Engine Control System & Fuel System - 3.6L - Description and Operation: Overview GMC Acadia I

Air Intake System Description

The MAF sensor measures the amount of air coming into the engine. This direct airflow measurement is more accurate than the calculated airflow information obtained from the other sensor inputs. The MAF sensor also houses an integrated intake air temperature (IAT) sensor. The MAF sensor uses the following circuits

1. An ignition voltage circuit
2. A signal circuit
3. A ground circuit
4. An IAT signal circuit
5. An IAT low reference circuit

The MAF sensor that is used on this vehicle is a hot film type and is used in order to measure the air flow rate. The air flow through the sensor passes over a temperature sensor, is then heated, and then passes over another temperature sensor. The difference in air temperature before and after the heater is measured. The air temperature difference is proportional to the amount of air flow. This air temperature difference also allow for determination of whether the air is flowing in the forward or reverse directions. As the air flow increases the delta temperature between the two sensors increases. The MAF sensor converts the temperature difference into a frequency signal that the ECM monitors. The ECM calculates the air flow based on this signal.

The ECM monitors the MAF sensor signal frequency and can determine if the sensor signal is too low or too high. The ECM can also detect airflow that is inappropriate for a given operating condition based on the signal frequency.

The scan tool displays the MAF sensor value in grams per second (g/s) and hertz (Hz). Values should change rather quickly on acceleration, but should remain fairly stable at any given engine speed. If the ECM detects a condition with the MAF sensor circuits, the following DTCs set

1. P0100 Mass Air Flow (MAF) Sensor Circuit
2. P0101 Mass Air Flow (MAF) Sensor Performance
3. P0102 Mass Air Flow (MAF) Sensor Circuit Low Voltage
4. P0103 Mass Air Flow (MAF) Sensor Circuit High Voltage

Scheme 25

The camshaft actuator system enables the engine control module (ECM) to change camshaft timing of all 4 camshafts while the engine is operating. The CMP actuator assembly (15) varies the camshaft position in response to directional changes in oil pressure. The CMP actuator solenoid valve controls the oil pressure that is applied to advance or retard a camshaft. Modifying camshaft timing under changing engine demand provides better balance between the following performance concerns

1. Engine power output
2. Fuel economy
3. Lower tailpipe emissions

The CMP actuator solenoid valve (7) is controlled by the ECM. The crankshaft position (CKP) sensor and the CMP sensors are used to monitor changes in camshaft positions. The ECM uses the following information in order to calculate the desired camshaft positions

1. The engine coolant temperature (ECT) sensor
2. The calculated engine oil temperature (EOT)
3. The mass air flow (MAF) sensor
4. The throttle position (TP) sensor
5. The vehicle speed sensor (VSS)
6. The volumetric efficiency

The CMP actuator assembly has an outer housing that is driven by an engine timing chain. Inside the assembly is a rotor with fixed vanes that is attached to the camshaft. Oil pressure that is applied to the fixed vanes will rotate a specific camshaft in relationship to the crankshaft. The movement of the intake camshafts will advance the intake valve timing. The movement of the exhaust camshafts will retard the exhaust valve timing. When oil pressure is applied to the return side of the vanes, the camshafts will return to 0 crankshaft degrees, or top dead center (TDC). The CMP actuator solenoid valve directs the oil flow that controls the camshaft movement. The ECM commands the CMP solenoid to move the solenoid plunger and spool valve until oil flows from the advance passage (11). Oil flowing thru the CMP actuator assembly from the CMP solenoid advance passage applies pressure to the advance side of the vanes in the CMP actuator assembly. When the camshaft position is retarded, the CMP actuator solenoid valve directs oil to flow into the CMP actuator assembly from the retard passage (3). The ECM can also command the CMP actuator solenoid valve to stop oil flow from both passages in order to hold the current camshaft position.

