Outer Limits 4x4 Board

Ok guys my mq has been overheating a little the last few weeks and was drinking water but couldnt find a leak anyway few nights ago it went straight to hot and still couldnt find a leak, it took 5 litres of coolant then had to stop again on way home and fill up with water, got home and found a small leak from a cap near the welch plugs on the side of the block, its leaking up in the top left corner of the block from this cap but i dont know what it is? is it just another welch plug or sumthing worse?
any help would be great cheers