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 Contamination Low fuel level |
Surges, chuggles, lack of power, sluggish, cuts out, and/or misses | Contamination |
Rough/unstable idle | Intake valve deposits (Note 1) Contamination |
Note 1: Intake valve deposits may result from unburned heavy-end hydrocarbons and lack of detergent additives. Another brand of gasoline with the proper detergent additives may correct the problem. | |
Diagnostic Hints
| • | Start 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 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 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 |
| • | Cuts 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 percent oxygen content |
Oxygenated Fuels
Beginning November 1, 1992, oxygenated fuels are required in 39 carbon monoxide non-attainment areas of the United States 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 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's 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, co-solvents and corrosion preventers must be added to the fuel. Five percent methanol boosts the gasoline's 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 additive.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-oxidant 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.
Excessive Exhaust Emissions Or Odors
Vehicle fails an emission test or vehicle has excessive "rotten egg" smell. Excessive odors do not necessarily indicate excessive emissions.
| • | Check the following: |
| - | . |
| - | Faulty cooling system components (fan, thermostat, PCM sensor) |
| • | If fuel system is running rich (long term fuel trim less than 118), refer to DTC P0172 . |
| • | If fuel system is running lean (long term fuel trim more than 138), refer to DTC P0171 . |
| • | If emissions test indicates excessive noxides of nitrogen (NOx), check for items which cause vehicle to run lean or too hot. |
| Important: Use of an emissions analyzer may help in locating points of fuel odor emissions. Even small amounts of fuel vapor will cause high levels of hydrocarbons to be indicated by the emissions analyzer. Slowly move the analyzer probe near the suspect area. The hydrocarbon level will increase as the point of emissions is approached. The probe must be moved slowly, as it takes time for the gases to travel through the tester's hose. |
| • | If emission test indicates excessive hydrocarbon (HC) and carbon monoxide (CO) or exhaust has excessive odors, check for items which cause vehicle to run rich. Refer to diagnostic chart and emission symptom chart. Check for the following: |
| - | Incorrect fuel pressure |
| - | Fuel loading of EVAP vapor canister |
| - | Positive crankcase ventilation (PCV) valve plugging, sticking, or blocked PCV hose |
| - | Check for fuel in crankcase. |
| - | Three-way catalytic converter lead contamination |
| - | Check exhaust back-pressure and removal of filler neck gas pump nozzle restrictor. |
| - | Improper fuel cap or damaged seal |
| - | EVAP vapor canister hose connections, condition and routings |
| - | Also inspect the vehicle's hood and front of dash seals for damage. |
| - | Faulty spark plugs, wires, coil/module, converter or ignition components |
