Leaking water from heater core 1999 gran prix

Go on U-tube and you can find several individuals that have videos that show step by step on how to change the heater core on your or any vehicle.

check a shop manual for the info

Make sure radiator is full and that all hoses are ok. Then make sure that the thermostat is opening and that your water pump is circulating coolant through the radiator which must not be clogged. Till you find issue run your heater full blast with window open to add additional cooling to engine.

May be none of those , just could be a temp door actuator.

behind glove box!
Inspect/Replace If it is necessary to remove the heater assembly, the cooling system must be drained before removing the heater core.
When a heater core leaks, a new heater core is installed or the old one repaired.
Heater Housing

• The heater housing is usually under the dash and must be removed to gain access to the heater core.
• Procedures for replacing the heater core vary with the year, make, and model of car. It threfore necessary to consult the manufacturer's repair manuals for the proper procedure for replacement.

The heater housing is disassembled to get to the heater core.
Heater Core

• Remove the access panels(s) or the split heater/air conditioning case to gain acess to the heater core.
• Remove the heater coolant hoses.
• Remove the cable and/or vacuum control lines (if equipped).
• Remove the heater core securing brackets and/or clamps.
• Lift the core from the case. Do not use force. Take care not to damage the fins of the heater core when removing.
• To reinstall the heater core, reverse the removal steps.

When the heater core leaks and must be repaired or replaced, it is a very difficult and time-consuming job primarily because of the core's location deep within the firewall of the car. For this reason always leak test a replacement heater core before installation.

If it is necessary to remove the heater assembly, the cooling system must be drained before removing the heater core.
When a heater core leaks, a new heater core is installed or the old one repaired.
Heater Housing

• The heater housing is usually under the dash and must be removed to gain access to the heater core.
• Procedures for replacing the heater core vary with the year, make, and model of car. It threfore necessary to consult the manufacturer's repair manuals for the proper procedure for replacement.

The heater housing is disassembled to get to the heater core. Heater Core

• Remove the access panels(s) or the split heater/air conditioning case to gain acess to the heater core.
• Remove the heater coolant hoses.
• Remove the cable and/or vacuum control lines (if equipped).
• Remove the heater core securing brackets and/or clamps.
• Lift the core from the case. Do not use force. Take care not to damage the fins of the heater core when removing.
• To reinstall the heater core, reverse the removal steps.

When the heater core leaks and must be repaired or replaced, it is a very difficult and time-consuming job primarily because of the core's location deep within the firewall of the car. For this reason always leak test a replacement heater core before installation.

The cabin filter might be clogged, try replacing it.

It sounds like either the heater valve which may be located inline with a 5/8" hose or 3/4" hose or the heater core is leaking, a heater core is like a small radiator.

There is a flat filter that sits between the firewall and the blower. Sometimes the filter is blocked and needs to be cleaned or changed. On older cars the filter is located on the firewall in the engine compartment infront of the blower. In the newer cars the filter is under the dash inside the car and much harder to get too.,

i do believe you have to take the passengers side dash apart behind the glove compartment. go to autozone and get a manual or go to the library and check the chiltons manual. this is not an easy job. how bad is the leak? if its not bad these heater cores are made of all aluminum. i would buy a bottle of that radiator additive and see if that takes care of the problem first. it only cost about 5 bucks a bottle and it may permanantly fix your problem.