Hi, wonder if anyone can shed some light on what they think my problem may be. I have a 1.6TDI 2014 and the temperature gauge is going up and down like a yoyo. Doesnt overheat but gets up to temperature then within a couple of minutes indicates cold, heater also blows cold at this point. Gets up to temperature again(heater now warm) and then goes back to cold, repeats.Does this every journey. The water expansion tank is pressurised too much and when the cap is removed at cold, water rushes back into it. Ive had the thermostat and sensor housing replaced, water pump also. It doesnt appear to be losing any water, water isnt milky or contain any visible nasties and the car runs perfectly. I'm just worried its going to blow a hose if i carry on driving it with a lot of pressure in the system, any ideas??

Sounds like an airlock try running the engine up to temp stationary and with the expansion tank top of you should see the  water level drop then top up repeat as needed if you are lucky you can keep it topped up through out the process avoiding the sudden drop in level which may give you an airlock again.

I'll give it a try, nothing to lose. Report back with my success or failure.

Jus tturned the car to face uphill and removed the cap while still cold. Water began to leak from underside of the expansion tank but stopped if i screwed the cap back on. Having removed the tank there is a small hole next to what i assume is the heater pipe. Looks like its meant to be ther but im wondering should it have a plug in it, why is it there? I thought expansion tanks were meant to be sealed and any pressure releif was done through the cap? Pictures of the bottle attached, anyone got any ideas?

Just realised its the outlet for any water getting past the pressure releif in the cap. Schoolboy mistake.

Bump, no one have any ideas at all? Could be time to get rid if she can't be fixed, might end up paying a fortune in garage bills :(

2 hours ago, foresterboy said:

Bump, no one have any ideas at all? Could be time to get rid if she can't be fixed, might end up paying a fortune in garage bills 😞

Have you tried my suggestion about bleeding it and air locks.

It would be worth flushing the system through with a garden hose, and then running it for a week without the thermostat to get rid of air locks. See Scotty Kilmer on YouTube for how to do this. I think the high pressure is due to trapped air expanding and going through the thermostat.

22 hours ago, Kevin#95 said:

It would be worth flushing the system through with a garden hose, and then running it for a week without the thermostat to get rid of air locks. See Scotty Kilmer on YouTube for how to do this. I think the high pressure is due to trapped air expanding and going through the thermostat.

It's the steam that cause the pressure build up.

Have you replaced the screw on cap on the expansion tank. It has a valve to release pressure above 15psi. It might be faulty.

Other possibility is there is a split in the head gasket allowing exhaust stroke air to be pushed into the water. You won't see any water in the oil or white smoke, but will get extra pressure in the water when cold and bubbles forming in the expansion tank when hot.

I think it is the head gasket is split. On the exhaust stroke compressed air is pushed into the water coolant and this causes it to froth with bubbles. The water pump then can't pump because the bubbles cause cavities. The water overheats and temp gauge goes high. The thermostat opens and steam is released into the expansion tank and pushes cold water from the radiator into the engine, and the temp gauge drops down. 

You might notice the exhause sound popping, but no white smoke comes out. If the split gets bigger water might get sucked into the cylinder and come out as white smoke.

2 minutes ago, Kevin#95 said:

I think it is the head gasket is split. On the exhaust stroke compressed air is pushed into the water coolant and this causes it to froth with bubbles. The water pump then can't pump because the bubbles cause cavities. The water overheats and temp gauge goes high. The thermostat opens and steam is released into the expansion tank and pushes cold water from the radiator into the engine, and the temp gauge drops down. 

You might notice the exhause sound popping, but no white smoke comes out. If the split gets bigger water might get sucked into the cylinder and come out as white smoke.

Get a sniff test done that will confirm the head gasket theory.

Thanks everyone for the suggestions, it will get me started. I replaced the cap, still the same. Ran it without a cap on, still the same. Its also been suggested that it may be the EGR cooler cracked letting exhaust gasses into the water system, so i have plenty to look at. If and when i get it fixed i'll post the cure here!

Replaced the EGR cooler, still the same, head gasket, still the same!! Sold the car in the end

Hey did you find out what this was at all or any other options... My car is doing exactly the same 

On 2/24/2022 at 5:49 PM, Dazman72 said:

Hey did you find out what this was at all or any other options... My car is doing exactly the same 

I am having a same issues too can't decide what to do on this thread my storey

Ford grand c-max heating issues | Ford C-Max Forum (cmaxownersclub.co.uk) 

On 4/11/2020 at 12:41 AM, Kevin#95 said:

Other possibility is there is a split in the head gasket allowing exhaust stroke air to be pushed into the water. You won't see any water in the oil or white smoke, but will get extra pressure in the water when cold and bubbles forming in the expansion tank when hot.

I agree... the cooling circuit is closed and works under pressure when it's hot, but the pressure should go down when it's cold. If there is extra air in your circuit, they only can come on it from the cilinders through split in the head gasket... I think the water expansion tank cap have an extra pressure valve. It will leak extra air (from exhaust) with a bit of water, specially after a while running at high rpm...

I have the same issue with my ford c max 2015 1.6 diesel coolant expansion tank pressure build up.

On 2/24/2022 at 5:49 PM, Dazman72 said:

Hey did you find out what this was at all or any other options... My car is doing exactly the same 

My is the same🤦