Fuel - Poor Driveability Using Premium
No: TSB-98-13-005DATE: June, 1998
MODEL: See below
SUBJECT:
POOR DRIVEABILITY USING PREMIUM FUEL
PURPOSE
This bulletin explains poor driveability Conditions that may be caused by using fuel with a high driveability index (DI). Premium fuel has a high DI.
AFFECTED VEHICLES
1997-98 Mirage models
1995-98 Eclipse models equipped with 2.0L DOHC 420A engine
1992-98 3000GT models equipped with SOHC engine
1992-98 Montero models equipped with SOHC engine
1997-98 Montero Sport models
1994-98 Galant
1996-98 Eclipse Spyder models equipped with SOHC engine
SYMPTOMS
Vehicles may experience long cold start times, sag during warm-up, and stalling. These symptoms are most noticeable and more prevalent at ambient temperatures between 40° F and 80° F.
BACKGROUND INFORMATION
Fuel with high driveability index (DI) can cause the above described driveability symptoms. DI is a measure of the gasoline's total volatility, or tendency to vaporize completely. A high DI number is less volatile than a low DI number. Most premium gasoline sold in the U.S. has a higher DI than regular or mid-grade gasoline. Use of premium gasoline is NOT recommended for vehicles designed to run on 87(R+M)/2 regular or 89(R+M)/2 mid-grade gasoline. High DI gasolines also cause higher emissions for the same reasons they cause driveability problems.
Octane Ratings
For vehicles that require an octane rating of 91 (R+M)12, premium gasoline is recommended, even possibly required. Using premium fuel with a higher than recommended octane rating is not recommended. Customers who experience fuel-related cold start and warm-up driveability problems should try a gasoline with the recommended octane rating or different brands of gasoline until they find one that provides good performance.
A gasoline's octane rating is only a measure of its resistance to spark knock. Using a higher than recommended octane gasoline under normal operating conditions does not improve startability, idle quality, fuel economy, driveability, acceleration, engine durability, or emissions. In fact, most higher octane gasolines available in the U.S. have higher DI values than regular gasoline.
Customers are more likely to experience poor driveability with premium gasoline than with regular.
Some vehicles, such as the 3000GT with DOHC or turbocharged engine and Montero with DOHC engine, have been specifically designed to take advantage of higher octane fuel. These vehicles may have higher compression ratios and/or more aggressive spark calibrations which provide optimum performance with the specified higher octane. However, other vehicles which are not specifically designed and calibrated for higher octane will not benefit from using higher octane.
In driving situations such as trailer towing or climbing steep grades, some vehicles may experience light spark knock. Under these conditions, a light knock or "ping" is not harmful. However, if the customer is concerned, using 89(R+M)/2 or even 91 (R+M)12 premium gasoline may be temporarily used. If a vehicle experiences heavy spark knock on fuel with its designed octane rating, it may indicate excessive combustion chamber deposits, or some other problem. Combustion chamber deposits can be removed with a combustion chamber conditioner. Standard diagnostic procedures may help identify other potential causes of excessive knock.
Fuel Additives
Some fuel brands advertise that their premium gasoline contains extra detergent additives. Under provisions of the Clean Air Act, all gasoline sold in the U.S. must contain effective deposit control additives. If fuel injector or intake valve deposits are suspected of contributing to poor performance, occasional use of a fuel injector cleaner is a much less expensive way to maintain engine cleanliness than using premium gasoline.