Fiat X19 1500 thermostat brain-teaser

[Guy Croft]

I spent half a day (with no factory info to refer to) unravelling the mysteries of the, er rather 'complicated' (but very clever) X19 thermostat. When I finally realised I had it upside down, things began to make sense and 10 kettles of boiling water later now I have it.

Can you figure it out? Clue - everything is there for a reason. If you don't know the car, it's mid engined, transverse. The two pipes on the housing go to the front of the car where the radiator is. The other big outlet is connected to the water pump on the inlet/ex manifold side located facing the rear end of the car.

Use Photoshop or similar to mark the coolant flows and stat behaviour at various temperatures. Don't try writing without pictures, even I don't have the time (patience?!) to fathom out replies that read 'it goes in the top hole and out of the other one' etc etc..!

The nice ending to the story is: having worked out the way it operates - I now have low temp stats available for competition units of that type.

GC

[James Bowen]

Guy,

OK I think I may have a chance with this one.....

The simple answer is that the thermostat not only opens to let water pass, but at the same time it closes or blanks off part of the thermostat housing. I have done the below photos with photoshop to illustrate

By the way, it might be more helpful to think in terms of the water pump pulling the water.

Thermostat Closed

X19%201500%20thermostat Closed copy.jpg (258.58 KiB) Viewed 15450 times

Thermostat Open

X19%201500%20thermostat Open copy.jpg (271.25 KiB) Viewed 15446 times

Top hose to the Rad is always open.

When the thermostat is closed, the pump draws water from the cylinder head through the pump into the engine block.

When the thermostat is open, it also closes or blocks the draw through the thermostat housing, and draws water instead from the Radiator bottom hose then through the pump to the engine block to the cylinder head to the Top hose to the rad etc etc...

Partial operation is a combination of the above. Can I have a discount from my thermostat that I will now buy from you please!!??

Regards, James

[DamirGTI]

Hello Guy !

Ok. i will try :

It's an "dual function" bypass type thermostat right ? it connects upper and lower radiator hoses via the bypass hole inside the thermostat housing .

So when the engine is cold , when we start the engine from cold , thermostat is in closed position (pic. 1) ,bypass passage is open and coolant circulates trough the engine via bypass passage and back to the water pump which then sends the coolant back into the engine without it being coold down by the radiator .
As the engine heats up the hot water slowly starts to open the thermostat which then goes up near to the bypass hole and slowly blends (balances) hot water from the bypass passage with the cold water from the radiator (pic. 2)
Then when the water heats up beyond 90deg. it opens the thermostat fully which then goes up to the top and closes the bypass passage allowing coolant (now fully blended and mixed with cold radiator water) to flow trough the radiator for maximum cooling and back to the water pump - block (pic. 3)

pic 1:

pic 2:

pic 3:


Right or wrong ?!

Regards !
Damir

[SteveNZ]

I have never seen an X19 thermostat before. It is different to every other Fiat thermostat I have seen as it is fitted in the return path of the coolant circuit rather than the outlet. There must be a reason for that as all other SOHC's have them fitted to the outlet. Maybe it makes it easier to bleed? Most manufacturers switched from outlet placement to return in the 90's or earlier.

The most important aspect of that design is to get a thermostat with the correct by-pass valve. A by-pass valve that is too short will cause overheating or give poor performance at least. You can not trust (afermarket) suppliers' listings on this. I have found them to be wrong a few times. e.g my Delta turbo engine. I alwasy measure them hot and cold

[TC1780]

This is a "shunt-valve" thermostate. It uses the principle of keeping the flow the pump is generating constant, and route the flow to the cooler or by-pass it according to the cooling demand in the moment. Common thermostats just choke the flow, only allowing flow through some pre-defined leak-through holes. Shunt valves are a very common in cooling and heating systems for industrial use, and are considered the correct way to regulate a cooling/heating circuit.

I've got an Ascona B with shunt regulated cooling. I scratched my head more than twice when I mounted that, as it came with an injection system I retrofitted.

[mdburchette]

Here's my take on it.
When cold, water flows from block to water pump.
When water gets to 172f to 183f (78c to 84c) t-stat partially opens and water flows to upper radiator hose and from lower radiator hose and from block to water pump outlet
When water gets to 194f (90c) t-stat fully opens and causes water from block to flow through upper radiator hose and water to come from lower hose to water pump.

Fully closed


partially open


Fully open

[Guy Croft]

Wow, great response, well done all!

Now I've printed off each reply I've got some marking to do!

GC

[Julian]

It is a fun little setup but it shouldn't have strained the grey matter too much (unless you happen to be my old car preparation expert in which case you blew up a few engines first and blamed Fiat for bad head castings)....

For bonus points (from me only) if you were to remove the thermostat and replace the mechanical water pump with an electrical equivalent where would you put it and why?

[Guy Croft]

Well it puzzled me a bit, or should I say annoyed me because I had to figure out how it worked on my own on a typically fraught and busy day when I'd much rather have doing 'paying' work!

Anyhow, smarting at my lack of 'grey cells' and in a rare breach of protocol (ie: faint attempt at humor )Julian I shall simply say that I don't know of any electric pumps that are the 'equivalent' of a mechanical one! Alternative to, maybe.

GC

[Julian]

I admit it - my sense of humour is typically best described as dry (as in dried out)...

An "alternative" water pump would indeed be a better description.