4-Stroke EFI Basics for Snowmobiles: Part 3

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4-Stroke EFI Sensors

By: Kent Lester

Here’s a list of the most important sensors used in common fuel injection systems and what they do. We’ve noted specifics about snowmobile applications:

* Throttle Position Sensor

The TPS monitors the throttle valve or “butterfly” position within the throttle body. This position actually determines how much air is being introduced into the cylinders. The ECU takes this info, considers the RPM level of the engine, processes the data and regulates the fuel flow accordingly.

It makes sense this data is in constant flux as your thumb moves the throttle randomly during acceleration and deceleration.

* Engine Speed Sensor

The ESS calculates the true speed or RPM the engine is operating at. It works in tandem with the Throttle position Sensor but the TPS reports up-to-date as your thumb presses down on the throttle while the Engine Speed Sensor is already reading the data present in the engine.

The calculation of where the engine RPM is at versus what the Throttle Position Sensor is demanding is the difference. Adjustments are made instantaneously.

* Coolant Temperature Sensor

This one is especially important during start-up and idle when precise fuel flow is critical to fire the cylinders and keep a cold engine running smoothly.

It reports the current operating temperature of the engine and makes adjustments to the fuel flow through the injectors so the engine doesn’t behave “cold blooded” or reject throttle inputs on cold days.

* Manifold Absolute Pressure Sensor

The MAP sensor measures the air pressure or vacuum in the intake manifold as the engine is running.

Manifold vacuum is a good indicator of what level the engine is producing power. Low vacuum means there’s lots of air flow, high pressure means less flow – kind of like the amount of water you can squirt through a garden hose versus a fire hose.

This is an after- the- fact measure of power and verifies with the CPU whether its inputs have been accurate. The CPU then makes further adjustments according to is data map.

* Mass Air Flow Sensor

The MAS sensor calculates how much air is entering the engine. It’s a very precise temperature reader and its guts are designed to measure temperature changes on the intake side many times per second as the engine is running. More air flow has a cooling effect and less air means a higher temperature is present.

When the ECU receives this info it responds by either lengthening or shortening the time the injector stays open. Although common in automobiles, in powersports applications like snowmobiles a Manifold Absolute Pressure sensor is used instead of a Mass Air Flow sensor.

* Oxygen Sensor

This one is located within the exhaust pipe or the exhaust manifold and monitors the amount of oxygen present in the exhaust after combustion is completed.

This helps the ECU determine how rich or lean the fuel mixture is after it’s been burned. If a lot of oxygen is present it can mean the engine is too lean; not much oxygen and it’s rich and probably not meeting emissions standards.

Oxygen sensors are critical tools in the regulation of exhaust emissions. Although EFI managed 4-strokes burn very, very clean, even without an exhaust oxygen sensor, most current 4-stroke snowmobiles use the more costly closed loop system (oxygen sensors in use) because 02 sensors are deemed to be necessary at this point in emissions regulation history.

The term “open loop” merely means the EFI system does not use an oxygen sensor.

Keep checking back for more on this 4-part series about the basics if Snowmobile EFI technology.

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