CEL + Code P0170 = Guzzling Gas?

Hey everyone! Thanks for checking out my question...

Ok - here's the info before the story:
1999 Mitsu Eclipse GST (Turbo)
Automatic transmission
136k miles
On the dash it says "Premium Fuel Only" but I mean, c'mon. I use 87.

You'll roll your eyes when you read this, but my CEL has been on for about a year now. I've been getting oil changes every 3k miles, as prescribed, and the check engine light has never been a problem, other than that it makes me a little uncomfortable. I had my beloved mechanic (honest! prompt!) run a check on the car and he gave me the code that is causing the light to come on = P0170 Fuel Trim bank 1. They said that they can't interpret the code there and should take it to a Mitsu dealership for service but I am not inclined to do that for a number of reasons - (far away, expensive, I love my local mechanic and would like to give them the business if they are able to do the repair).
I did some internet work on that code and what came up was information about the fuel system. Fine. Ok.
It wasn't really a problem at all for ages but now things are starting to act a little wonky when I drive...

The new problems:
So, before I took it for it's last oil change (1/5/09) at 136k, I started noticing this funny "feeling" when I would accelerate on the road. Though this is an automatic, I drove stick for years (loved my Galant!) and it feels like something isn't "sticking" between 1st and 2nd or 2nd and 3rd. You know when you shift and the car seems to slow for a millisecond and the tachometer drops until you stick it in the new gear? That's sort of what happens. I had my guy look at the transmission when I last took her in and they said everything looks ok...
So that's the first part. The other part, which I fear/hope is related...is the mileage I've been getting as of late. ABYSMAL.
Ordinarily, as you know, the mileage on this guy is not all that great. The turbo really takes its toll. However, recently this thing has been sucking down gas faster than a fat kid with a milkshake. I'm getting about 200+/tank city or highway. Yikes! I'm thinking it's got something to do with that P0170 code.

So, the question:
what do you suppose is wrong here and am I going to have to take it to a Mitsu retailer or can I have my local guy take care of the work? Am I making things worse by waiting?

THANKS so much and sorry for the long-windedness!

P0170 code means your car is running lean. at 136k miles, I wouldn't be surprised if it was a bad O2 sensor. Have you ever had to replaced the front or rear O2 sensors? It is more than likely the front one.

If it's not your O2 sensor then my next guess would be possibly a bad MAF.

Also, I would suggest running premium in any turbo vehicle. They recommend it for a reason. Running 87 once in a while won't kill it, but running 87 all the time just isn't healthy for the motor at all. The reason for this is because lower octane fuel is more like to to pre-detonate than higher octane. Pre-detonation is also known as knock. Ask any car guy and they'll tell you knock is not good for your motor at all. Turbo vehicles are more likely to suffer from knock due to the higher amount of air being forced into the motor. More air means it needs more fuel. The reason it needs more fuel is to keep it from running lean... Which brings up full circle back to your CEL. It could be running lean due to a bad O2 sensor or bad MAF. So the fact that it's running lean and your running 87 octane is just a bad combo for tons of knock and the overall lifespan of your motor... But that's just my :2cents: :wink:

yea especially if you are running lean the extra octane will go a long way for ya. Unfortunately these two problems sound to me like they are a bit independent of one another. If it is fixed with the o2 then great but my sneaking suspiscion is that you got two problems one tranny related and the other fuel related.

I failed to read the second part of the post... That just goes to show how lazy I really am. lol...

The tranny issue could go several directions with that kinda mileage. Being an automatic, first thing I would try is put a new tranny filter in with some fresh fluid. Drive it around a bit and see if it corrects the issue. If not, then it may be something more serious like the tranny wearing out or the TCU going bad. (If the TCU is going bad then it will stick in a gear for several seconds before shifting to the next gear)

That's about as far as my knowledge goes with the automatic trannys on these things. Hopefully its a start to finding a solution though.