You sound pretty careful and observant, so forgive me for this line of questioning. Users sometimes pose a real stumper, and after a week it turns out they omitted some grossly significant fact. Are you or the car's other users doing something different? For example:
Did you start driving a different route?
Is there a new stop light on the old route?
Did you install a roof rack for your new surf boards?
Did someone buy four 50lb bags of cat food and forget to take them out of the trunk?
Did you start a carpool or a cinderblock delivery service, and now carry more weight than before?

Did "10% Ethanol" stickers appear on your gas pumps around that time? I would have thought that O.C. was using E10 for several years now, but maybe not...

 

Just a thought.
Vacuum fitting on the manifold.
Oxygen sensor.
Coolant temp sensor.

 

Any of those things should have set a trouble code, no?

 

it wouldn't be the oil level right?

 

One more thought -- the 12V battery might not be holding charge, so the system tries to constantly charge it...

 

If the dealer said they did this, I would run to a new service provider ASAP. To the best of my knowledge, the last car to be sold with a carburetor in the U.S. was the Yugo. There is certainly NOT a carburetor in your Prius.

The spark plug was someone's long shot but bizarre things do happen.
The people I know that had the 2nd 2004 Prius in the state had zero problems until 1 day the engine started running really rough. Turned out that (I'm not sure exactly) one of the plugs had backed out, the wire came off or something. In any case it was related to a spark plug.

Even if you did unknowing start using E10 (we are stuck with it here now ), that would only account for about 1 MPG from your usual 44.

BTW, you can easily blow your MPG simply by adding a 1-2 mile daily trip. Did you happen to start taking a kid somewhere for an activity that you didn't do when you were getting 44 MPG??

 

Well, there *is* a throttle plate, and it *can* get dirty, so modulo some imprecision in terminology the dealer may have been perfectly honest.

The question about E10 was motivated because when retailers first start selling E10 it can liberate water which had collected in the bottom of the storage tanks. The initial sales of E10 can have significant water in them which can more significantly affect MPGs. That quickly gets cleared out and thereafter the MPG hit versus non-ethanolated fuel is only about 3%, as you wrote.

 

Check for loose moldings or body panels (especially on the skirt under the engine) Drag from this can drop mileage.

 

My friends and I suffered a sudden, persistent MPG drop simultaneously - and coincident with our state's impementation of an E10 (ethanol) mandate.

We are trying to get our government, our newspapers, consumer reports, and anyone else to do a scientific study of ethanol mileage, with no luck so far. That in itself is amazing. We've bet our country's future, and our sacrificing people to worldwide hunger, on ethanol without actually testing it???

You can ask about fuel changes in your area, but be forewarned. Oil comanies have been putting Ethanol into fuel, in small amounts, for years. It's almost as though if they snuck it in little by little no one would notice that it hurts mileage. But as it stands, your gas station attendant may not even know what's in the gas.

 

This is a well known problem with ethanol. What is less well known is how to test the gas yourself to measure the actual ethanol level as well as the optimum mix, the one that gives the most miles per dollar spent (appears to be E30.)

Bob Wilson

 

I filled up 6 days ago with E10. My mpgs are going south, my car idles rough for the first time in 3 years and the engine revs higher. I will never use E10 again if I can get anything else - even premium.

 

If it was the first time that gas station had E10 there may have been some old water in it from their tank. Just wait a little. I've been running mine on E10 full time for two years and part-time for two years before that, and it works fine.

It's physics and chemistry: ethanol has 70% of the energy of the same volume of gasoline, so one gallon of E10 has 97% of the energy of a gallon of gasoline. In other words you will get 3% less MPGs using E10 versus the same octane gasoline. No more, no less. Any other differences you see are coincidence, caused by different outside temps or different routes used on the different fillups, or unconscious changes in driving technique in support of the expected difference.

Ethanol is inherently high octane, and because it includes an oxygen atom it enhances combustion, which keeps both our engines and our air cleaner. Finally, E10 is reality now in all large US cities. Trying to avoid it by driving miles out into the boondocks to buy gas would be a foolish waste of time and fuel.