P2078

IMT Valve Pos Sensor/Switch Circ High

Indicator

The PCM has detected a problem with the intake manifold tuning valve or circuit.

Code Set Parameters

If the PCM detects a voltage reading from the mass airflow sensor or barometric pressure sensor that indicates the intake manifold tuning valve is out of position, a code will be stored and a malfunction indicator lamp may be illuminated. Voltage and resistance readings from the intake manifold tuning valve may also cause a code to be stored and a service engine soon lamp to be illuminated.

Symptoms

Symptoms may include a rough engine idle, an engine surge, a slight reduction in low end engine torque, a code will be stored and a service engine soon lamp may be illuminated. Since this is considered a “soft code” the service engine soon lamp may flash until multiple failure cycles are accomplished. If the system corrects the conditions for setting the code, the PCM will reset the malfunction indicator lamp. Hard codes will remain until the conditions are rectified.

Common Causes

Two common causes of this code being stored come to mind from personal experience. The first is due to a defective intake manifold tuning valve. The second cause is due to carbon deposits which form on the manifold tuning valve. The carbon deposits make it impossible for the tuning valve to open and/or close properly. However, if there are mass airflow codes or barometric pressure sensor codes present; either one of those sensors could have contributed to this code being stored as well. Broken, disconnected, shorted, or open electrical circuits, broken, disconnected, or corroded electrical wiring or connectors could also be causes of this code being stored.

Common Misdiagnosis

Technicians report that the intake manifold tuning valve is frequently replaced in error when excessive carbon deposits are at the root of the problem (and vice-versa). PCM failure is rare and all other possibilities should be exhausted before condemning the PCM. Replacement of the PCM will require reprograming.

Diagnosis

  • Several tools will be needed to diagnose this code
  • A suitable scanner, a digital volt/ohmmeter, a hand held vacuum pump, and a manufacturer’s service manual will prove instrumental in performing a successful diagnosis of the conditions which have caused this code to be stored
  • Begin with a visual inspection of all wiring and connectors
  • Repair or replace damaged, disconnected, shorted, or corroded wiring, connectors, and components as necessary
  • Don’t forget to inspect vacuum lines and hoses where applicable
  • Always retest the system after repairs are completed to ensure success. If all system wiring, connectors, and components (Including fuses) appear to be in normal working order, connect the scanner (or code reader) to the diagnostic connector and record all stored codes and freeze frame data
  • This information can be extremely helpful in diagnosing intermittent conditions that may have contributed to this code being stored
  • Continue by clearing the code and operating the vehicle to see if it returns
  • This will help to determine whether or not the malfunction is intermittent
  • After the codes are cleared, test drive the vehicle to see if the code returns
  • If the code fails to immediately return, you may have an intermittent condition
  • Intermittent conditions can prove to be quite a challenge to diagnose and in extreme cases may have to be allowed to worsen before a correct diagnosis can be made. The intake manifold tuning valve system is designed to improve the quality of the air/fuel mixture in the intake manifold in order to affect better fuel atomization, improve low-end engine torque, reduce exhaust emissions, and promote better fuel efficiency
  • Vehicles which utilize an intake manifold tuning valve system to create a specific degree of air turbulence in the intake manifold use a combination of a computerized controller, electrical voltage, and an electric actuator to control system operation
  • The tuning valve is typically mounted on top of the intake manifold and its appearance and operation resemble that of the throttle plate
  • Due to the design and position of the intake tuning valve itself, mechanical failures that occur in the intake manifold often result in catastrophic engine failure
  • The intake manifold tuning valve and retaining bolts are sometimes “sucked” into the engine combustion chamber resulting in valve, piston, or cylinder wall damage
  • The intake manifold tuning valve is opened and closed at varying degrees using a mechanical arm actuated by an electrical motor. The PCM calculates input values from the mass airflow sensor, throttle position sensor, manifold air pressure sensor, and barometric pressure sensor to determine the position of the intake tuning valve flap
  • Once the desired position of the intake manifold tuning valve is determined by the PCM, it sends an output signal to the intake manifold tuning valve motor, in varying voltage levels
  • These varying degrees of voltage determine the exact position of the intake manifold tuning valve
  • The intake manifold tuning valve then directly affects flap position via a mechanical apparatus
  • Once the PCM has sent the output signal, it monitors the mass airflow sensor, barometric pressure sensor, and manifold air pressure sensor to determine whether or not the intake manifold tuning valve is opening and closing properly
  • If the PCM fails to detect the proper amount of manifold air pressure variation, a code will be stored and a malfunction indicator lamp may be illuminated. Use the scanner to activate intake manifold tuning valve operation and observe the intake manifold tuning valve flap to see if it is operating properly
  • Suspect carbon build-up around intake manifold tuning valve to create problems in high mileage engines
  • If the intake manifold tuning valve fails to operate, check for a voltage and signal at the intake manifold tuning valve motor
  • If there is a voltage and a ground signal present, remove the intake manifold tuning valve to determine if it is operating as commanded by the scanner
  • If the intake manifold tuning valve fails to operate as commanded, replace the valve
  • If the electrical motor functions properly, then suspect a stuck or binding flap or shaft in the intake manifold
  • Replace intake manifold tuning valve as required
  • Clear codes and test drive the vehicle after repairs are completed to ensure that the problem has been rectified. If either the voltage or ground signals are absent from the intake manifold tuning valve connector, disconnect the electrical connector from the PCM and perform a continuity and resistance test of all system circuits
  • Compare your actual findings with manufacturer’s specifications and repair or replace system circuits, connectors, or components as necessary, then clear codes and test drive the vehicle to ensure a successful repair. If all system circuits comply with the manufacturer’s recommendations, suspect a defective PCM or programming error
  • PCM failure is rare and will require reprograming by qualified personnel