Coolant over pressurising

[Overfinch]

Hiya guys I wonder if anyone here can shed some light on my problem. I have a 2014 C4 GP 1.6 e-hdi and the coolant keeps over pressurising. Now, it's been in the garage several times and they have done every test possible to try and find out why. They have done pressure test, sniff test, compression test you name it it's been done which they have all passed. I have replaced the thermostat housing (twice), heater matrix, radiator, expansion tank cap twice and still it is over pressurising.
My last port of call to try is replacing the expansion tank and the cap again which I will do on Saturday. If that don't work then I'm at a real loss. Took it 2 garages and it's left them scratching their heads. They have flushed it out god knows how many times, they have refilled the coolant following the correct procedure and bled it by both bleed caps.
So now I'm at a real loss and I don't really want to get rid of the car as it's a great car. Any bit of info on this would be a great help. Thank you. Oh and on a side note, when I open the cap there's lots of pressure and the tank fills up again and can hear the coolant draining from the radiator.

[mickthemaverick]

What gearbox have you got and does it have a coolant fed oil cooler?

[xantia_v6]

What were the results of the pressure test?

[PaulC5]

Is it overheating, is the engine fan running, was the coolant pump replaced with the timing belt, have you watched the coolant temperatures using a code reader as the engine is running to see if the sensor is giving correct values ?

[RichardW]

Head gasket....

Not common but not unknown on this engine. If it's holding pressure when cooled down after a run, and / or it pressurises from cold start before it heats up, combustion gas is escaping into the coolant.

[mickthemaverick]

That was my first thought Richard but he said the garage had done sniff test and compression test and all was ok. Hence me moving to the gearbox oil cooler if he has one!

[Overfinch]

Hiya Mick, it's got the etg6 gearbox if that's any help. Xantia I have no idea, they just said all is ok. Paul, temperature gauge goes up to normal, they checked it on diagnostics and temperature was going between 90-98, it's not overheating at all. Fan kicks in ok low speed and high speed.
Richard, I can leave it running up to temperature and it don't pressurise, I go for spin then it does. When cold the pressure doesn't seem so high. Still get hot air through the vents, Aircon works great.
Anything to do with gearbox cooler?

[Overfinch]

Ah sorry, not sure if it's got oil fed cooler for gearbox and yes the had the water pump changed when the cambelt was done. This actually all started back in October when we had all the floods. I was at my sister's caravan in welshpool and went through a fair few puddles. Lost all heat and gauge went to zero, next morning gauge was erratic but got heat back, it was then the thermostat housing was changed but then the gauge kept going up and down by a notch or 2 either way.

[RichardW]

No box cooler on the EGS box.

Sniff test is not always that indicative on a diesel as they make no CO.

Could be the EGR cooler, but I would expect this to fill the coolant with soot.

Normally there is no pressure cold, pressure rises as the coolant heats up so there is pressure hot, then it goes away as the coolant shrinks as it cools. If there's pressure when it's cold, then some gas gas been introduced to the coolant circuit, and there's only really one place that can come from....

[Overfinch]

Thanks Richard, that had sprung to mind but a compressing test was carried out, both hot and cold but come back all good. Egr cooler could be another good point as I have read they do sometimes leak and a real pita to change. I got car back from garage yesterday with new coolant so I could keep an eye on it and see if any soot appears.

[moizeau]

A leakdown test would be more conclusive than a compression test.