Insight Engine Problems

I have a 2001 Insight (5 speed manual) that went in for service on Oct of 05. The main battery pack went out and it took 3 weeks or so to replace. After getting the car back it sputters when driving at a
constant speed. It feels like it is misfiring, even though that is not the case. After taking the head mechanic on a test drive, they hooked it up and found nothing wrong. With no lights and no problem codes, as far as they are concerned, there is nothing wrong. One possible problem they suggested is that I don't drive my car 'hard enough' and there is a carbon build-up, which is my fault. Ok... so the manual and all the fancy computer components and lights that tell you how to best drive your vehicle are wrong??? Anyway... I digress. Has anyone had a similar problem and, if so, what was the issue? Possible suggestions (oxygenated gas, wrong oil, etc) would also be very helpful.
Thanks all,
Michael
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I realize the Insight is a different system than I am accustomed to, but there are a couple possibilities...
One is that the problem is not with the engine, but it is the IMA being weird. I especially wonder about that since the battery pack was just replaced. Add to that that the "check engine" light isn't telling you something is wrong with the combustion and it makes me say, "hmm...."
Otherwise, I would suspect the usual sorts of things - any tune-up parts that are out of date, cleanings that the owner's manual never mentions like throttle body, PVC, injectors, etc.
But first it would be good to determine that the IMA is not leading you down the wrong path.
Mike
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Those battery packs have an 8 year warranty so since it was a warranty repair tell them to call the district rep if they can't figure out what they did wrong when fixing your car.

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