The ECM operates the CMP actuator solenoid valve by pulse width modulation (PWM) of the solenoid coil. The higher the PWM duty cycle, the larger the change in camshaft timing. The CMP actuator assembly also contains a lock pin (14) that prevents movement between the outer housing and the rotor vane assembly. The lock pin is released by oil pressure before any movement in the CMP actuator assembly takes place. The ECM is continuously comparing CMP sensor inputs with CKP sensor input in order to monitor camshaft position and detect any system malfunctions. If a condition exists in either the intake or exhaust camshaft actuator system, the opposite bank, intake or exhaust, camshaft actuator will default to 0 crankshaft degrees.

CMP Actuator System Operation

Electronic Ignition System Description

The electronic ignition (EI) system produces and controls a high-energy secondary spark. This spark is used to ignite the compressed air/fuel mixture at precisely the correct time. This provides optimal performance, fuel economy, and control of exhaust emissions. This ignition system uses an individual coil for each cylinder. The ignition coils are mounted in the center of each camshaft cover with short integrated boots connecting the coils to the spark plugs. The driver modules within each ignition coil are commanded ON/OFF by the engine control module (ECM). The ECM primarily uses engine speed, the MAF sensor signal, and position information from the crankshaft position and the camshaft position sensors. This controls the sequence, dwell, and timing of the spark. The EI system consists of the following components

Engine Control Module Description

The Engine Control Module (ECM) interacts with many emission related components and systems, and monitors emission related components and systems for deterioration. OBD II diagnostics monitor the system performance and a diagnostic trouble code (DTC) sets if the system performance degrades. The ECM is part of a network and communicates with various other vehicle control modules.

Malfunction indicator lamp (MIL) operation and DTC storage are dictated by the DTC type. A DTC is ranked as a Type A or Type B if the DTC is emissions related. Type C is a non-emissions related DTC.

The ECM is in the engine compartment. The ECM is the control center of the engine controls system. Review the components and wiring diagrams in order to determine which systems are controlled by the ECM.

The ECM constantly monitors the information from various sensors and other inputs, and controls the systems that affect vehicle performance and emissions. The ECM also performs diagnostic tests on various parts of the system. The ECM can recognize operational problems and alert the driver via the MIL. When the ECM detects a malfunction, the ECM stores a DTC. The condition area is identified by the particular DTC that is set. This aids the technician in making repairs.

EVAP System Operation

The evaporative emission (EVAP) control system limits fuel vapors from escaping into the atmosphere. Fuel tank vapors are allowed to move from the fuel tank, due to pressure in the tank, through the EVAP vapor tube, into the EVAP canister. Carbon in the canister absorbs and stores the fuel vapors. Excess pressure is vented through the vent hose and EVAP canister vent solenoid valve to the atmosphere. The EVAP canister stores the fuel vapors until the engine is able to use them. At an appropriate time, the engine control module (ECM) will command the EVAP purge solenoid valve ON, allowing engine vacuum to be applied to the EVAP canister. With the normally open EVAP canister vent solenoid valve OFF, fresh air is drawn through the vent solenoid valve and the vent hose to the EVAP canister. Fresh air is drawn through the canister, pulling fuel vapors from the carbon. The air/fuel vapor mixture continues through the EVAP purge tube and EVAP purge solenoid valve into the intake manifold to be consumed during normal combustion. The control module uses several tests to determine if the EVAP system is leaking or restricted.

Fuel System Overview

The fuel tank stores the fuel supply. The fuel pump module, located in the fuel tank, supplies fuel through the fuel feed pipe to the high pressure fuel pump. The high pressure fuel pump supplies fuel to a variable-pressure fuel rail. Fuel enters the combustion chamber through precision multi-hole fuel injectors. The in-tank fuel pump is controlled with a fuel pump control module (FPCM). The engine control module (ECM) enables the FPCM and sends the desired fuel line pressure to the FPCM via GMLAN. The FPCM sends a pulse width modulated (PWM) voltage to the in-tank fuel pump allowing the fuel line pressure to be varied as commanded by the ECM. The high pressure fuel pump, fuel rail pressure, fuel injection timing, and injection duration are controlled by the ECM.

