I had been reading the FSM, my Hayes, and forums for the better part of a year trying to track down a coolant leak. After much effort I discovered it in the "injection" system and finally solved it. If you are confused looking at the FSM's this may be helpful.

The obvious place to start with a coolant problem is.. the coolant system. The FSM and others label several hoses you've probably already checked, e.g. in out of the radiator, water pump side, heater core in/out etc. You've also probably read up on weep hole and maybe even replaced your water pump to no avail.

Water started collecting on the driver side of the engine and thought it was an issue in the heater core return or *gasp* an engine gasket issue. I didn't have telltale signs of water in the combustion but was confused by a thermo sensor there that I thought might be my leak above the heater core return port, note the FMS and forums don't label or identify that part well either so it's hard to diagnose.

After some time the hose started to really fail and I could hear a hissing sound after running the engine under the injection system. For me it was not an obvious place for coolant to circulate but there are two rubber hoses at the top, middle of the rear of the injection manifold going into the "water thermovalve throttle body". F1-35 in the FSM is the only place I really see these hoses labeled. (pic attached). I'd assumed incorrectly the hoses were fuel related.

NOTE: Trying to get the clamp off of the hose without removing the injection system isn't possible. You need to remove the injection system to get to the hose that attaches vertically to a port underneath the manifold. I thought I'd had a vacuum leak but I readjusted my throttle cable and things more or less returned to normal. Good luck and ganbatte!