Not sure if already mentioned but try starting with the cap off and checking for bubbles. If bubbles present a fairly bad sign that the head gasket has failed.
While you have the cap off wash it under the tap then check the spring still works well, also try blow through it from the seal side...
I have recently picked up a 1994 FZJ80 mostly as a tow wagon for my track car but have been enjoying a few trips out the dunes and tracks and am getting more in to this as a side hobby.
My issue with the 1FZ is it chews gas(like nearly 20L/100km) and seems gutless for a 4.5L
I have been thinking...