Fuel and Its Effects on Driveability

Diagnostic Chart:

DIAGNOSTIC HINTS
^ Start diagnosis by verifying the complaint. Keep the vehicle overnight before verifying a cold driveability problem.
^ Remember that the symptom(s) may indicate a fuel delivery/pressure or fuel control problem. Use the Diagnostic Circuit Check and the fuel system diagnostic procedures in the Service Manual for diagnosis.
^ If the engine is functioning properly, suspect fuel volatility/fuel quality.
- Determine if the customer is using premium grade fuel. If so, suggest trying a different brand of fuel or regular fuel except when a premium-grade is recommended because of operating conditions. Some premium grade gasolines may actually cause cold engine driveability problems because of their lower volatility in the warm-up range. Inform the customers that it may require more than one tankful of a different brand of fuel to correct a fuel quality problem.
- Determine if the customer has old fuel in the tank. If so, the volatility may be too low because the "light-ends" have evaporated.
- Consider the possibility that an unseasonable cold snap has made the fuel volatility wrong for the weather.

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)

REFORMULATED GASOLINE
Reformulated gasolines (1995-1996) are gasolines with a revised formula that:
^ lowers Reid Vapor Pressure
^ lowers benzene content
^ contains a minimum 2% oxygen content