No coolant flow

Coolant Valve? Don't believe there is one.

http://trailers.com/tow-capacity/1995_Pontiac_Grand_Prix_1620.html
have look to this link
hope it helps

Yes, the coolant flows from the lower rad hose to the water pump.

Your coolant could indeed be low. Double check the level. Often when the cooling system appears to be full, it drops when the thermostat opens and the coolant flows into the engine.

CHECK COOLANT LEVEL.IF COOLING FAN TURNING ON COOLANT TEMPERATURE SENSOR WORKING.BECAUSE IT CONTROLS AND TURN ON COOLANT FAN AT A SET TEMPERATURE.SOUND LIKE ENGINE RUNNING TOO HOT. HAVE RADIATOR FLUSHED.REPLACE THERMOSTAT. HAVE ENGINE BLOCK FLUSHED.REPLACE RADIATOR PRESSURE CAP ALSO.SEE WHAT HAPPENS.

Maybe a backflushing of the cooling system can help to make it cool more efficiently.

Sediment can build up in the cooling system causing a lack of flow of coolant.

Also, the thermostat could be stuck closed, as others have suggested.

maybe a water pump.

you nave to determine where the problem is. This could be caused by many things here is a list that I can give you but can't be sure as I don't know what was all done when the pump was replaced. Some mechanics don't do the same things when they repair or replace water pumps. Here is possible causes, Plugged catalytic converter Could cause overheating. This could of caused the pump to fail as it would of caused the engine block to get hot and boil the coolant out from the block. As well when the mechanic changed the pump he should of done the following if he inspected the following.

1. Rod out radiator and use scaler for engine before putting in antifreeze.

2. Replace water hoses that may have separated and collapsed

3. Replace Radiator cap Because if the cap has a weak spring allowing coolant to flow into overflow tank and thus, after several cooling and heating periods over a few days causes you to lose coolant.

4. A loose water pump belt. Or a spring tensioner worn out.

5. If it has a automatic Transmission could have plugged oil cooler which could cause the engine to overheat.

6. Oil cooler plugged can cause this as well.

7. A/C Malfunctioning causing the condenser or evaporator to heat up water.

8. Plugged condenser with foreign material it it not allowing correct air flow.

9. on some models fan clutch worn out.

And the list could go on this is not a full list of what could cause it but gives you an ideal how far ranging the problem could be. Hope this helps thanks for using fix-ya.

Hi:
The mass air flow sensor is a round sensor that is mounted in the air intake, between the air cleaner box and the engine throttle body.

change the sending unit to the fan, if that doesnt work check for power too the fan or even a bad fan