Having been on this forum for about 2 years now, I’ve asked, read and answered a lot of questions. As Polaris keeps making more machines with EFI, it stands to reason that more and more of the the problems & questions are related to the EFI system, its sensors and electronics. That being said, I thought I would try to explain in general how an EFI system works on an ATV. Hopefully this helps take the mystery out of how EFI operates. In the right hands, an EFI system can be checked and diagnosed with a fuel pressure gauge, a multimeter, a factory manual, and access to forums such as this one for the special tool workarounds. In fact, most of this information is right out of the factory manuals for a 2006 700 Twin Cylinder EFI and a 2007 500 EFI.
First and only quote from the manual: "80% of all EFI problems are caused by wiring harness connections."
ELECTRONIC FUEL INJECTION 101:
The central component of the system is the ECU which manages system operation, determining the best combination of fuel mixture and ignition timing for the current operating conditions. An in-tank electric fuel pump is used to move fuel from the tank through the fuel line and in-line fuel filter. The in-tank fuel pressure regulator maintains a constant system operating pressure and returns any excess fuel to the tank. At the engine, fuel is fed through the fuel rail and into the injectors, which protrude into the intake ports. The ECU controls the amount of fuel fed to the engine by varying the length of time that the injectors are energized. The controlled injection of the fuel occurs on each crankshaft revolution, or twice for each 4-stroke cycle. One-half the total amount of fuel needed for one firing of a cylinder is injected during each revolution. When the intake valve opens, the fuel/air mixture is drawn into the combustion chamber, ignited, and burned.
The ECU controls the amount of fuel being injected and the ignition timing by monitoring the primary sensor signals for air temperature, barometric air pressure, engine temperature, engine speed (RPM), and throttle position (load). These primary signals are compared to the programming in the ECU computer chip, and the ECU adjusts the fuel delivery and ignition timing to match the values.
The ECU has the ability to compensate on-the-fly for changes in overall engine condition and operating environment. This allows it to maintain the ideal air/fuel ratio regardless of temperature, demand, or altitude.
During certain operating periods such as cold starts, warm up, acceleration, etc., a richer air/fuel ratio is automatically calculated by the ECU.
Fuel Pump –
Located inside the tank, this electronic pump draws fuel through a 30-micron filter sock on the bottom of the tank and maintains a constant flow of about 25 liters per hour. That’s about 7-8 gallons per hour which far exceeds demand. The fuel pump operates on as low as 7 volts.
Fuel Pressure Regulator –
Located just above the pump on the inside of the tank. It maintains a constant pressure of 39 +/- 3 psi by diverting the excess fuel from the pump back into the tank. This is considered a “closed” system as opposed to an “open” system that has the regulator on the fuel rail and a return hose to the fuel tank. Unlike a car, there is no vacuum reference port so the fuel pressure doesn’t rise and lower with demand.
Fuel Lines –
Hard-sided hoses with quick-disconnects on each end. There are two: one from the tank to the inline filter, and one from the filter to the fuel rail.
Fuel Filter –
Located behind the frame member that is just aft of the radiator cap. A cylinder about 4 inches long and 2 inches in diameter with quick-disconnect receptacles on each end. Fuel passes through the 10-micron element inside.
Fuel Rail –
A metal tube fitted on top of the fuel injector(s) and attached to the cylinder head. Fuel passes through the fuel rail on the way to the injector(s) and the rail holds the injectors in place. It has a Schrader valve (bicycle valve) at the end where you can attach a fuel pressure gauge or push the valve in and relieve the pressure from the system.
Fuel Injector(s) –
Located on the cylinder head. These are spark-plug sized mechanisms with a wiring harness attached. They open when energized for 1.5-8.0 milliseconds (called a “pulse width”) to let fuel pass through the spray nozzle. If removed, you will see a rubber O-ring on each end and a small filter screen on the intake end.
Battery/Charging System –
Efficient operation of the EFI system depends on a fully charged battery and properly operating charging system. The Ignition Coils
paragraph will explain why.
Ignition Coils –
Located under the left front fender mounted on the frame. On an EFI machine the DC/CDI system relies on battery power for ignition. Instead of generating DC voltage via magnetic induction (a.k.a. the stator), a 12 volt DC current is supplied directly to the ECU from the battery. 12 volt DC current charges an internal capacitor to build up the initial ignition charge. An A/C signal from the Crank Shaft Position Sensor is processed by the ECU, which determines ignition timing by calculating from a point pre-determined in the crankshaft rotation. This signal releases the electrical charge which saturates the coil for ignition. DC/CDI systems have the ability to ignite with as little as 6 volts of power.
Engine Control Unit (ECU) –
Located against the frame directly below the headlight pod. The ECU is the brain of the entire EFI system and requires a minimum of 7.0 volts to operate. The memory in the ECU is operational the moment the battery cables are connected. Sensors continuously send data to the ECU. Signals to the ECU include: ignition (on/off), crankshaft position/speed (RPM), throttle position, engine coolant temperature, air temperature, and intake manifold air pressure and battery voltage. The ECU compares the input signals to the programmed maps in its memory and determines the appropriate fuel and spark timing requirements for the immediate engine cycle. The ECU then sends output signals to set the injector duration and ignition timing for that engine revolution. That equates to one output signal from the ECU for every RPM, which can be over 6,000 times per minute, or 100 times per second!
