My 2003 4.8 Lt is loosing coolant out of the vent in the reservoir.

1. Engine Coolant Temperature.
If the thermostat in the cooling system is getting stuck in the open or partially open position rather than opening and closing as needed to keep the coolant at the required operating temperature then you will get the results you have described here with the coolant temp dropping as you drive. The radiator is providing optimum cooling when the car is moving and there is plenty of cold air flow through it.
I am assuming the temp gauge in the vehicle is reading correctly.

Thermostats in the cooling system do become faulty in time and it may well never have been changed (or only changed once) since the car was built and is possibly gummed up with debris. With those symptoms I would definitely change the thermostat. Make sure a full flush of the cooling system is done at the same time and refilled with fresh coolant.

2. In Car Heater.
I would not worry about the interior heating until you change the thermostat on the cooling system. However if you continue to get cold air from the heater then there will be an issue with hot coolant from the cooling system being restricted in flowing through the heater core.

AS FAR AS THE OVERHEAT ISSUE BLEED THE AIR OUT OF THE COOLING SYSTEM. FOR THE NO HEAT CHECK OPERATION OF TEMP BLEND DOOR AND FOR CLOGGED HEATER CORE

sounds like air in system ...
Leave the radiator cap off, turn the engine on and let it run until the radiator "burps": You will see the coolant level drop and may see or hear a large air bubble come to the top as the system burps.
Keep an eye on the temperature gauge throughout this process

Refill the radiator to the top and coolant reservoir as needed. Put the radiator cap back on. Note that if the engine runs hot after this procedure there may have been another pocket of air that "burped." Let the engine cool down and then add more coolant to both the radiator and the coolant reservoir.

Which hose came off? If it was the top radiator hose, is the nipple on the radiator still intact? The failure mode for these plastic radiators is that the top hose nipple crumbles from the inside and blows off.
Did you bleed the cooling system through the bleed screw next to the fill point on the radiator? Normally, when these cars overheat after coolant is changed, it is because there is air in the system. You need to turn on the key and turn the heater all the way up so the heater valve opens, then add coolant to the reservoir until pure water comes out the bleed screw hole. The reservoir will appear to be overfilled, but any excess coolant will push out the first time you drive the car (oops, is EPA listening?)
These cars do tend to loose thermostats more often than other cars for some reason. The way to check that without taking it apart is to pull the top radiator hose off at the radiator and put it over a bucket, and start the car. (do this only after you have bled all the air out of the system) About the time the temperature gauge starts to climb, water should start to trickle out the top radiator hose.
If it appears to be bad, after you replace it, boil it in a pan of water to see if it opens. If it does not open in boiling water, you know that you found and fixed your problem.

You probably need to bleed the cooling system properly.

Open the cooling system bleeder screws.

Slowly fill the cooling system with a 50/50 coolant mixture until coolant runs freely out of bleeder screws.
Close the cooling system bleeder screws
Install the coolant pressure cap
Start the engine.
Run the engine at 2,000 - 2,500 RPM until the engine reaches normal operating temperature. (midway on gauge)
Allow the engine to idle for 3 minutes.
Shut the engine off.
Allow the engine to completely cool.
Top off the coolant as necessary.
Run engine again at 2000 - 2500 RPM until the engine reaches normal operating temperature.
Allow engine to idle until cooling fans come on. It should come on with the gauge about 3/4 of the way to red. - if it gets to where is touches the red, shut down immediately.
When the fan comes on, put your hand behind the fan and observe the temperature of the air coming from the fan. It should feel hot. If it is blowing cold air, the coolant is not circulating through the radiator like it should. This could be caused by lots of things including an improper/defective thermostat, corroded water pump impeller, or a blown head gasket.

Welcome to FIxYa.com

How to Fill Coolant

If the coolant system has been flushed, first add 100 percent ethylene glycol in

the amount listed for the appropriate engine application. Then slowly add clean

water to the system until the level of the coolant mixture has reached the base-of

the radiator neck. Wait for two minutes and recheck the level of the coolant

mixture. add clean water if necessary to restore the coolant mixture level to the

base of the radiator neck

When refilling the engine cooling system, add three engine coolant supplement

sealant pellets, GM RN 3634621 or equivalent, and GM approved new Ethylene

Glycol Coolant DEX-COOL GM RN 9985809.