Scheme 26

The fuel storage tank is made of high density polyethylene. The fuel storage tank is held in place by metal straps that are attached to the under body of the vehicle. The tank shape includes a sump in order to maintain a constant supply of fuel around the fuel pump strainer during low fuel conditions or during aggressive maneuvers.

On-Board Refueling Vapor Recovery (ORVR) System

The on-board refueling vapor recovery (ORVR) system is an on-board vehicle system to recover fuel vapors during the vehicle refueling operation. The flow of liquid fuel down to the fuel tank filler neck provides a liquid seal. The purpose of ORVR is to prevent refueling vapor from exiting the fuel tank filler neck. The ORVR components are listed below, with a brief description of their operation

1. The fuel tank - The fuel tank contains the modular fuel sender assembly.
2. The fuel filler pipe - The fuel filler pipe carries fuel from the fuel nozzle to the fuel tank.
3. The evaporative emission (EVAP) canister - The EVAP canister receives refueling vapor from the fuel system, stores the vapor, and releases the vapor to the engine upon demand.
4. The vapor lines - The vapor lines transport fuel vapor from the tank assembly to the EVAP canister and engine.
5. The check valve - The check valve limits fuel spit-back from the fuel tank during the refueling operation by allowing fuel flow only into the fuel tank. The check valve is located at the bottom of the fuel filler pipe.
6. The modular fuel sender assembly - The modular fuel sender assembly pumps fuel to the engine from the fuel tank.
7. The fuel tank pressure (FTP) sensor is located on top of the fuel sender assembly.
8. The FLVV - The FLVV acts as a shut-off valve. The FLVV is located in the fuel sender assembly. This valve has the following functions: Controlling the fuel tank fill level by closing the primary vent from the fuel tank Preventing fuel from exiting the fuel tank via the vapor line to the canister Providing fuel spillage protection in the event of a vehicle rollover by closing the vapor path from the tank to the engine
9. The pressure vacuum relief valve - The pressure vacuum relief valve provides venting of excessive fuel tank pressure and vacuum. The valve is located in the fuel fill cap.
10. The vapor recirculation line - The vapor recirculation line is used to transport vapor from the fuel tank to the top of the fill pipe during refueling to reduce vapor loading to the enhanced EVAP canister.

Scheme 27

In order to prevent refueling with leaded fuel, the fuel filler pipe has a built-in restrictor and a deflector. The opening in the restrictor will accept only the smaller unleaded gasoline fuel nozzle which must be fully inserted in order to bypass the deflector. The tank is vented during filling by an internal vent tube inside of the filler pipe.

Scheme 28

The fuel tank filler pipe is equipped with a turn to vent, screw on type cap, which incorporates a ratchet action in order to prevent over-tightening.

The turn to vent feature allows for fuel tank pressure relief prior to removal. Instructions for proper use are imprinted on the cap cover. A vacuum safety relief valve is incorporated into this cap.

Scheme 29

The fuel sender assembly consists of the following major components

1. The fill limit vent valve (1)
2. The fuel feed pipe (2)
3. The fuel tank pressure sensor (3)
4. The fuel level sensor (4)
5. The fuel pump electrical connector (5)

The modular fuel sender assembly mounts to the threaded opening of the plastic fuel tank with a seal and a retainer ring. The reservoir, containing the exterior inlet strainer, the electric fuel pump and the fuel filter, maintains contact with the tank bottom. This design provides

1. Optimum fuel level in the integral fuel reservoir during all fuel tank levels and during driving conditions
2. An improved tank fuel level measuring accuracy
3. An improved coarse straining and added pump inlet filtering
4. More extensive internal fuel pump isolation for noiseless operation