Diagnosing Fuel Quality Problems

Symptoms
A fuel-quality problem may be indicated by the following driveability symptoms:
^ Spark knock/detonation
^ Hard cold start and garage stalls
^ Poor cold performance
^ Tip-in hesitation, sag, or stumble (momentary lack of response during initial acceleration)
^ Surges and/or chuggles
^ Lack of power, sluggish, or spongy
^ Cut out or misses
^ Rough/unstable idle (intake or exhaust valve deposits)