If the cooling system is being refilled ONLY (no flush), a 50 percent ethylene

glycol and 50 percent clean water mixture should be used to fill the system. Fill

the system until the level of the 50150 mixture has reached the base of the radiator

neck. Wait two minutes and recheck the level of the coolant mixture. add a 50/50

ethylene glycol water mixture as necessary to restore the coolant mixture level to

the base of the radiator neck.

Install the radiator cap making certain the arrows line up with the overflow tube.

Close all air bleeds. DO NOT over torque the air bleed valve. The air bleed valve is

made out of brass.

Fill the coolant reservoir with a 50 percent ethylene glycol and a 50 percent clean

water coolant mixture. The proper amount of this mixture to be installed into the

coolant reservoir is listed below:

Add 0.65 liters (0.68 quarts) of a 5010 mixture of ethylene glycol and clean water

to the coolant recovery reservoir.

CAUTION: The low coolant indicator lamp may come on after this procedure. After

operating the vehicle so that the engine heats up and cools down three times. if at

any time the "TEMP' warning indicator comes on. Immediate action is required.

Air bleed vent is at the thermostat house

Thank you for using FixYa.com

Regards, Lee Davidian

I dont think the coolant is boiling, I think you have an air bubble in your system wich is causing a air lock and youll need it flushed out. Your reservior cap should be tight always.. never loose this is an air tight system and your allowing it to get air in it. I have a 2002 linc ls and just had to pay 1000.00 to replace the system because I let an air lock go for too long and had so much back pressure I blew up my reservior.

You could possibly have air in your cooling system. If the dash gauge temperature sensor does not have coolant flowing past it, it will give a false temperature reading while your engine is actually overheating. If you recently drained and refilled your cooling system, you might need to bleed the air out of that system. If you didn't drain it recently, you might have a leak that's letting out coolant and letting in air.

A complete drain and refill of the cooling system at 50,000 miles (80, 000km) and afterwards at least every 30,000 miles (48,000 km) or 3 years will remove the accumulated rust, scale and other deposits. The recommended coolant is a 50/50 mixture of ethylene glycol and water for year-round use. Choose a good quality antifreeze with water pump lubricants, rust inhibitors and other corrosion inhibitors, along with acid neutralizers.

1. Raise the vehicle and support it with jackstands.
2. Remove the splash shield from the front of the front sub frame and body.

Before opening the radiator petcock, spray it with some penetrating lubricant.

1. Place a suitable container under the petcock.
2. Remove the reservoir bottle pressure cap.
3. Attach a length of rubber hose to the draincock. This will help to direct the coolant flow into the drain pan.
4. Drain the existing coolant by opening the radiator petcock.
5. Close the petcock and install the splash shield.
6. Lower the vehicle.
7. Determine the capacity of the coolant system, then properly refill the cooling system with a 50/50 mixture of fresh coolant and water, as follows:
   1. Fill the reservoir bottle with coolant until it reaches the top of the cold fill mark on the bottle.
   2. Reinstall the bottle pressure cap.
   3. Select the maximum heater temperature and blower motor speed settings. Position the control to discharge air at the A/C vents in the instrument panel.
   4. Start the engine and allow it to idle until hot air is coming from the A/C vents.
   
   
8. If the air discharge from the A/C vents remains cool and the temperature gauge does not move, the engine coolant is low in the engine and must be filled. Turn the engine OFF , allow the engine to cool and add coolant through the reservoir bottle as described earlier and replace the cap.
9. Start the engine and allow it to idle. Hot air should come from the A/C vents. The engine coolant temperature gauge should maintain a stabilized reading within the normal range and the upper radiator hose should feel hot to the touch.
10. Shut off the engine and allow it to cool.
11. If available, install a pressure tester and check for leaks. If a pressure tester is not available, run the engine until normal operating temperature is reached (allowing the system to naturally pressurize), then check for leaks.

