Ford Zetec Cooling System Diagram

Updated 26 May 2019

Having disturbed all of my cooling hoses to perform a full coolant system flush, some of the older rubber hoses have now degraded, resulting in coolant seepage. As we sourced some of the rubber hoses from the "parts bin" at the local motor factors, we have no idea what the hoses were originally from. Therefore, I have been researching silicone hose options, especially as it's now 18 years since I built my car and the Zetec installation has now been undertaken by many more people and manufacturers.

During my research, I came across several forum posts linking to this page. Whilst many were positive, with diagrams of effectively the same cooling system arrangement, some of the responses were saying the hose arrangement I documented is wrong and that it overcools the water in the system.

As the system I provided diagrams for is a mirror of the coolant arrangement in the Ford installation (Mondeo or Focus), this either implies a lack of understanding of how the OEM Zetec cooling system works, or is based on an incorrect assumption this is the same layout as early Westfield Zetec installations used, which does overcool the water in the system.

Therefore, I have updated the page to detail exactly the systems' operation to aid that understanding.

In the interests of transparency, the original document is available by clicking here.

The Thermostat Housing

The thermostat and housing on the Zetec are key to how water flows around the system as the various stages of operation, so we'll begin with some annotated pictures and associated commentary.

The thermostat housing is in two sections, the main body and the top hose/bleed outlet. The thermostat sits between these two sections and has a rubber U shaped seal (not pictured) which is clamped in place when the two sections are joined to prevent leaks.

The main body bolts against the cylinder head and has a captive rubber seal to prevent leaking. This section has two hose outlets:

  • The one towards the exhaust side of the engine (left on the photograph) is 21mm in diameter and feeds the bypass circuit. The dashed red line shows how this is routed internally.
  • The one towards the inlet side of the engine (right on the photograph) is 19mm in diameter and is the feed to the heater matrix. This passes underneath the housing.

The outer section has the main top hose outlet, which is 34mm in diameter and a bleed/overflow which is 8mm in diameter.

Thermostat Housing - Outside

Inside the housing, we can see the flange to which the thermostat seals. To the bottom left, highlighted, is the outlet to the heater. This is "always open", so water can always flow through the heater. The lower temperature sender screws in via this hole so it sits in the main water flow.

The "D" shaped highlighted section is the outlet to the bypass hose and the circle around this is the bypass flange to which the thermostat seals when it is open.

Thermostat Housing - Inside

The thermostat sits between the two sections of the housing. It is 52mm in diameter and has a U section rubber seal (not pictured) to seal the two sections of housing. To the base of the thermostat is a metal disc 35mm in diameter which sits against the bypass flange when the thermostat is fully open.


Water Pump

Impeller blades within the water pump suck water from the bottom hose, through the various channels in the engine block and head and out through the thermostat housing. The direction of water pump rotation depends upon the variant of the Zetec engine you have, but it's operation is still the same.

When looking from the cambelt end of the engine, the crank rotates clockwise. The water pump rotation direction depends upon which variant of the Zetec you have.

  • Phase 1/2 "Silver Top" (all but Escort/Fiesta 1.6/1.8): Pump rotates anti-clockwise and is driven by the back of the belt. This can be via an idler pulley, or mounting the alternator as high up under the inlet manifold and using a short drive belt.
  • Phase 1/2 "Silver Top" Escort/Fiesta 1.6/1.8: Pump rotates clockwise and is driven by the inside of the belt. It is possible to fit this waterpump to any Silver Top variant.
  • Phase 3 "Black Top": Pump rotates anti-clockwise and is driven by the back of the belt using an idler pulley.

Coolant Circuit

Top Hose

The top hose connects between the thermostat and the top of the radiator. Water flow to the top hose is controlled via the thermostat. When the water is cold, the thermostat is closed and no water flows via the top hose. When the water is up to temperature, the thermostat begins to open and water starts flowing via the top hose to the radiator.

There is a small bleed valve (circa 3mm diameter) in the thermostat which allows air and water to pass through which in turn allows the system to auto-bleed.

Bottom Hose

The bottom hose connects between the water pump and the bottom of the radiator and also acts as a junction point for other circuits within the system, notably the bypass hose and the heater return. On some installations, the main header tank feed connects into the bottom hose, on others this connects directly to the bottom of the radiator.

