So I have been looking at this for a while. First, that is a lot of ports and hoses and people thought Fiats were complex cars.
I have never played with one of these cars so what I say could well be complete BS, but I have thought about it pretty carefully and I think I understand the general behavior of the system.
So we have the block, the head, the thermostat housing and the return pipe as the primary exits and returns for the system. We have two primary external elements, the radiator and its associated pressure cap and overflow and the vehicle's HVAC system. There are also two secondary engine mounted systems served with coolant, the oil cooler/heater and the intake system.
The water pump feeds coolant directly into the block. The block feeds coolant into the head and has one outlet into the oil heater/cooler besides the distribution into the head.
The pump is fed by the coolant return pipe. The coolant return pipe has two returns along its length, one from the oil heater/cooler and one from the intake with a large feed from the thermostat housing at the end.
- The coolant collected in the return pipe which comes from a variety of sources, the liquid returned directly into the pipe itself is cooled by the intake manifold and the oil cooler/heater. Eventually these two sources get to a fairly steady state of temperature as the engine reaches operating temperature.
- The end of the coolant return pipe is fed by the thermostat housing and has the greatest temperature variation of the coolant due to the variety of sources feeding into the thermostat housing.
The thermostat housing has four returns into it, the return from the radiator on the end of it and three returns into the vicinity of the thermostat's spring and regulation wax housing. The returns in the vicinity of the thermostat's wax housing are the means of introducing the heated water to cause the thermostat to open.
- Starting nearest the thermostat is a return from the head, the primary bypass, this delivers hot water directly from the head onto the wax capsule of the thermostat and has the largest effect on its operation.
- Next to it is the heater return line, this line either delivers no flow if the heater is off and therefore no effect on the thermostat or it delivers cooled water to dilute the hot water from the main bypass and causes the thermostat to open more slowly.
- Lastly there is the bypass from the head directly that ensures there is flow at all times from the firthest part of the head right next to the heads outlet to the radiator.
- The largest return into the thermostat housing is the radiator return directly onto the base opening of the thermostat body and flow is enabled when the thermostat opens.
The head has five feed ports from it that feed two bypasses, the heater core, the intake system and the largest outlet, the radiator. It also has an air bleed port.
- The largest outlet goes to the radiator. The radiator has the filler with pressure cap and overflow bottle, the hot feed from the engine and the cooled return to the thermostat housing.
- The next largest outlet is the primary bypass that feeds directly onto the thermostat in the thermostat housing.
- The other two outlets are the same size. One feeds the intake system and returns the coolant into the main return tube to the water pump.
- The other outlet feeds hot water from the hottest end of the cylinder head into the thermostat housing just beyond the heater return.
I won't delve further into the intake or the oil heater/cooler system as they don't have much effect on the thermostatic system.
So how does it work?
When cold, there are two direct high speed flows from the head into the thermostat housing and directly onto the thermostat, which as the engine heats up causes the thermostat to open. The thermostat opening allows coolant from the radiator to flow back into the engine and causes significant flow (at a slower rate) into the return tube to the water pump. This flow is tempered by the hot bypasses from the head, however due to their constrained diameter, they deliver much lower flow once the thermostat is open and have less effect until the thermostat starts to close due to the cold return coolant effecting the thermostat.
So how should the system be plumbed to the X's existing system. This is my view and how I would start working towards a solution:
- Connect all the hoses in the standard Honda way to and from all of the outlets and inlets of the standard system.
- Connect the primary exit from the head to the the existing X pipe to the radiator.
- Connect the thermostat housing return to the the existing X returnpipe from the radiator.
- Connect the head heater outlet to the heater to the existing X heater supply
- Connect the heater return hose from the X to the thermostat housing's existing heater return.
This all makes sense and is pretty simple. So how do you fill the system and manage the pressure cap which on an X is part of the expansion tank?
On a Honda the radiator has a pressure cap and a non pressurized expansion tank that captures expelled coolant and also allows that coolant to go back into the system. On an X we have a tank with some expansion space with an air volume capped by a pressure cap and an overflow that just loses coolant when the system exceeds its volume or pressure.
You could omit the X expansion tank which has a primary larger hose and a smaller return pipe. I don't think deleting this is a good idea as all coolant expands and contracts so eventually without the tank something will burst. So it needs to be kept.
Where in the system should the expansion tank go?
- I would plumb it into the outlet from the head that leads to the radiator and the smaller diameter return I would plug or possibly plumb a small line into the return line from the radiator. Why? This is the part of the system which will see the largest amount of expansion and contraction as well as the greatest heat variation. I am tempted to suggest that one shouldn't use the small return line into the expansion tank. If you don't use the small return line I would add a fill point as I previously suggested on the heater outlet from the head to fill both sides of the system.
- Alternatively I would plumb the expansion tank into the main outlet from the head to the large hose inlet to the expansion tank and I would then plumb the small line of the expansion tank into the return line of the heater return line close to the thermostat housing. This ensures that there is pressure relief from the high temperature side and a return from the low temperature side of the system which goes to the highest point (just below the cap of the expansion tanks pressure cap) that could allow trapped air in the thermostat housing to escape into the expansion tank as the system is filled.
I would tend to do the latter alternative (main head outlet to the larger expansion tank inlet and a hose to the heater return to the small return on the expansion tank) rather than the former. I think it would be the closest to the way the X system currently works and would allow the system to self regulate the trapped air issues.
This is my understanding, you may see this differently. I hope it helps.
Karl
