Leaking injector sleaves in the head are most likely to be your problem. To test the sleaves you need to remove the fuel lines at the injectors.Do this on one side or the other of the engine, remove radiator or coolant jug lid apply air into the injector you should get bubbles in the coolant, do both sides to make sure the sleaves in both heads are good or bad. The only fix is to remove and replace all sleeves on that side. Hope I have been helpful in your ventures let me know what you find and if I can help let me know.
