Driveability Symptoms Versus Fuel Quality (Engine Systems Functioning Properly)
DRIVEABILITY SYMPTOM | FUEL QUALITY - CAUSE |
|---|---|
Excessive spark knock | Octane number too low |
Hard cold starts | Volatility too low contamination |
Garage stalls and/or tip-in hesitation | Volatility too low contamination intake valve deposits (Note 1) Low fuel level |
Poor cold performance | Volatility too low |
Surges, chuggles, lack of power, sluggish, cuts out, and/or misses | Contamination |
Rough/unstable idle | Intake valve deposits (Note 1) Contamination |
NOTE: Intake valve deposits may result from unburned heavy-end hydrocarbons and lack of detergent additives. NOTE: Another brand of gasoline with the proper detergent additives may correct the problem. | |
Diagnostic Hints
| • | Start the diagnosis by verifying the complaint. Keep the vehicle overnight before verifying a cold driveability problem. |
| • | Remember that the symptoms may indicate a fuel delivery/pressure or fuel control problem. Use DTC codes and 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 a premium grade fuel. If so, suggest trying a different brand of fuel or regular unleaded fuel except when a premium-grade is recommended because of operating conditions. Some premium grade gasoline 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 gasoline is gasoline with a revised formula that:
| • | Lowers reid vapor pressure |
| • | Lowers benzene content |
| • | Contains a minimum 2 percent oxygen content |
Oxygenated Fuels
Beginning November 1, 1992, oxygenated fuels are required in 39 carbon monoxide non-attainment areas in the U.S. during the winter months. By slightly increasing the oxygen in the air/fuel mix, combustion can be improved and carbon monoxide emissions reduced.
Oxygenated fuels contain oxygen in their chemical makeup. Common oxygenated fuels are methyl tertiary-butyl ether (MTBE) and ethanol. When blended with gasoline, oxygenated fuels increase the octane rating and lower the carbon monoxide emissions.
Methyl tertiary-butyl ether (MTBE) -- MTBE is an ether manufactured by reacting methanol and isobutylene. Present laws permit fuel to contain a maximum of 15 percent MTBE. Fuel with 15 percent MTBE will raise the pump octane by about 2.5 octane numbers, will not increase RVP, is not sensitive to water, will reduce tailpipe CO emissions, and is not harmful to the engine.
Ethanol -- Ethanol (grain alcohol) is a renewable resource produced from corn or sugar cane. Present laws permit gasoline to contain a maximum of 10 percent ethanol. Ten percent ethanol boosts the gasoline octane by 2.5-3 octane numbers, raises RVP by 1 psi, and lowers tailpipe CO emission levels.
Ethanol has the ability to absorb water and thus prevent fuel line freeze-up. However, excessive water causes the ethanol to separate from the gasoline.
Methanol -- Methanol (wood alcohol) is produced from natural gas or coal. Methanol contains about 50 percent oxygen. Fuel that is more than 5 percent methanol is bad for the engine and should not be used. It can corrode metal parts in the fuel system and damage plastic and rubber. To avoid these problems, cosolvents and corrosion preventers must be added to the fuel. Five percent methanol boosts the gasoline octane by 2.5-3 octane numbers but raises RVP by 3 psi. This higher RVP results in more fuel evaporating into the atmosphere.
Additives
Additives are blended with gasoline for better performance. Gasoline typically contains several of the following:
| • | Antiknock agents -- MTBE and ethanol (oxygenated fuels) are the 2 most common antiknock agents. Toluene (aromatic hydrocarbon) is another common antiknock. |
| • | Detergent/deposit control agents -- Detergents and deposit control agents are found in most fuels to help prevent and, in some cases, remove deposits from the throttle body, intake manifold, ports, fuel injectors, and valves. |
| • | Anti-oxidants -- Anti-oxidants additives stabilize fuel during storage and shipment, which helps reduce gum formation. |
| • | Metal deactivators -- A metal deactivator inhibits the oxidation of the fuel which is catalyzed by trace metals such as copper. |
| • | Corrosion inhibitors -- Corrosion inhibitors prevent the corrosion of iron. |
| • | Dyes -- Dyes are used to identify different fuel grades and brands (by color). |
| • | Anti-icing agents -- Anti-icing agents help prevent ice formation that may cause restrictions and possible stalls. In cold weather, as humid air passes over a throttle plate, moisture can condense and freeze on the throttle plate. |
