01 neon problem

:banghead: i have just purchased a 2001 plymouth neon and have some issues that need resolved. i have this hissing sound that started from the passenger side of vehicle, then water flooded my passenger floorboard. Can anyone offer some trouble shooting for me as to what it may be?

First check whether the water on the floor is just water - or is it really engine coolant (ethylene-glycol/water mix). If its engine coolant, you either have a leaking heater core, or a leak in one of the tubes running to it. I would have expected it to cause a steamy cloud when it happened, and to have had a noticeable 'sickly-sweet' smell. And you need to ensure the car still has coolant in the motor, or else add some. Ours is a 97, but I think the later ones are similar: The coolant overflow bottle at the back of the engine compartment should still have liguid in it, and when you take the radiator cap off - it should be FULL under there, with greenish coolant.
I guess the hissing sound was on the inside of the car? The most common hiss is from a vacuum leak in the heat/a.c. controls, like from a hose coming off, etc. Vacuum acutators are what open and shut the heater doors to control where the warm/cool air blows out.
If the water on the floor was just water - I would presume it leaked in from the cowling at the base of the windshield. This can happen after heavy rains if there are leaves or such blocking the usual drains (down into the fender usually). It may have occured suddenly from the heating system switching from the default 'off' position to the 'heat' position - which will happen when the motor is started up.
If what is on the floor is engine coolant, then the hiss could have been the sound of it coming out - under pressure and boiling hot.
If my car had a cooling system leak in the heater, I would do an interim fix: by connecting together (under the hood) the two rubber tubes running from the side of the motor, back toward the firewall. Getting a heater core fixed means removing the dashboard, and heater/a.c. evaporator - not rocket science, but it sure consumes the hours and causes back aches.
If my car had a leak at the cowling under the windshield, I would poke around to see if there was a mass of dead leaves blocking a drain.
There is also a water drain for the air conditioning system. I think it can be reached from underneath when the car is raised up on a lift - and the end of the rubber fitting can be cleaned out.

this may not help much, but i had something similar to that with my 2001 neon. i would hear a sizziling sound that was identical to water being put onto a hot pan, then i would notice water on the floorboard and it only happened after it would rain. after inspecting my windsheild, i noticed that the rubber lining that fits between the glass and the hood wasnt in place. so as soon as i reinstalled it, the sound and the water disappeared.