Introduction
FIM: Fast Idle Mechanism
BACV: Bypass Air Control Valve
*
AWS: Accelerated Warmup System (aka ASV)
Baseline: Based off Air Adjust Screw
The Second Generation RX-7's idle system is a hot topic because many run into idle problems at one time or another. This page provides a general overview of the stock idle control systems, and links to other idle trouble-shooting pages. The graph on the right represents the probable idle expected when an FC is cranked at a given coolant temperature. The idle will "track along" whatever path the RX-7's components dictate as coolant temperatures increase. If the engine is cranked within the 17-second range (gray region) and the BACV or AWS is present, for the first 17-seconds after cranking the idle will be raised, then drop to either the FIM or baseline if the FIM is not present.
*Note: at this time the AWS's specific RPM contribution is unknown, and the graph assumes 750 RPM - a minimum value. The AWS probably contributes closer to 1k RPM. If you find the value through experimentation, let stevej88na know and he can update the graph accordingly. This would have to be done by testing idle speed with and without the AWS, when no BACV is present.
Air Intake Path
Air's path into the engine.
On the right is a diagram that shows the path air travels on its way into the engine. An important note is all systems use air coming from the Air Flow Meter (AFM) - that is, the ECU accounts for all air in the system by metering through the AFM (such air referred to as "metered air"). A leak in the system throws the ECU's accounting off, causing idle problems. This is a common cause of bad idle, and is why checking for vacuum leaks is important.
After the AFM, air takes one of three paths: through the throttle body, Bypass Air Control valve, or Air Bypass Solenoid valve.
Throttle Body
The throttle body (or throttle chamber, TB, or TBody) controls the majority of air flowing into the engine. Within the TB are various butterfly valves, including the primary throttle valve, and dual no. 1 and no. 2 secondary valves.
Sidenote 1: when you press the gas pedal, you open a butterfly valve in the TB to let more air into the engine; it's up to the ECU to compensate with more gas to optimize the air to fuel ratio (a/f ratio).
Sidenote 2: The popular TB mod removes the dual no. 2 secondary throttle valve, whose purpose is to prevent hesitation during acceleration at the expense of slightly obstructed air flow.
The thermo wax raises cold idle temperatures.
Two idle systems are present on the TB. The first is the air adjust screw (AAS), located on the top of the TB, which determines how much air bypasses the butterfly valves through a small port (more air raises idle). For information on setting the AAS properly, see
[Idle Speed and AFR Adjustment]. The second idle system on the TB is the fast idle mechanism, which includes the thermo wax device and a fast idle cam, both located on the rear-facing side of the throttle body. When the car's coolant is cold, the fast idle cam holds the butterfly valves in the TB slightly open, raising idle. As the coolant warms and expands the thermo wax, a rod pushes the fast idle cam, gradually lowering idle from 1500 to 750, as the graph on the right depicts. This is why adjusting idle when the car fully warmed is important.
Bypass Air Control Valve
The BAC looks the same, but is in different
locations between S4 and S5.
The BAC valve is an ECU controlled device that allows metered air to bypass the throttle body, in effect, giving the ECU its own small gas pedal. The BAC is located on the driver's side of the dynamic chamber (DC) in S4s, and on the passenger side upper intake manifold (UIM) on S5s. Beyond providing an air bypass amount based on Crank angle Sensor (CAS) signals, throttle position, ignition switch, and boost sensor signals, the BAC's purpose is to stabilize idle when the engine sees various loads, like air conditioning, electrical loads, etc. During warmup, covering coolant temperatures above 59F and below 98.6F, the BAC valve, along with the air bypass solenoid valve below, contributes to a 3,000rpm idle for the first 17-seconds after cranking. The ECU controls the BAC based on the following signals:
S4 Bypass Air Control Signals
- Boost sensor
- Ignition coil: engine speed
- Crank angle sensor (CAS): engine speed
- Throttle position sensor (TPS): throttle valve opening angle (see [TPS Adjustment])
- Water thermo sensor: engine coolant temperature
- Water temperature switch: radiator temperature
- Intake air temperature switch: intake air temperature
- Ignition switch: cranking
- Power steering switch: p/s active
- Air conditioner relay: a/c active
- Initial set switch: for maintenance*
S5 Bypass Air Control Signals
- Intake Air Thermo Sensor (engine)
- Throttle Sensor, full and narrow range: (see [TPS Adjustment])
- Crank angle sensor (CAS)
- P/S Switch
- A/C Switch
- Test Connector*
- Blower Switch (part of electrical load control system)
- Rear Defrost Switch (part of electrical load control system)
- Headlight Switch (part of electrical load control system)
- Fog Light Switch (part of electrical load control system)
- M/T: Neutral Switch
- M/T: Clutch Switch
- A/T: Inhibitor Switch
- Water Thermo Sensor
- Ignition Switch
*Sidenote: When adjusting idle, jump the initial set coupler to keep the BAC from compensating for your tuning, and remove the jumper when finished!
Air Bypass Solenoid Valve
During warmup, covering coolant temperatures above 59F and below 98.6F, the air bypass solenoid valve helps increase air flow beyond what the BAC can do by itself, raising idle to 3,000rpm for the first 17 seconds of cold operation; this the Accelerated-Warmup System (AWS). Below 59F the 17-second idle increase is disabled due to high oil viscosity.
S4 Air Bypass Solenoid Valve Signals
- sensors needed
S5 Air Bypass Solenoid Valve Signals
- Throttle Sensor (full range)
- M/T: Neutral Switch
- M/T: Clutch Switch
- A/T: Inhibitor Switch
- Water Thermo Sensor
- Ignition Switch
Idle A/F Ratio
The air to fuel ratio at idle is set with the variable resistor located in front of the passenger-side suspension tower, and is marked with an
R and
L to show which direction to turn for richer and leaner idle. This is only for setting idle a/f ratios; outside of idle, no a/f ratios can be tuned without an aftermarket fuel map of some sort. See
[Idle Speed and AFR Adjustment].
See Also...
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