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Hopefully i can help..
If there has been no previous work done on the coolant system to maybe suggest and airlock somewhere then going by your description with the excess pressure via the cap "check the condition of that" could most likly be the early stages of headgasket failure..quite often if coolant is being forced out via the bottle or there is an increase in pressure in there or bubbles seen..then it can suggest that cylinder compression/exhaust gases via the breached section on the headgasket are finding there way into the coolant system and thus you get over-pressurizing of the coolant system thus the cap being safety release valve will or should allow the excess pressure/coolant to force it way out there or any other weak point in the system ie: blowing a hose of or bursting the radiator...One test apart from a normal cylinder compression test. is to have a chemical block/Dye test done on the coolant system..this will test for any presence of exhaust gases in the coolant system by fitting the kit which the blue test dye goes into over the expansion tank neck then fill it with the blue test fluid..run the engine and watch to see if the fluid turns green for diesel engines.if so then you can confirm the headgasket is at fualt and needs replaced..the head itself will also need to be pressure checked too...so it maybe worth having the block test done first and then the cylinder compression test done on all 4 cylinders and see what results that comes up with...Alternatively you could have the system pressure checked to ensure the pump is actually performing as it should as it isnt unknown for the pump impeller to fail on these / or the system drained or flushed out of any blockages in the system especially the radiator back flushed /thermostat replaced and re-filled with the correct ratio of antifreeze and water the system re-bled of any air pockets and see wether the fault re-occurs..after doing that..if the system is still pressurizing then the dye test etc would be next to check for the above..I do hope this helps
Thank you for your response. I will come back to you after continuing with further diagnostic checks.