I have a 1994 6X4 with the Kawasaki FD620D-JS11 and had the same problem. After I removed the water pump cover to check the impeller, I could never get it to circulate the coolant properly. I must have filled the radiator 5 times, ran it with bleed screw cracked opened to let the air out. I squeezed the hoses, parked it on a slope while filling it with coolant and it still ran hot. So I could not use the Gator for the weekend. I ended up correcting the problem when I got back home by:
1) Parking the Gator on a flat surface
2) Went to the Dollar General Store (or similar place) and purchased a plastic squeeze condoment bottle that has the removable pointed nozzle on top. It might have cost $2.50. Then snip the nozzle tip open enough where it will fit down into the bleed screw hole and seal.
3) Open and remove the vent screw on the on the rear head of the engine. Do not start the engine !
4) Fill the radaitor and top it off several times while letting the air escape through the open radiator . Leave cap off for now. (very important)
5) Fill the coolant recovery tank at least half full so the coolant level is at least half way up the pickup tube.
6) Take the plastic condoment bottle filled with coolant and insert the nozzle into the bleed screw hole. Repeatedly squeeze the coolant from the bottle into the bleed hole. You might have to refill the bottle a couple of times.You should hear the radiator burp when the the air pocket escapes.
7) You may have to repeat step 6 two or three times to get all the air out.
8) Reinstall bleed plug and tighten.
9) Make sure radiator is completely full of coolant and put the cap back on. Check recovery tank level as well.
10) Crank and run the engine at a fast idle and after a couple of minutes, check to see if the hoses to and from the radiator start to get warm. It will take several mintes for them to heat up.
11) Do not remove the radiator cap as that will introduce more air back in system.
12) Make sure the recovery tank is at least half full. Adjust the level if needed as the radiator can draw coolant from the recovery tank if it needs it.
13) The radiator fan should kick on in about five minutes and cycle as normal.
I learned this by trial and error. I kept taking the radiator cap off to see if it was moving the coolant. Well every time I did that, it let air back into the system and would never bleed out. Then I thought about how I could force coolant into the system and bam !
I had been at my place in the mountains of North Carolina when this happened. When I got home I read my JD Tech manual and learned about not opening the radiator. I came up with the idea of using the squeeze bottle myself. Next time I was at my dealer talking to the shop foreman, I told him what I had done and he said that my way sounded a lot easier than jacking the front up and all. It turns out that it has to do with the radiator and the water pump being on almost the same plane and that allows air pockets to form when the cooling system is opened up.
Let us know if this remedies your problem. It did for me as I have not had an issue since !
Thanks for reading and happy Gatoring !
Bob