The MG ZR, ZS and ZT use four distinct cooling system architectures, one for each major powertrain. K-series cars (ZR 105/120/160 and ZS 110/120) use the small-capacity Rover Group system inherited from the donor 25 and 45, with around 5.5 litres of coolant, a belt-driven mechanical water pump at the front of the engine, an inlet-side thermostat and a metal coolant rail running across the rear of the block. The KV6 in the ZS 180 and ZT 160/180/190 has a substantially larger system with a plastic thermostat housing in the V of the engine and a twin-rotor water pump driven from the front timing belt. The L-series and M47R diesel cars share much of the K-series cooling layout in the ZR/ZS but with a larger radiator and (on the CDTi from 2002) an intercooled charge-air route. The ZT 260 V8 has a unique cooling and heating installation built around the longitudinally mounted Ford Modular V8, with its own radiator, fans and heater matrix not shared with the FWD ZT range.
A hose, pump or thermostat correct for one engine is rarely correct for another.
K-Series Cooling and the Pressure-Relief Thermostat
The 5.5-litre capacity is small for the engine's output, which makes the system unforgiving of slow leaks: a small loss can empty the expansion tank quickly, induce an air lock, and lead to localised overheating that the system pressure cap and elastomer head gasket together cannot accommodate. The standard cure is methodical: inspect the radiator, the inlet manifold gasket coolant passages, the steel coolant rail at the rear of the block (corrosion is the typical failure), the thermostat housing, and the expansion tank cap, which is a wear item and one of the cheapest fixes for unexplained coolant loss. The genuine Febi-supplied OE expansion cap is the correct replacement for ZR, ZS and ZT 1.8 cars; non-OE caps with incorrect pressure ratings should be avoided.
From 2003 the K-series in the MG TF and shortly after in the 1.8 K-turbo ZT 160 received a Land Rover-developed Pressure-Relief Thermostat (PRT), relocated downstream of the engine, opening with pressure as well as temperature, and addressing the thermal cycling that contributes to head gasket stress. The PRT is a sound retro-fit on earlier cars, though on cold mornings it can extend warm-up time. The system should be filled with genuine OAT (pink) coolant and never mixed with older blue IAT coolant.
KV6 Cooling and the Plastic Thermostat Housing
The KV6 cooling system is dominated by one well-known wear item: the plastic thermostat housing in the V of the engine. The housing is constructed in two halves joined by an ultrasonic weld, and the weight of the large coolant hoses places the joint under continuous flexing stress as the engine moves on its mounts. With age, the weld cracks and coolant pools in the valley between the cylinder banks, the classic telltale being pink staining around and beneath the inlet manifold. The housing itself is inexpensive but the job is labour-intensive: the upper inlet manifold and plenum must be removed for access, and the elbow pipe (which can also crack at its mating face) and all associated O-rings are typically replaced as a set.
Reinforced epoxy-strengthened housings are now available as a longer-life upgrade. The KV6 also requires careful attention to its inlet manifold gaskets, the water pump seal (which leaks behind the cam covers and is hard to spot), and the expansion tank cap (the same Genuine Febi OE part as the K-series). Bleeding the system after any cooling work is essential, the V configuration creates air pockets that must be vented through the cooling rail bleed screw.
Diesel and ZT 260 Cooling, and the Heater Side
The L-series diesel in the ZR TD and ZS TD uses an enlarged version of the Rover Group cooling system to handle the higher thermal load, with a larger radiator, intercooler and revised hose routing. The BMW M47R CDTi shares its core cooling architecture with the Rover 75 CDTi installation and from 2002 includes an intercooler circuit (the badge change from CDT to CDTi reflects this). M47R cooling components are unique to the transverse Rover/MG application and not interchangeable with the longitudinal BMW M47D20. The ZT 260 V8 has its own radiator, revised coolant routing for the longitudinal Ford Modular installation and a substantially redesigned heater matrix and climate control system, widely acknowledged as a weak point, with the Visteon FS10 air-conditioning compressor and the heater blend valve both routine wear items.
Reinforced replacement blend valves are now produced by independent specialists.