Installation manual

90-898305

1-58

Theory of Operation - V6 Models

Air Induction Through Crankcase

Combustion air enters the cowl through holes located in the top aft end of the

cowl. The cowl liner directs this air to the bottom of the powerhead. This limits

the exposure of salt air to the components inside the engine cowl.

Once inside the cowl the air enters the plenum through the throttle shutter(s)

which are located in the plenum assembly The air then continues through the

reed valves and into the crankcase The throttle shutter(s) are actuated by the

throttle shaft. Mounted onto a separate shaft are two throttle position sensors

(TPS) These sensors tell the engine control unit (ECM) the position of the

throttle.

1997/1998/1999 MODELS

Two TPS’s are installed on the OptiMax engine for safety redundancy. If one

TPS should fail, the dash mounted CHECK ENGINE light will flash and the

warning horn will sound. The 1997 Model engine will continue to perform

normally; the 1998/99 models engine speed will be reduced to 3000 RPM. If

both TPS’s should fail, engine speed will be reduced to idle.

To reduce emissions, OptiMax engines require large amounts of air into the

cylinders at idle speed. To accomplish this, the throttle shutters are partially

open at low engine speeds. The dual TPS system reads the shaft movement

in both directions, one reads movement up (increasing resistance), while the

other reads the same movement as down (decreasing resistance). The ECM

reads both and calculates the throttle shutter position.

2000 MODELS & NEWER

One TPS is used on these models, if the TPS fails, the ECM will default to the

MAP sensor. If both the TPS and MAP sensors fail, the engine is limited to

idle speed.

Notes

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