Hiya, 

In the Youtube video link https://youtu.be/XTPvOdnKrkA you can observ please  that I loose water/coolant over the coolant tank cap on Fiesta 2004. Some details :

- in only happens when the car is parked with the engine on for more than ~ 5 minutes after a short drive. There is no lights coming on the dashboard, temp display says normal. 

- while driving there are no issues regarding engine temp and the car runs perfect. No lights coming up on dashboard, temperature says normal. If I stop the engine right after a drive, there is no boiling and the cooling tank can be touched, it's not mega hot. 

- there are no other coolant leaks anywhere on the car, just trough the cap

- the cap is properly tightened 

- if I leave the engine on for more then 10 minutes while parked, the whole water boils away trough the cap and I have to refill. 

- weirdly, after about 20 miles of driving the gearbox gets stiff. It's possible that the coolant /cooling system heating up transfers some of the heat to the gear box? 

Cheers 

Cheap and easy try a new OEM cap. Eliminate the easy one first.

Yeah, found one at £8. Should I also try to run the engine while stationary without the cap on? 

1 hour ago, Sss said:

Yeah, found one at £8. Should I also try to run the engine while stationary without the cap on? 

If you think there might be air trapped in the system, yes. Otherwise probably not. 

Make sure the engine is cold before doing this. 

Replaced the cap with a new one. Now I observed that there is a small crack in the coolant expansion tank. The crack is on top right below the cap. Again, after aprox 10 minute while parked and engine ON the coolant starts to rise and comes out trough the crack. Is the small crack the reason for coolant rise because there is a pressure issue? Or the thermostat is defective too? 

Check that the water is returning to the tank ok through the small pipe , buy a new expansion tank there actually not that expensive quite a common fault

How do I check if water comes trough the small hose? 

Found a tank on Amazon for £17. I think I could change it myself. Just 2 screws and 2 hoses? Will have to empty the water in a bucket from the large hose in the bottom? 

2 hours ago, Sss said:

Replaced the cap with a new one. Now I observed that there is a small crack in the coolant expansion tank. The crack is on top right below the cap. Again, after aprox 10 minute while parked and engine ON the coolant starts to rise and comes out trough the crack. Is the small crack the reason for coolant rise because there is a pressure issue? Or the thermostat is defective too? 

First, you need to purchase and fit a new tank then run the car again. The fact the tank is split the system can't pressurise. 

If the tank can't pressurise then water will boil at 100 deg C.    Under normal pressure it boils at a much higher temperature.   Similar to the way a pressure cooker works.   This is why you should never remove the cap when the engine is hot.   If you do, the water will immediately boil and shoot out under pressure.

Just pull the small black pipe off at the expansion tank and put it lower than the tank start the engine and watch for the water coming out of it , that will let you know the water is circulating ok 

Is it easy to change the tank? Can  I do it? Just ordered a new one, should be here on Monday. 

yes should be no hassle for you easy as to do ,  do not connect the small return pipe till you have added the antifreeze and start the engine and let it run topping it up till the colour of the antifreeze comes out the small pipe , this eliminates the need to bleed the system as the small pipe is the last part of the coolant return system , don't bother draining it down just pull the expansion bottle lower pipe off and connect the new bottle ,good luck

Still waiting for the new tank to arrive. I pulled out the small hose and it evacuated out all the water from the tank with engine ON, so this means the water pump works OK? And how can I check if the thermostat is working properly? The temperature on the dashboard is always OK. When starting the car in the morning it's way low on the graph and slowly ascends to normal  after a few minutes of driving. 

Changed the cooling tank with a new one. I left the car with engine ON in idle for 5 minutes, but I'm afraid to leave it more. If the boiling wasn't due to the old cracked  tank, can the pressure destroy the new tank or hoses? 

Problem solved. It ran almost 1 hour at the MOT test, no issues with the water having the new coolant expansion  tank fitted. Thanks everyone. 

Water bottle cap pops off changed the thermostat and fan switch but same thing still happens