The water pump on every MGF and MG TF K-series engine is driven by the main timing belt, with no separate auxiliary drive, which makes the pump's condition directly relevant to timing belt safety, as a seized water pump bearing can strip teeth from the timing belt or cause it to come off, with the interference-engine consequences described in the Timing Gear section. For this reason the correct service practice is to replace the water pump at every timing belt service, which is why the timing belt kits include the water pump as a bundled item. The pump mounts to the engine block via two locating dowels and is secured by four screws and one bolt, with a single O-ring sealing the pump to the block, included with the assembly.
Water Pump Production Change
The water pump was revised from approximately 2001 to incorporate an evaporation chamber in the pump body, visible as a small additional chamber on the later casting, designed to allow steam or trapped air to escape rather than accumulate in the pump cavity, reducing the chance of cavitation or airlock. Both the earlier pump and the later pump with evaporation chamber are catalogued, and the later pump is suitable as a replacement on cars that originally had the earlier specification, being backward-compatible, so either specification can be fitted to any MGF or MG TF.
Engine-Mounted Thermostat & Housing
Every MGF and every pre-2003 MG TF has the thermostat mounted in a housing bolted to the engine in the traditional manner. The thermostat is an 88°C wax-pellet design that opens progressively as coolant temperature rises, permitting flow to the front-mounted radiator. The housing is an aluminium casting sealed to the engine by one large and two smaller O-rings and secured by three screws and one bolt, and also carries the water pump outlet flange as an integrated part of the casting. A thermostat that sticks closed causes overheating, critical on the K-series where head gasket integrity is sensitive to thermal stress, while one that sticks open causes the engine to run cold, producing poor fuel economy, elevated emissions, and MOT failure on the emissions test.
Thermostat replacement is straightforward once the housing is accessed, the housing unbolting from the engine with the thermostat and seals renewed as a set, and reusing compressed original O-rings usually results in a leak within weeks, so new O-rings should always be fitted.
MG TF Pressure Relief Thermostat (PRT)
From January 2003 intermittently, and from September 2003 onwards in full production, the MG TF adopted the Pressure Relief Thermostat system. This is not a simple thermostat replacement but a substantially different cooling circuit arrangement, the original engine-mounted housing being retained but with the thermostat replaced by a restricting flange that maintains correct water pump flow governance, the actual PRT itself being a separate sealed assembly mounted remotely in the hose run to the underfloor pipes, and the radiator's direction of coolant flow reversed. For MG TF cars with PRT, the correct engine-mounted housing is a different part that accepts the restricting flange in place of a thermostat. The PRT system was introduced specifically to reduce thermal shock on the head gasket by softening the cycle of cold and hot coolant through the engine during high-speed running, is considered a meaningful improvement, and can be retro-fitted to MGFs and pre-2003 TFs, many specialists recommending this as part of a head gasket rebuild or cooling system overhaul.
MG TF automatic Stepspeed CVT cars have their own PRT layout with different hose runs to the manual cars.
Coolant Outlet Elbow & Ordering
The engine coolant outlet elbow sits at the rear of the cylinder head and connects the coolant outlet to the main hose run, sealed to the head by a dedicated gasket and secured by two screws. The elbow and gasket are frequent service items on older cars, as the gasket can perish and produce a weeping coolant leak from the back of the head, while the elbow can corrode where it interfaces with the aluminium head, so replacing both together at any major cooling service is sensible. For ordering, the key identifiers are model and year to determine PRT versus non-PRT housing on the MG TF, production date for the water pump though the later evaporation-chamber pump is the sensible default, and transmission type on MG TF for cars already fitted with PRT. The PRT itself, its dedicated hoses, and the automatic-specific assembly are catalogued under Cooling, this section dealing exclusively with the water pump, the engine-mounted thermostat or flange housing, and the associated seals and fixings.