Tiger supply a fabricated aluminium tube 32mm in diameter with two 19mm diameter take-off's. This is referred to as the "Side Exit Pipe", "Twin Exit Pipe" or "Double-Tee Pipe"

Header Tank / Bleed

The header tank should be the highest part of all the coolant circuit. Coolant mixture is added to to the header tank in order to fill the system. The bleed hose allows the displaced air to return back to the top of the header tank. The level in the header tank is then maintained between the minimum and maximum levels (when cold) and allows the system to auto bleed. The header tank also features a pressure relief cap. This enables the system to become pressurised within safe limits, increasing the boiling point of the coolant. Should the pressure exceed the safe limits, the pressure relief valve opens and allows air to escape.

It is important that, at cold, the coolant is not above the maximum level, If it is, there may be insufficient air in the system to allow for the expansion and boiling coolant may therefore be ejected via the filler cap.


The radiator connects between the top hose and the bottom hose, it may also have a connection to the header tank for filling the system and may itself have a bleed connection. Water should only flow through the radiator when the thermostat has opened and coolant is flowing through the top hose. It then passes through the coolant cores, is cooled down and then goes back into the engine at a lower temperature.

A temperature controlled switch is often placed within the radiator to control a radiator fan. The purpose of this is to get extra airflow through the cores of the radiator to aid cooling. This is especially usefull when in slow or stationary traffic when there is little natural airflow through the radiator. Alternatively, the radiator fan can be controlled by a temperature controlled switch installed in the top hose, or via the engine management unit.

Bypass Circuit

The bypass circuit is essential to warning up the engine quickly. When the engine is cold, coolant flows through the bypass hose, to the bottom hose and back into the engine, heating it up quicker. When the engine is up to temperature, the thermostat opens to allow flow via the top hose. At the same time, the disc on the end of the thermostat closes off the bypass hose, preventing coolant flow from bypassing the radiator ensuring it is cooled

Heater Circuit

The heater matrix sits in-line between an "always open" connection on the thermostat housing and the bottom hose. A valve on the heater matrix controls how much coolant flows through the matrix and a fan then passes air over the matrix to the ducting systems to deliver warmed air to the passenger compartment. It is entirely possible for the valve in the heater matrix to be completely closed and therefore permit no coolant to flow via this circuit.


Cooling System - Engine Cooling Animation

Engine Cold

When the engine is cold, the thermostat is closed preventing coolant from flowing through the top hose and radiator. All coolant is sent via the smaller diameter bypass hose to the bottom hose. Here, the same coolant is circulated around the engine, rapidly heating it up.

Cooling System - Water Cold, Thermostat Closed, Bypass Open
Cooling System - Water Warming Up, Thermostat Closed, Bypass Open

Engine Reaching Operating Temperature

As the temperature of the coolant rises, the thermostat will begin to open. At this point, some coolant flows via the radiator and some continues to flow via the bypass hose. The coolant from the radiator mixes gradually with the warmer coolant within the bypass hose, warming up all coolant within the system. This slows the process of the water heating, but also prevents thermal shock whereby cold water in the radiator is suddenly forced into the hot engine.

Cooling System - Water Warm, Thermostat Part Open, Bypass Open

Engine At Operating Temperature

When the engine is at operating temperature, the thermostat opens/closes as necessary to maintain the flow via both the bypass and the radiator. Should the system start to get too hot, the thermostat opens completely and blocks all flow through the bypass circuit, forcing it via the radiator to be cooled. There, should the temperature continue to rise, the thermostatic switch turns on the cooling fan to add extra airflow and hopefully reduce the temperature of water in the system back to operating levels.

Cooling System - Water Hot, Thermostat Open, Bypass Closed

Coolant Plumbing Diagrams for Kit Cars

Cooling System Diagram (Revision 3)

The diagram above shows a physical representation of the cooling system as it should be. As this updated section is still "work in progress", my original diagram is below:

Cooling System Diagram (Revision 2)

With a heater unit in place, if the valve is open, you get a level of cooling from the heater matrix. If the valve is closed, you get no flow through the heater circuit. Therefore, if you do not plan on installing a heater unit, there are generally two recommended options with regard to this circuit:

1) Block or heavily restrict the flow of water through the pipe. This effectively simulates a closed, or almost closed, heater valve.

2) Block the right hand thermostat housing and the corresponding 'side exit pipe' connections (or just use a tee pipe) so there is no connection between the two. This simulates a completely closed heater valve.

Please note, this diagram was modified in 2004 and is now correct for the operation of the heater and radiator bypass circuit used during warm-up. The previous version on my site prior to this (without the "Revised" title) showed the heater connected to the wrong side of the thermostat housing.

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