If you have a 980C the engine is a 3406. The older 980's have D342 (I think thats what the number was) with external injectors similar to the 3306. This injector sleeve has a seal around the top to hold the water in the head as the sleeve is cooled that way.
Even if it is a 3306 the same applies to pre-combustion chamber heads.
A special tool is used to remove the sleeve or the cup, they are not expensive. The sleeve or cup has a metal washer at the bottom and should be replaced also.
If the engine is old or has been running hot these seals will fail and cause coolant leak. On the 3406 it will allow coolant into the oil since it is under the valve cover. A 3306 the cups are out side the valve cover and just leak outside.
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Thank for the info, maybe somebody change the engine because acording to the information you give me the cup its outside the valve cover and just its leaking outside
Another expert has told me, if this is pre-1983 it is a D336, this is the grandfather of the 3306. It is similar but, not the same engine. The cups are the same and seals are too. The tool is still needed to install them to the correct torque.
Thanks to Boxerman 1969