CAUTION If you are checking for leaks with the system at normal operating temperature, BE EXTREMELY CAREFUL not to touch any moving or hot engine parts. Once the temperature has been reached, shut the engine OFF, and check for leaks around the hose fittings and connections which were removed earlier.

1. Check the level of protection with an antifreeze/coolant hydrometer
2. If this is helpful please leave positive feedback.
3. Thanks, snugglpants

I Suspect that you have AIR in your COOLING SYSTEM Fallow Procedures BELOW to Insure there is NO AIR IN COOLING SYSTEM


Draining and Filling Cooling System

Draining Procedure

Important: Do NOT add any engine coolant supplement sealant pellets to the cooling system. Drain and refill the engine cooling system every 5 years or 240,000 km, (150,000 mi) , whichever occurs first.

1. Park the vehicle on a level surface.
2. Remove and clean the coolant recovery reservoir.

Caution: Refer to Radiator Cap Removal Caution in Service Precautions.

1. Remove the radiator cap when the engine is cool:

3.1. Slowly rotate the cap counterclockwise to the detention tab. Do not press down while rotating pressure cap. 3.2. Wait until any residual pressure, indicated by a hissing sound, is relieved. 3.3. After all hissing stops, press down on the radiator cap and rotate the cap counterclockwise in order to remove the cap.
Important: Recover and store used coolant in a used coolant holding tank and submit the used coolant for recycling on a regular basis. Do NOT pour used coolant down the drain. Ethylene glycol antifreeze is very toxic. Disposing of ethylene glycol into the sewer system or ground water is both illegal and ecologically unsound.

1. Place a drain pan under the radiator drain valve in order to collect all of the used coolant.
2. Open the radiator drain valve located at the bottom of the radiator tank. If only partial cooling system draining is required, drain enough coolant to perform the procedure.
3. Open the air bleed valve on the thermostat housing 2 full turns
   1. Open the air bleed valve above coolant pump 2 full turns.
   2. Place a drain pan under the engine block drains in order to collect the used coolant.Important: Allow the coolant to drain completely.
      
      
      1. Remove both of the engine block drains.
      • The front left engine block drain is located between the starter motor and the engine oil filter.
      • The rear right engine block drain is located near the crankshaft position sensor.
      Filling Procedure
      Reinstall the engine block drains
      
   3. Close the radiator drain valve,
      1. Install the coolant recovery reservoir. Notice: When adding coolant, it is important that you use GM Goodwrench DEX-COOL or HAVOLINE DEX-COOL coolant. If Coolant other than DEX-COOL or HAVOLINE DEX-COOL is added to the system the engine coolant will require change sooner; at 50,000 km (30,000 mi) or 24 months.
      2. Slowly add a 50/50 mixture of GM Goodwrench DEX-COOL or HAVOLINE DEX-COOL and clean, drinkable water to the radiator until the coolant level is at the base of the radiator fill neck.
      3. Wait for 2 minutes and reinspect the level of the coolant in the radiator. If necessary, add coolant to the radiator until the coolant level is at the base of the radiator fill neck.
      4. Install the radiator cap.Ensure that the radiator cap arrow points toward the radiator overflow tubeImportant: The air bleed valves are brass. Do NOT over-tighten the air bleed valves.
         
         
         1. Close the air bleed valve above the coolant pump.
            1. Close the air bleed valve on the thermostat housing.
            2. Fill the coolant reservoir with a 50/50 mixture of GM Goodwrench DEX-COOL or HAVOLINE DEX-COOL and clean, drinkable water. Caution: Refer to Radiator Cap Removal Caution in Service Precautions. Important: If the coolant temperature indicator turns ON, turn the engine OFF immediately and allow the engine to cool. Do NOT remove the radiator cap.
            3. Start the engine and allow the engine cooling fan to cycle on and off 3 times. If the low coolant indicator does not turn OFF and the coolant level is above the Full Cold mark on the coolant recovery reservoir, refer to Engine Coolant Temperature Gage Inaccurate or Inoperative.
            4. Turn off the ignition.
            5. Allow the engine to cool.
            6. Ensure that the engine coolant concentration is sufficient.