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Like it or not here is the in depth Roadster 3.7 V6 cooling system explained v2.0

I've added an extra image of the lower inlet manifold moulding shown upside down and rear to the left to illustrate coolant rail, interfaces to cylinder heads and heater feed union.





The Roadster 3.7 V6 cooling system can be explained with the help of this Mustang engine cutaway image and that of lower inlet manifold. This appears to be moulded from a hard composite and is bolted in the V of the engine to seal with heads. It incorporates on underside a top coolant rail (tube) running horizontally down it's middle, mating to water jackets on rear of each cylinder head. Hot coolant flows forwards along the rail protruding to seal via "O" ring to the thermostat casting bolted to front face of engine (identified by blue label on cutaway) passing coolant via union to large bore top hose and radiator. The inlet manifold also has a hose union at rear fed from coolant rail to accommodate rubber hose feed to cabin heater matrix inlet (bottom union). In this way the hottest coolant in the engine reaches both the radiator and heater in most expeditious manner without the need for long rubber hoses. The thermostat casting provides the narrower bore permanent radiator bypass (part with cast-in text) running down to a mixing chamber below (shown as cutaway) to provide quickest warm up. The bottom hose of rad feeds to the thermostat housing with the temperature sensing side of thermostat facing inwards to the mixing chamber and directly onto bypass flow. This chamber also takes coolant from the return of permanently fed cabin heater circuit (see hose union) via a T piece which accomodates a head of coolant from the lowest point of header tank above.

When engine coolant warms sufficiently the stat begins to open allowing a cooled flow from radiator to be mixed in with the bypass and heater return. The turbulence from various flows into the mixing chamber mitigating the temp of very hot coolant from bypass circuit, the greatest influence being from the radiator cooled flow as the larger bore rad plumbing gives a preferred path for coolant, providing stat is open or partially so. The chamber outlet interfaces directly to pump casting inlet with actual water pump below, then pushing coolant upwards from lower part of engine block waterways. In this way the stat is able to control coolant temperature through a wide range of engine power demands (confirmed by near constant temp gauge) whilst always providing maximum temperature coolant to heater matrix.

The header tank accomodates two very narrow bore bleed/vent hoses from top of rad and top coolant rail with connection to latter seen adjacent to blue label. These assist bleeding and equalise levels throughout the system as coolant expands. The tank has markings for max and min levels with plenty of room above coolant level to allow for expansion when pressure within will rise, hence need for its pressure release filler cap, should system be overfilled. The upper mark should be checked to match top of radiator level since the plastic tank is a re-purposed moulding from a Ford Focus installation. Experience suggests coolant level is not critical although overfilling should be avoided.

The radiator, whether OE plastic side tank or conventional design taller after market all aluminium replacement, is further cooled when necessary by a two speed cooling fan. The only difference is it will run far less often with the latter, and when it does is far quieter due to fan assembly being attached directly to radiator without the OE cowling interface. The two fan speeds are ECU triggered by excessive coolant temperature via one of two relays dedicated to each fan speed, with a high wattage Ford resistor being brought into circuit to limit motor current for lower.

It seems very likely that earlier Roadsters and even recent 4 cylinder Fords work in a similar manner with hot coolant feed taken from rear of engine and thermostat sitting within housing opened with bypass coolant to allow cooled flow from bottom hose and on to water pump inlet. If so then it seems prudent to maintain the coolant flow when cabin heater bypassed to aid coolant mixing when improving air-con operation.

The S1 Roadster with Mondeo ST220 engine designed for transverse mounting, suffered not only from sump shape ground clearance issues but seemingly a plethora of extra or longer rubber hoses due to longitudinal mounting in Mog and as many cooling system components were fitted and fed external to engine block.