MODULATED DISPLACEMENT DRIVING CHARACTERISTICS

In 1981, Cadillac offers a revolutionary new engine, the modulated displacement V-8-6-4. This engine features a sophisticated new fuel control system, digital fuel injection, which allows the engine to operate in 8, 6 or 4 cylinder modes. This fully automatic cylinder selection optimizes fuel economy and exhaust emissions as well as the power requirements demanded by today's customer. The engine provides the fuel economy benefits of a 4 or 6 cylinder engine during periods of reduced engine load and delivers the power of an 8 cylinder engine during periods of increased power demands.

To obtain the desired levels of fuel economy and power, the driving characteristics of the 1981 modulated displacement 8-6-4 engine are unique from past model V-8 engines. An additional set of "shifts" that feel similar to transmission gear changes, may be detected as the electronic control module selects the optimum cylinder operation.

The engine mode changes or "shifts" are designed to be as indistinguishable to the driver as possible. These mode changes, although noticeable to a trained observer, will be apparent to only a few customers. When diagnosing or discussing with owners the operation of modulated displacement equipped vehicles, it is important to understand the following normal, specific operating conditions.

A. A slight delay or lag in vehicle response may occur when both the 8-6-4 engine and transmission shift at the same time. This can occur when the vehicle is operating in the 4 cylinder or 6 cylinder mode with the transmission in 3rd gear. If the throttle is depressed, indicating a need for more power, the engine will shift to 8 cylinder operation and the engine speed and spark timing will change to provide an output level equal to the driver's needs. This rapid change in engine operating conditions may cause momentary spark knock. The spark knock will cease once the engine stabilizes in 8 cylinder operation. The transmission may also downshift, depending upon the specific power requirements. This double shift of the engine and transmission may be evident to some drivers.

B. When operating in cruise at 50-55 mph, a slight speed change may be experienced as the vehicle goes up and down gradual inclines. This occurs as the engine is selecting the optimum operating mode. When this mode selection occurs, a change takes place in engine torque which results in a variation in power output. This variation may be interpreted by the driver as a surge. It is most noticeable when Cruise Control is engaged. This condition is a characteristic of this engine.

C. A slight sensation may be experienced by some owners when the engine operates in the 6 cylinder mode for extended periods of time. This sensation may be similar to a slightly unbalanced wheel or an irregular road surface and is most noticeable on very smooth roads. The engine firing sequence is "uneven" in the 6 cylinder mode and a sensation transmitted through the vehicle may be felt by some drivers. This condition will not cause any mechanical damage to the engine and is considered normal. Do not attempt correction of this normal condition. Once the engine enters the "even" firing 4 or 8 cylinder modes the sensation ceases.

D. As a new vehicle goes through the break-in period, the engine load will be greater than after the accumulation of several hundred miles. This will cause the engine operating characteristics to change as the vehicle mileage increases. When a vehicle is new, it will operate more in the 8 and 6 cylinder modes and less in 4 cylinder mode than will be the case when the engine is "broken in".

To best experience these new driving conditions it is recommended that all dealership personnel involved in customer contact or diagnosis and repair of modulated displacement equipped vehicles drive them to become totally familiar with its operation. To closely associate these driving conditions with modulated displacement cylinder selection, the "active cylinder" mode should be selected on the MPG sentinel. By associating the above conditions with the cylinder changes displayed on the sentinel, it will be easier to understand and appreciate the vehicle performance of modulated displacement vehicles. This experience will also be advantageous when explaining these conditions to those owners who question its operation. Avoid inconvenience to the customer by not scheduling repairs for conditions described in this bulletin.

Innovation in automotive design is often accompanied by operational changes offering added convenience, reliability and efficiency. Most drivers, when detecting the above characteristics, will associate its operation with new technology and appreciate the V-8-6-4 feature for the fuel economy benefits it offers.

All Service and Sales personnel are urged to develop a total awareness of the V-8-6-4 engine and all it has to offer the motoring public.

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.