Three standard throttle body specifications were fitted to MGF and MG TF production cars. The earliest cars used a plastic 48mm bore throttle body, manufactured by SU Carburettor Ltd, the plastic construction was a cost-reduction measure but proved vulnerable to distortion from over-tightened hose clips. From approximately 2001 (MY2000 facelift and later), MGF and MG TF non-performance variants use an alloy 48mm bore throttle body, same bore as the plastic item but stronger and more durable. Finally, the MGF Trophy 160 SE, MG TF 135, and MG TF 160 were fitted as standard with an alloy 52mm bore throttle body, a genuinely larger bore supporting the higher-power engines.
The 52mm alloy throttle body can be fitted as an upgrade to 48mm-equipped engines. Expectations should be realistic: the larger bore improves throttle response by allowing faster airflow change during pedal movement, but does not deliver a significant peak power increase on standard-tune engines because the intake system and engine management are not the limiting factor at standard output. The upgrade is chosen for character, sharper response, more eager feel, rather than for measurable power gain.
Throttle Position Sensor (TPS)
The throttle position sensor is a rotary potentiometer mounted on the throttle body shaft, providing the MEMS ECU with a continuously variable voltage signal representing throttle opening angle. The ECU uses this signal, together with manifold absolute pressure, coolant temperature, and engine speed, to calculate fuelling and ignition timing for every engine cycle. A failing TPS produces erratic voltage output, the symptoms include flat spots under acceleration, hesitation, rough idle, and occasionally fault codes registering in MEMS diagnostics. The TPS is common across MGF, MG TF, and MG ZS applications of the same throttle body type.
Replacement requires only the removal of two screws and a wiring connector.
Idle Air Control Valve (IACV), The Stepper Motor
The idle air control valve is a stepper motor that controls a metered bleed of air around the closed throttle butterfly, allowing the MEMS ECU to maintain stable idle speed regardless of engine temperature, electrical load, and air conditioning demand. The system maintains the warm idle within approximately 500 rpm of the manufacturer's specification, a requirement that also applies at MOT test. Symptoms of an IACV problem include erratic or high idle speed, stalling when coming to rest, and hunting (rhythmic rising and falling) at idle. The IACV passages can accumulate carbon deposits and oil mist from the crankcase breather system, and cleaning is often the correct first step before considering replacement.
The IACV connects to the throttle body via a short hose, which is separately catalogued for replacement where the hose itself has deteriorated.
Fuel Injectors
The fuel injectors are solenoid-operated spray valves that atomise fuel from the pressurised fuel rail at the correct timing for each cylinder. The standard MPi injector fits MGF and MG TF non-VVC applications plus the MG ZR/ZS parts-bin equivalents. Injector flow rates vary by engine variant, VVC and Trophy/TF 160 engines use higher-flow injectors matched to their higher power output, and these are not interchangeable with the standard MPi specification. Injector failure usually presents as a misfire on the affected cylinder, often registering as a diagnostic fault code.
More commonly, injectors degrade gradually through spray-pattern deterioration, uneven fuelling across cylinders without a triggering fault code. Symptoms include rough idle, poor fuel economy, and marginally increased emissions. Ultrasonic cleaning can restore moderately-degraded injectors; replacement is the definitive solution for end-of-life items.
Injector O-Rings and Seals
Each injector seals into the fuel rail at the top and into the inlet manifold port at the bottom via rubber O-rings. These O-rings harden with age and heat exposure, and a weeping seal allows fuel to seep at the connection or, at the manifold end, allows unmetered air to be drawn into the cylinder. A fuel leak at an injector seal in the mid-engine bay is a serious fire risk and must be addressed immediately, any fuel smell from the engine bay warrants an immediate investigation. When injectors are removed for cleaning or replacement, new O-ring seals should always be fitted; reusing compressed original O-rings invites a leak within weeks.
Performance Upgrades
Alongside the standard service components, this section carries two established performance upgrade paths. The K&N 57i performance induction kit replaces the factory air box with an open-element cone filter drawing cooler air from a relocated intake position.