If I were you I would run by my local GM dealer and pick up a can of "top end cleaner". Follow the directions on the can exactly as described and clean out the carbon deposits that are on your pistons.

As Steve B. pointed out in a previous post, find a secluded area to run the car after the soak period, or else your neighbors may phone the EPA

After that, give it an El Wapo (Italian) tuneup by going out on an open freeway and open the engine up a bit.

Besides that, you may want to diagnostically check your O2 sensor and lambda, to make sure your fuel/air mixture is optimized.

Good luck!

~Paul

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'91 300E, 214K miles