Cat 966B misfiring

Hi recently I posted regarding my 966B loader was misfiring, first thanks to all who replied, I posted that I started up my loader and it was misfiring, I had previously topped up the radiator and while it was running water started to overflow thru the top, thinking this was because I put to much coolant in it was the reason for the overflow,,, so perhaps it's a injector and cracking the nuts on the fuel pump the one on the left made no difference to the running if the idling misfiring engine,,, now I restarted it and still misfiring , however the radiator water did the same , that is overflow with cold engine after ,90 seconds if starting, there was bubbles but more like one big one pushing water upwards,,, I looked on the dipstick and no visible signs of water and nothing under the oil filler cap, very clean actually,, no oil in radiator water...one member said it may be the combustion chamber... . Now I am thinking Head gasket or the worst, cracked block or head... The loader was running perfectly when I stopped it a week before...never gets hot, can near in hold your hand on the engine and radiator water is not all that hot....... ...