How the cooling circuit works (and the thermostat)

The basic flow (2.2)

• The heater circuit is the bypass. In the factory set-up it flows all the time. Until the thermostat opens, coolant goes round the engine and heater matrix only. You adjust the cabin heat with the air flap, not by regulating the coolant through the matrix (one owner, 2010; another found the same reading).
• The thermostat is opened by the heater return. The heater return pipe flows into the thermostat housing and warms the wax element. A remote thermostat housing must have the heater return connected too, or it will not be warmed properly (2009 research by one owner).
• The expansion tank is a dead leg. One owner says flow shouldn't go through the header tank. It degasses and sets the system pressure at the pump inlet. The small feed to the tank only flows once the thermostat opens a little, because until then it connects to the main radiator return pipe, which is blocked. So "no flow into the tank at idle" does not by itself mean a dead pump. Note that a garage told one owner the opposite (that there should always be a steady flow), which the thread argued about and did not settle.
• A blocked heater matrix is dangerous. If the return flow is blocked, the thermostat only sees block heat and opens late. One owner says very bad things would happen, and mentions softened and warped heads from local overheating, which can happen quickly. See High coolant temperatures.
• The oil cooler is water-cooled, so its heat goes into the engine coolant (the Turbo thread covers this).

Where the thermostat sits

• One owner researching in 2009 concluded that the 2.2 has its thermostat on the pump inlet, and that the Turbo has it on the engine outlet (as on some cast-iron engines), which holds engine temperature more constant. His understanding was unsure, so check yours before relying on it.
• He speculated that the inlet position could build pressure and stress the radiator, but offered no evidence. Another owner replied that he had never heard of temperature failures on the 2.2.
• That owner moved his thermostat to the engine outlet as a personal project, blanked the original block and relocated the engine temperature sensor, and planned an electric pump and a larger heater bypass. Nobody posted a result, so this is not a recommendation.
• Another owner questioned whether it was worth it: "if it works fine, don't touch it".

Thermostat ratings and running without one

• Lower-rated thermostats. One owner with an 82°C thermostat says the spring is too weak: above about 3,500 rpm the pump pushes it open and dumps cold water into the circuit, so he went back to the original. Another with an 82°C thermostat saw the temperature fall to 66°C in under a minute at 3,000 rpm on the motorway.
• No thermostat. One owner says it takes the engine longer to warm up and uses more fuel while it does, but once at temperature there is no real difference if the original thermostat was working. Another suggested running without one to avoid pressure build-up. Neither reports long-term results and the thread has no evidence either way, so this is opinion. See the overheating guide for one owner who tried it while chasing high temperatures.
• The coolant temperature sensor feeds fuel and ignition maps as well as the gauge, so a low-reading engine runs differently (one owner's comment about sensors, from the Turbo thread).

The second (electric) pump

• A 2007 post says every VX has a second water pump, not integral to the engine, shown as part 11 in TIS and located near the fuel pump, fitted to help circulation because the blocks come from front-engined cars. Another owner thought it was Turbo-only, and a third says the NA doesn't have it as standard.
• On the Turbo it is often assumed to cool the turbo after shutdown, but one owner who traced the hoses in 2016 says it only circulates water from the cylinder head to the heater matrix. See Turbo cooling circuit and upgrades.

Related: Changing the coolant and bleeding the system, Water pump replacement, Radiator replacement.