93 CHEVY BLAZER S-10 4X4 I GET POOR MILEAGE ABOUT 9 MILES TO THE GALLON SMOKES REALLY BAD MAKES THIS FUNNY SQUEAKING NOISE WHEN IDLEING and I got the hard shift from 1st to 2nd gear Is there anything that I can do to improve performance? P.S. I change the oil , do an engine flush, change oil filter, clean and/or replace the spark plugs all on a regular basis and for this truck thats about every 2 to 3 months (The Spark plugs becomes fouled out often and quickly)

 

Sounds like overhaul time.

 

Check your fuel pressure.

Also, when was the last time that the O2 sensor was bad. Bad plugs can foul an O2 sensor just as quickly as a bad O2 sensor can foul good plugs.