What does it mean?
OBD II fault code P0661 is a generic code that is defined as “Intake manifold air control solenoid, bank 1 -circuit low”, or sometimes as “Intake Manifold Tuning Valve Control Circuit Low Bank 1”, and is set when the PCM (Powertrain Control Module) detects an abnormally low input voltage in the control and/or signal circuit(s) of the intake manifold tuning valve on the bank of cylinders that contains cylinder #1.
The purpose of an intake manifold tuning valve is to either increase or decrease the total volume of the intake runners to increase an engine’s volumetric efficiency at high engine speeds. In practice, intake manifolds that use tuning valves have what amounts to two sets of intake runners- one set being shorter than the other.
In terms of operation, the two sets of intake runners are separated by a rotatable flap that depending on its position can close off one set of runners, while allowing the intake air to enter the engine via the other set. The advantage of this is that since the flow rate of the inlet air is relatively low at low engine speeds, the PCM can rotate the tuning valve to close off the longer intake runners.
Doing this decreases the total volume of each runner, which accelerates the flow of air, thus allowing more air to be drawn into the cylinders than would have been possible through the longer runners. The result is therefore improved power delivery at low engine speeds without a fuel consumption or increased emissions penalty. Conversely, at high engine speeds, the intake air has less time available to travel through the runners and into the cylinders.
Thus, when the shorter intake runners are closed off and the longer runners are opened the total volume of the runners is increased, which increases the mass, and therefore the velocity of the intake air through the runners. This again results in increased power delivery, without a concomitant increase in fuel consumption and emissions. In terms of operation, the manifold tuning valve is operated by a PCM-controlled stepper motor, or a vacuum operated actuator that uses engine vacuum to work.
As a practical matter, the PCM uses input data from the engine speed sensor, throttle position sensor (or throttle pedal position sensor on drive-by-wire systems), and others to determine the engine load. Based on the engine load in combination with the engine speed, the PCM will then determine the appropriate position for the manifold tuning valve In a fully functional system, the PCM can control the manifold intake tuning valve to alternate between the long and short intake runners seamlessly.
However, when malfunctions, defects, or failures occur in the tuning valve’s control and/or signal circuits that prevent the PCM from controlling the tuning valve effectively, the PCM recognizes that it cannot control the flow of the intake air and it will set code P0661, and may illuminate a warning light. Note that on some applications, several failures need to occur before a warning light will be illuminated.