Fuels - Effect of Volatility On Driveability Conditions
NO.:93-1-9
DATE:
Nov. 1992
GROUP:
6
CORP. NO.:
249128R
SUBJECT:
INFORMATION ON THE EFFECT OF FUEL VOLATILITY ON DRIVEABILITY CONDITIONS ON ALL YEARS, ALL MODELS
Background Information
Recent changes in EPA regulations have effectively lowered the maximum allowable fuel volatility. Volatility is the ability of gasoline to change from a liquid to a vapor. It directly affects the amount of evaporative emissions released from the fuel. Lower volatility results in fewer unburned hydrocarbons being released into the atmosphere during fuel storage and transfer. Unfortunately, reducing fuel volatility can pause problems during cold engine operation since low temperatures limit the ability of fuel to vaporize and burn.
The two terms used to describe volatility characteristics are:
Distillation Curve:
^ The distillation curve is a graph showing the relationship between temperature and the percentage of fuel evaporated. The fuel components that boil at relatively low temperatures (below about 90~F) are known as "light ends" and are essential for good cold engine performance. "Heavy ends" boil at about 300~F and contain the most energy but are more difficult to burn. Laboratory analysis is usually required to determine the distillation curve of a gasoline sample.
Reid Vapor Pressure (Rvp):
^ RVP is the pressure (psi) vaporized fuel exerts within a sealed container when heated to 100~F The higher the RVP the higher the fuel volatility. While RVP may be easily tested in the field, fuels containing the same RVP can have different distillation curves and cold driveability characteristics.
Fuel volatility will vary depending on geographic location and time of year (fuel intended to be used in higher ambient conditions is formulated with lower volatility). This can make cold driveability as major a problem during summer months as during winter. There may be additional variation in the volatility of pump gasoline caused by the differences in fuel manufacturers, blends, and storage times. As fuel volatility is lowered as required by EPA regulations, variations among further reduce volatility and become a critical factor which may severely affect cold engine performance. No matter how thoroughly the relationship between fuel quality and cold driveability is understood, eliminating fuel quality as a cause can be difficult due to the following:
TOOLS WHICH TEST FUEL VOLATILITY MEASURE ONLY RVP NOT THE DISTILLATION CURVE.
^ The distillation curve has a greater effect on cold driveability than RVT.
THE CUSTOMER MAY PERCEIVE DISCUSSING FUEL QUALITY AS AN ATTEMPT TO EVADE HIS PROBLEM.
^ This may cause difficulty in getting accurate fuel usage information.
WATER AND OTHER TYPES OF FUEL CONTAMINATION MAY CAUSE CONTINUED DRIVEABILITY COMPLAINTS.
^ The problem symptoms may remain even after the vehicle has been refueled several times with quality gasoline.
Customer Concerns
CONDITION:
Poor Cold Engine Operation symptoms may include - hard start/extended crank, stalling, backfiring, hesitation, rough idle, detonation, and/or lack of power.
POSSIBLE CAUSE:
Fuel of low volatility will not vaporize sufficiently to allow normal combustion.
CORRECTION:
Replace fuel.
Diagnostic Procedure:
1. Perform basic system checks in Section 6E of the Service Information Manual.
2. Check for Service bulletins which relate to cold driveability conditions specific to the problem vehicle.
^ An updated engine control calibration or service procedure may be available to make the engine less susceptible to low volatility fuels.
3. If basic checks do not reveal a vehicle fault, advise the customer that fuel quality may be the cause. Recommend the following actions:
^ Change brands of fuel
^ Use only 87 octane gasoline unless the vehicle is designed to require the use of premium gasoline.
^ Empty the fuel tank as much as practical before refilling.
^ Use a minimum of three tanks of new fuel before returning for service.
4. If above steps are ineffective, do not proceed with additional vehicle diagnosis and/or parts replacement until the fuel tank has been drained and refilled with a known good quality gasoline AT THE DEALERSHIP
5. If the problem remains, refer to Service Information Manual, Service bulletins, and/or Technical Assistance System.
The owner letter explains, in basic terms, the effects of different fuels. This page may be copied and distributed to owners you feel may have a fuel problem.
Dear Cadillac Owner:
As with many other things you put in your vehicle, the fuel you use affects the way your vehicle operates. You may have noticed that your engine runs differently for a while after filling up at a gas station you don't normally use. Unfortunately, the fuel you use can even cause problems such as hard starting, stalling, backfiring, hesitation, rough idle, lack of power, or spark knock.
Although fuel may look the same everywhere, all fuels are not the same. Fuels are blended differently based on the time of year the fuel is blended, by whom, and in what part of the country. Even after the fuel is blended, it can change based on where and how long it is stored. Also, the Environmental Protection Agency recently changed the guidelines used to blend fuels. All of these factors affect fuel properties which in turn affect the way your vehicle performs.
Consequently, if you bring your vehicle in with a driveability concern, like one of those listed above, your dealer may replace the fuel in your tank or ask you to change the gas station you use for a period of time to determine if the fuel you are getting is the cause of the condition. If your dealer mentions fuel is the cause of a concern or suggests switching gas stations to see if fuel is the cause, please follow their instructions. Your dealer is best equipped to determine whether or not fuel is the cause of the condition.
As always, the aim of Cadillac and your Cadillac dealer is to satisfy you, the customer, with our vehicles. If you have any questions or concerns about your Cadillac, we stand ready to serve you.