Throttle Body / Intake Manifold –
The throttle body is located in the forward end of the throttle body intake boot. It houses the throttle cable attachment cam, the throttle plate (butterfly), and the TPS. On single cylinder engines, it also houses the IAC. The intake manifold is located between the cylinder head and the output side of the throttle body.
Check Engine Light (MIL or Malfunction Indicator Light) –
During operation, the ECU continually performs a diagnostic check of itself, each of the sensors, and system performance. If a fault is detected, the ECU turns on the Check Engine Light (Malfunction Indicator Light) on the speedometer and stores the fault code in its fault memory. Depending on the significance or severity of the fault, normal operation may continue, or “Fail-Safe” operation (slowed speed, richer running) may be initiated. Some call this "limp mode." You can access the stored fault code using a “blink code” diagnosis flashed out through the instrument cluster.
Wire Harness Assembly –
If I need to tell you what this is, perhaps I can refer you to a dealer.
With the exception of the CPS, the sensors are variable resistors that allow a certain voltage to pass through them and back to the ECU. They start with a reference voltage that comes from the ECU. As pressure or temperature increase and decrease, the amount of sensor resistance varies along with it. The ECU’s data is derived from the difference between the reference voltage and the voltage allowed to pass through the sensor back to the ECU. The ECU then translates that data into a value on its fuel map, combines it with inputs from the other sensors, and determines how much fuel to inject and when to release the coil charge to ignite the fuel.
Coolant Temperature Sensor –
Mounted on the cylinder, the engine temperature sensor measures coolant temperature. The engine temperature sensor is a Negative Temperature Coefficient (NTC) type sensor, as the temperature increases the resistance decreases. Coolant passes through the cylinder and by the sensor probe, creating a resistance reading which is relayed to the ECU. This signal is processed by the ECU and compared to its programming for determining the fuel and ignition requirements. The data from this sensor, when combined with the air pressure and temperature sensor data, gives an EFI the ability to richen the fuel automatically during a cold start, just as a manual choke would. The ECU also uses this signal to determine when to activate the fan during operation. If for any reason the engine temperature sensor circuit is interrupted, the fan will default to ’ON’. Some models have an Engine Temperature Sensor for EFI inputs, and a separate Coolant Temperature Sensor mounted on the lower part of the radiator for fan operation.
Throttle Position Sensor (TPS) –
Located on the throttle body and operated directly off the end of the throttle shaft. The TPS works like a rheostat, varying the voltage signal to the ECU in direct correlation to the angle of the throttle plate (engine load). This signal is processed by the ECU and compared to the internal pre-programmed maps to determine the required fuel and ignition settings for the amount of engine load. The initial position of the TPS is established and set at the factory. If the TPS is repositioned, replaced or loosened it must be initialized.
Crankshaft Position Sensor (CPS) –
Located on the aft side of the stator cover. Also known as the engine speed sensor, the CPS is essential to engine operation, constantly monitoring the rotational speed (RPM) of the crankshaft. A magnetic, 60-tooth ring gear with two consecutive teeth missing is mounted on the flywheel with the inductive speed sensor (Hall Effect sensor) mounted immediately next to it. During rotation, an AC pulse is created within the sensor for each passing tooth. The ECU calculates engine speed from the time interval between the consecutive pulses. The two-tooth gap creates an “interrupt” input signal, corresponding to specific crankshaft position for PTO cylinder. This signal serves as a reference for the control of ignition timing by the ECU. On twin cylinder engines, synchronization of the CPS and crankshaft position takes place during the first two revolutions each time the engine is started. On single cylinder engines, synchronization also includes the Manifold Air Pressure Sensor. The CPS must be properly connected at all times. If it fails or becomes disconnected for any reason, the engine will quit running.
Intake Air Temperature (IAT) –
Found on single cylinder engines, the IAT is located on the rear of the air box. It protrudes into the box to measure charge air temperature for the ECU.
Manifold Air Pressure Sensor (MAP) –
Found on single cylinder engines, it is located on the throttle body intake boot. It measures air pressure for fuel/air mixture calculations. It also identifies to the ECU which stroke is the intake stroke, which was determined during synchronization.
Intake Air Temperature – Barometric Air Pressure Sensor (T-BAP) –
On the twin cylinder engines, the IAT & MAP sensors are combined into one sensor called the T-BAP, which is located on the throttle body intake boot.
Based on air temperature and pressure, the ECU can calculate the density of the air and adjust the fuel output accordingly. This is what gives the EFI system the ability to automatically adjust for altitude changes—a distinct advantage over carburetors.
Idle Air Control Motor (IAC) –
Used on single cylinder engines and located on the throttle body. The IAC is used to stabilize the idle quality of the engine. The IAC is a stepper motor that receives varying voltage inputs from the ECU. The amount of voltage determines the IAC plunger setting, which in turn controls the amount of air bypassing the closed throttle body for idle control. If the IAC is disconnected or inoperative, it will remain at its last setting.