GM Cadillac 1967-1989 Repair Guide

Oil and Fuel Recommendations



Gasoline Engines

It is important that you use fuel of the proper octane rating in your car. Octane rating is based on the quantity of anti-knock compounds added to the fuel and it determines the speed at which the gasoline will burn. The lower the octane, the faster the gas burns. The higher the octane, the slower the fuel burns and a greater percentage of compounds in the fuel prevent spark ping (knock), detonation and pre-ignition, and post-ignition (dieseling).

All 1971 and later models covered in this guide will perform happily on unleaded regular gasoline. Owners of 1967-70 Cadillac cars may opt for unleaded premium fuel of at least 91 octane to protect against detonation. Since many factors such as altitude, terrain, air temperature, and humidity affect operating efficiency, knocking may result even though the recommended fuel grade is being used. If persistent knocking occurs, it may be necessary to switch to a higher grade of fuel. Continuous or heavy knocking may result in engine damage.

Your engine's fuel requirement can change with time, mainly due to carbon buildup, which will in turn change the compression ratio. If your engine pings, knocks, or diesels (runs with the ignition off) switch to a higher grade of fuel. Sometimes just changing brands will cure the problem. If it becomes necessary to retard the timing from the specifications, don't change it more than a few degrees. Retarded timing will reduce power output and fuel mileage, in addition to making the engine run hotter.

Diesel Engines

See Figure 1

Fuel makers produce two grades of diesel fuel, No. 1 and No. 2, for use in automotive diesel engines. Generally speaking, No. 2 fuel is recommended over No. 1 for driving in temperatures above 20°F (-7°C). In fact, in many areas, No. 2 diesel is the only fuel available. By comparison, No. 2 diesel fuel is less volatile than No. 1 fuel, and gives better fuel economy. No. 2 fuel is also a better injection pump lubricant.

Two important characteristics of diesel fuel are its cetane number and its viscosity.

The cetane number of a diesel fuel refers to the ease with which a diesel fuel ignites. High cetane numbers mean that the fuel will ignite with relative ease or that it ignites well at low temperatures. Naturally, the lower the cetane number, the higher the temperature must be to ignite the fuel. Most commercial fuels have cetane numbers that range from 35-65. No. 1 diesel fuel generally has a higher cetane rating than No. 2 fuel.

Viscosity is the ability of a liquid, in this case diesel fuel, to flow. Using straight No. 2 diesel fuel below 20°F (-7°C) can cause problems, because this fuel tends to become cloudy, meaning wax crystals begin forming in the fuel (20°F (-7°C) is often called the cloud point for No. 2 fuel). In extreme cold weather, No. 2 fuel can stop flowing altogether. In either case, fuel flow is restricted, which can result in a no start condition or poor engine performance. Fuel manufacturers often winterize No. 2 diesel fuel by using various fuel additives and blends (No. 1 diesel fuel, kerosene, etc.) to lower its winter-time viscosity. Generally speaking, though, No. 1 diesel fuel is more satisfactory in extremely cold weather.

No. 1 and No. 2 diesel fuels will mix and burn with no ill effects, although the engine manufacturer will undoubtedly recommend one or the other. Consult the owner's manual for information.

Depending on local climate, most fuel manufacturers make winterized No. 2 fuel available seasonally.

Many automobile manufacturers (Cadillac for example) have published pamphlets giving the locations of diesel fuel stations nationwide. Contact the local dealer for information if they are still available.

Do not substitute home heating oil for automotive diesel fuel. While in some cases, home heating oil refinement levels equal those of diesel fuel, many times they are far below diesel engine requirements. The result of using dirty home heating oil will be a clogged fuel system, in which case the entire system may have to be dismantled and cleaned.

One more word on diesel fuels. Don't thin diesel fuel with gasoline in cold weather. The lighter gasoline, which is more explosive, will cause rough running at the very least, and may cause extensive damage if enough is used.

It is normal that the engine noise level is louder during the warm-up period in winter. It is also normal that whitish-blue smoke may be emitted from the exhaust after starting and during warm-up. The amount of smoke depends upon the outside temperature.

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Fig. Fig. 1: Diesel engine fuel cetane versus gasoline engine octane ratings - the higher the cetane number the faster the fuel burns