Your Vehicle: 2001 Ford Escort ZX2 L4-2.0L DOHC VIN 3
Vehicle » Powertrain Management » Fuel Delivery and Air Induction » Throttle Body » Description and Operation » Throttle Body System Overview  
  Throttle Body System Overview  

The throttle body system meters air to the engine during idle, part throttle, and Wide Open Throttle (WOT) conditions. The throttle body system consists of an Idle Air Control (IAC) valve assembly, idle air orifice, single or dual bores with butterfly valve throttle plates and a Throttle Position (TP) sensor. One other source of idle air flow is the Positive Crankcase Ventilation (PCV) system. The combined idle air flow (from idle air orifice IAC flow and PCV flow) is measured by the MAF sensor on all applications.

During idle, the throttle body assembly provides a set amount of air flow to the engine through the idle air passage and PCV valve. The IAC valve assembly provides additional air when commanded by the Powertrain Control Module (PCM) to maintain the proper engine idle speed under varying conditions. The IAC valve assembly mounts directly to the throttle body assembly in most applications, but is remote-mounted to the intake manifold in some applications. Idle speed is controlled by the PCM and cannot be adjusted.

NOTE : The traditional idle air adjust procedure as well as throttle return screw are no longer used on OBD II applications.

Throttle rotation is controlled by a cam/cable linkage to slow the initial opening rate of the throttle plate. The TP sensor monitors throttle position and provides an electrical signal to the PCM. Some throttle body applications provide an air supply channel upstream of the throttle plate to provide fresh air to the Positive Crankcase Ventilation (PCV) or IAC systems. Other throttle body applications provide individual vacuum taps downstream of the throttle plate for PCV return, Exhaust Gas Recirculation (EGR) , Evaporative Emission (EVAP) , and miscellaneous control signals.

Throttle Body System Hardware
The major components of the throttle body assembly include the TP sensor, IAC valve assembly, and throttle body housing assembly.

Throttle Position Sensor
The TP sensor monitors throttle position and provides an electrical signal to the PCM. It is monitored by the On-Board Diagnostic (OBD) II system for component integrity, system functionality, and faults that can cause emissions levels to exceed standards set in government regulations. For additional information on the TP sensor, refer to Electronic EC System Hardware-PCM Inputs.

Idle Air Control Valve

Nippondenso Idle Air Control (IAC) Valve Assembly

Hitachi Idle Air Control (IAC) Valve Assembly With Vent/Filter

The Idle Air Control (IAC) valve assembly (Figure 117) and (Figure 118) controls engine idle speed and provides a dashpot function. The IAC valve assembly meters intake air around the throttle plate through a bypass within the IAC valve assembly and throttle body. The PCM determines the desired idle speed or bypass air and signals the IAC valve assembly through a specified duty cycle. The IAC valve responds by positioning the IAC valve to control the amount of bypassed air. The PCM monitors engine rpm and increases or decreases the IAC duty cycle in order to achieve the desired rpm.

Hitachi Idle Air Control (IAC) Valve Assembly With Air-Assist Injectors

On applications with air-assisted injectors, the IAC valve (Figure 119) also supplies a small amount of air into the path of the fuel injectors. The jet of air causes an increase in fuel atomization at low speed and light load conditions.


The IAC valve (part of throttle body assembly) has an internal diode on some applications. If the internal diode is measured in crossed terminal position with a digital multimeter, there will be an incorrect or negative reading. It is important that the mating component and harness connectors correctly oriented. Diagnostic procedures emphasize this importance.

The PCM uses the IAC valve assembly to control:

  • No touch start
  • Cold engine fast idle for rapid warm-up
  • Idle (corrects for engine load)
  • Stumble or stalling on deceleration (provides a dashpot function)
  • Over-temperature idle boost.
  • Air Assist to Injectors.

Throttle Body Housing
The throttle body housing assembly is a single piece of aluminum casting with an air passage and a butterfly throttle plate with linkage mechanisms. When the throttle plate is in the idle (or closed) position, the throttle lever arm should be in contact with the Throttle Return Stop. The throttle return stop prevents the throttle plate from contacting the bore and sticking closed. The setting also establishes the amount of air flow between the throttle plate and bore. To minimize the closed plate air flow, a special coating is applied to the throttle plate and bore to help seal this area. This sealant/coating also makes the throttle body resistant to engine intake sludge accumulation.

Typical Attention Decal Locations

Features of the Throttle Body Assembly include:

  1. Idle Air Control (IAC) valve assembly mounted directly to the throttle body assembly (some vehicles).
  2. A pre-set stop to locate the WOT position.
  3. An air supply channel upstream of the throttle plate to provide fresh air to the PCV system (some vehicles only).
  4. Individual vacuum taps for PCV, EGR, Evaporative Emission (EVAP) and miscellaneous control signals (some vehicles only).
  5. PCV air return (it applicable).
  6. A throttle body-mounted Throttle Position (TP) sensor.
  7. A sealant/coating on the throttle bore and throttle plate makes the throttle body air flow tolerant to engine intake sludge accumulation. These throttle body assemblies MUST NOT BE CLEANED and have a white/black attention decal (Figure 116) advising not to clean.