Fuel System - Fuel Effects on Driveability
BULLETIN NO.: 93-T-07AISSUE DATE: November, 1993
GROUP/SEQ. NO.: Engine - 02
CORPORATION NO.: 286304R
SUBJECT:
Fuel and How it Affects Driveability
Due to revisions in the affected model years, this bulletin supersedes bulletin 93-T-07, which should be discarded.
MODELS AFFECTED:
1991 - 1994 Saturn's
PURPOSE:
A number of inquires have been received from Retailers concerning fuel and how it affects driveability. This bulletin describes how gasoline and fuel additives are constantly changing and gives details about diagnosing fuel-related driveability concerns.
DRIVEABILITY SYMPTOMS VERSUS FUEL QUALITY (ENGINE SYSTEMS FUNCTIONING PROPERLY)
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 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.
Determine if the customer is using premium grade fuel. If so, suggest trying a different brand of fuel or regular fuel except when a higher grade is recommended. 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 fuel volatility is incorrect for an UN-seasonable weather change.
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
VOLATILITY
Volatility is a gasoline's tendency to change from a liquid to a vapor. Volatility is crucial to engine performance. Liquid gasoline will not burn; only vaporized gasoline burns. Once liquid fuel is injected into an intake port and mixed with air, some of it should start evaporating immediately.
Any liquid's rate of evaporation changes with temperature. Temperatures in an engine vary widely. During operation, the combustion chamber temperatures may rise to more than 822°C (1500°F). Before start-up, on a very cold morning, the temperature may be zero. To burn well across the broad range, a fuel must consist of a mixture of hydrocarbons that vaporize at different temperatures.
The hydrocarbons consisting of smaller molecules vaporize at lower temperatures and those consisting of larger molecules vaporize at higher temperatures. The easily vaporized hydrocarbons, which are easy to burn, are called the "light-ends." They help with quick starts when the engine is cold. The less volatile "heavy-ends" contribute more to engine power, but because they are harder to burn, they can coat the cylinder walls without burning and in extreme cases may dilute the oil in the crankcase.
Lower volatility is desirable in the summer to prevent excessive premature evaporation of the fuel. In the winter, high volatility is desirable because the fuel must vaporize quickly for cold starts and good cold drive ability.
Volatility Affects on Driveablity and the Environment
OCTANE
A gasoline's octane number is a measurement of its anti-knock capability. Increasing the octane from 87 (regular) to 89 (mid-grade) or 91 (premium) does not increase power or miles per gallon, nor does it keep the engine's internal parts cleaner. Increasing the octane only increases a fuel's resistance to spark knock.
Back in 1985, when port fuel injection was new, the plugging of tiny passages in the injectors with fuel deposits became a serious concern. Because deposit control additives were not available in many regular-grade fuels, higher grades were sometimes recommended to keep injectors clean. Today, redesigned injectors are more resistant to plugging, and deposit control additives are generally used in all grades of fuels.
With today's fuels, there is absolutely no reason to use high-octane fuel except to reduce spark knock or pinging.
SPARK KNOCK
Knock occurs after the spark and before all of the air-fuel mixture has burned smoothly across the combustion chamber. The increasing heat and pressure may cause spontaneous ignition of the last 3 to 5% of the air-fuel mixture before normal combustion consumes it. This mini-explosion causes a vibration that is heard as a ping or knock. Excessive spark knock over a prolonged period of time can damage the engine.
One reason most of today's engines do not need premium gasoline is the Electronic Ignition Control system. The ignition control system's knock sensor (detonation sensor) reports any pinging to the Engine Control Module (ECM), which retards ignition timing. Delaying the spark produces less build-up of heat and pressure - so the end gas does not explode, but burns properly. However, retarding the timing reduces engine efficiency; so the system operates only when necessary.
Another way to reduce knock is to use combinations of the basic hydrocarbons in fuel that offer more resistance to knock. Adding lead to the gasoline will also increase the octane but fouls catalytic converters. Another way to increase octane is to add "oxygenates," that is, chemicals composed of hydrogen, carbon. and at least one oxygen atom. Increasing the octane enables the fuel to withstand greater pressure and heat. Under normal conditions, regular unleaded 87 octane fuel is sufficient for today's vehicles.
REID VAPOR PRESSURE (RVP) TEST
The Reid Vapor Pressure (RVP) test indicates the fuel's tendency to evaporate early. The test also indicates how the engine may perform during a cold start and warm-up. The higher the RVP the higher the volatility.
RVP is measured using a Reid Vapor Pressure Tester. A gasoline sample at 32°F is placed in a sealed container that is attached to a pressure gage. The container is then placed in a 100°F water bath for two to five minutes. The pressure gage reads the pressure in pounds per square inch (psi) exerted by the vaporized fuel within the sealed container. This test can be performed using the Saturn Service Fuel Test Kit SA9213NE (or equivalent).
According to more stringent standards effective on June 1, 1992, except in Alaska and Hawaii, the EPA limits RVP to 9.0 psi maximum between May and September 15. In "ozone non-attainment" in the areas in the southern and western U.S., the EPA limits RVP to 7.8 psi maximum between June 1 and September 15. The EPA permits these maximum vapor pressures to be 1.0 psi higher for gasoline-ethanol blends that contain (by volume) 9 to 10% ethanol.
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