2002 Pontiac Grand Prix is loosing coolant and having to add every 3-4 days. No leaks are found. After running car for some time there seems to be no pressure under the radiator cap and its cold.

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.

Do you have any check engine light on or maintainance light on? On some occasions the fuel filter are clog up or vacuum leaks.

You should look under the car after it sits a while and see if there is antifreeze on the ground. If so try to see where it could be leaking under the hood directly over the puddle. You could have a leaking water pump. If so there would be dampness under the right side of engine compartment. Also check the radiator hoses at front of car.

you may have a bad temp light sensor in the motor or gauge.170 is not hot at all for these cars.

You need to replace the radiator thermostat and water temperature sensor and flash out old coolant.

your water pump is starting to leak from weep hole

check you climate control vacuum hose. it may have come loose or crimped. its located beneath and under battery mount box. also check to see if your coolant resvouir is full as well as your resivour cap is tight.

It is likely the heater core.It is possibly a 4 hour or less job

Did u check ur front floor rug? Coolant is circulated all threw the car there are two hoses that attach to (motor) wall that can leak inside.

I assume it's a V6, I know from working on Fords and when we would refill the radiator on a V6, we would take the upper hose loose from the radiator and back fill it that way. Hope this helps