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Mitsubishi CEL: P0170 & Bad Mileage?

Hey there! I’ve been trying to call into your lousy show for weeks now with no luck!

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!

First, your using the wrong gas, and it is hurting the engine. This is a turbo car, and the cylinder pressures will be higher than the econo boxes 87 octane is good for. The engine is doing it’s best to compensate for your cheapness by retarding the ignition timing to keep from blowing the cylinders apart. This is leading to long-term damage. The engine is probably so carbon-ed up from incomplete burn that it is choking on the inside.

Second, the code means that the fuel trim is malfunctioning, meaning it cannot correctly adjust the proper metering of fuel. The code is most likely set because the computer is wildly trying to trim the fuel properly, but cannot find the right number. It sets it low, and the sensors tell it to go higher. It adjusts it up, then the sensors tell it to go low. This is all because your using the wrong fuel for this car, and the computer cannot fix it for you.

If you want to salvage this car, get good gas mileage again, and get the good performance back, START USING THE CORRECT FUEL, PREMIUM. Ask your mechanic, and explain the same thing you told us about using 87 octane in your turbo car.

The DTC (Diagnostic Trouble Codes) aren’t there to inform you of the engine performance, directly; but, indirectly they can indicate some of the things which are happening which can affect performance. The sensors, and other things, are there to monitor and control EMISSIONS. It’s coincidence that that information can be used to gauge engine performance factors.
The use of low octane fuel could be most, or all, of the problems with your car.
Would you use kerosene in it if it would run on it (by reducing the ignition timing to 0 degrees, or lower)?

Good point hello, How can we convince the public that when your car is working as designed in regards to emissions its also working as designed as far as performance.
Unless you don’t agree.
The public does not believe that a car with low emissions is a healthy car,or that a emissions check is a look into the health of your car (now don’t conclude that I am a fan of mandated emission checks).

For the OP it is best to post info regarding mpg in MPG format not miles per tankful.As a mechanic I would be ashamed to turn a customer to the Dealer over a fuel trim code,what about his reputation?

check the upstream O2 sensor for slow response (with a graphing multimeter) REAL time. #2 check the turbo enrichment solenoid.then run the proper fuel.

#3 the trans is on its way out,normal at this milage,actually way high on milage for trans issues.they usually go at 60k due to the turbo stress.


and people wonder why I’d never buy a used turbo/super charged vehicle