MGF and MG TF Engine Mounted Thermostat and Housing
The thermostat fitted to MGF models and pre-2003 model year MG TF models is of a traditional type located inside a housing bolted to the engine. The thermostat can be replaced in its housing, in time honoured fashion, as required. It may be found in Engine Mounted Thermostat & Housing.
From September 2003 onwards (from VIN 4D) and intermittently from January 2003, a Pressure Relief Thermostat (known variously as ‘PRT’ or ‘PRRT’ - pressure relief remote thermostat) was introduced on the MG TF. Its purpose was to reduce thermal shock on the engine (and subsequent strain on the cylinder head gasket) by introducing a softer cycle of cold and hot coolant through the engine as the thermostat opened and closed during high speed running.
The PRT’s location was further away from the engine inside a sealed assembly inserted in the hose run to the underfloor pipes. As the ‘sealed’ suggests, this assembly must be replaced as a single unit - it can be found under 'Pressure Relief Thermostat & Hoses'.
When the PRT was introduced, the old engine-mounted thermostat housing was retained in the cooling system, but a restricting flange was fitted inside it in place of the thermostat to ensure water pump flow was still governed correctly. At the same time, the water flow through the radiator was reversed. The radiator was not modified, since being a crossflow type it can operate either way, but considerable changes in the cooling system routing were required.
Also from January 2003 the MG TF automatic (CVT Steptronic) was given a PRT, but the layout and hose runs differ significantly to those on the manual cars, throwing further complexity into the already complicated MGF/TF hose layout history.
The PRT system was successful, and can be retro-fitted to MGFs and earlier TFs if desired. A good understanding of cooling system principles is however required to correctly carry out the necessary modifications to the engine bay pipework - unfortunately not a job for the average DIYer.