I have the par max plu, 6gpm with 3/4in quick disconnects. I would like to add a expansion tank to the system. Which is the correct one for this unit?

First of all, there is a recall for gas tank replacement for your vehicle. The dealer will replace it free. The link will get you to the document in PDF form
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It looks like there is a bracket on one end of the tank hidden by a heat shield

FUEL SUPPLY SYSTEM Fits Buick Rendezvous

In a closed circuit the expansion tank is supposed to fill up, then, as the engine cools it sucks coolant back from the expansion tank. Drive the car and let it find the correct level, probably too full if you filled the system and the expansion tank.

I WOULD CHECK BOTH RADIATOR HOSES TO MAKE SURE THEY ARE HOT. THERMOSTAT COULD BE STICKING SLIGHTLY CLOSED.CHECK COOLANT LEVEL MAKE SURE COOLANT NOT LOW.IF COOLING NOT BOILING OUT OF THE EXPANSION TANK. I WOULD HAVE CAR HOOKED UP TO A DIAGNOTIC SCANNER AND CHECK COOLANT TEMPERATURE SEE IF TEMP. OVER 230 DEGREES IF SO RADIATOR COOLING FANS NOT WORKING.COOLANT THERMO SWITCH BAD BOTH FANS SHOULD BE RUNNING AT LOW SPEED.IF ALL LOOKS GOOD YOU COULD HAVE FAULTY LOW COOLANT SENSOR IN EXPANSION TANK.MAKE SURE COOLANT IS IN THE EXPANSION TANK BECAUSE COOLANT LEVEL COULD BE LOW.ADD COOLANT IN THE EXPANSION TANK UNTIL YOU AT MAX LEVEL LINE ON THE EXPANSION TANK.DONT OVER FILL THE EXPANSION KEEP ADDING COOLANT UNTIL COOLANT LEVEL STAYS AT THE MAX LEVEL LINE ON THE EXPANSION TANK.

Hi!!
On the bottom of the radiator, there is a drain valve, just open it and drain fluid into a container, if you want faster drain, remove bottom radiator hose.
NOTE: DO NOT OPEN SYSTEM WHEN HOT, COOLANT IS POISONOUS FOR PETS, KEEP IT AWAY FROM THEM.
Good Luck!! Rate this post!!

NOTE Always maintain cleanliness when servicing fuel system components.

1. Relieve the fuel system fuel pressure.
2. Drain the fuel tank.
3. Raise the vehicle.
4. Disconnect the fuel tank wiring harness connector at the body pass through.
5. For 3.4L engines, disconnect the quick-connect fitting at the inlet side of the in-pipe fuel filter.
6. For 3.4L engines, disconnect the quick-connect fitting on the fuel return pipe, near the fuel filter.
7. Disconnect the fuel tank evaporative emission (EVAP) pipe quick-connect fitting near the fuel filter.
8. Remove the fuel feed, the fuel return, and the EVAP lines from the fuel pipe retainer.
9. Remove the retaining nut for the EVAP canister bracket.
10. Loosen the fuel tank filler pipe hose clamp.
11. Disconnect the fuel tank filler pipe from the fuel tank.
12. For 3.5L engines, disconnect the EVAP vent pipe near the fill pipe hose.
    NOTE Do not bend the fuel tank straps. Bending the fuel tank straps may damage the straps.
    
13. With the aid of an assistant, support the fuel tank and remove the fuel tank strap attaching bolts.
14. Remove the fuel tank from the vehicle and place the fuel tank in a suitable work area.
15. If the fuel tank is not being replaced, go to the Installation Procedure.
16. Disconnect the fuel tank wiring harness from the fuel sender assembly.
17. Remove the fuel sender shield from the fuel tank.
18. Disconnect the fuel feed and return pipe quick-connect fittings at the fuel sender.
19. Disconnect the EVAP vapor pipe assembly from the fuel sender.
20. Remove the fuel and/or EVAP pipe assembly from the fuel tank.
21. Remove the fuel sender assembly.
22. Remove the EVAP vent solenoid valve.
23. Remove the EVAP canister.

To install:

1. Install the EVAP canister.
2. Install the EVAP vent solenoid valve.
3. Install the fuel sender assembly.
4. Install the fuel and/or EVAP pipe assembly.
5. Connect the EVAP vapor pipe to the fuel sender assembly.
6. Connect the fuel feed and fuel return pipes at the fuel sender.
   NOTE Use the correct fastener in the correct location. Replacement fasteners must be the correct part number for that application. Fasteners requiring replacement or fasteners requiring the use of thread locking compound or sealant are identified in the service procedure. Do not use paints, lubricants, or corrosion inhibitors on fasteners or fastener joint surfaces unless specified. These coatings affect fastener torque and joint clamping force and may damage the fastener. Use the correct tightening sequence and specifications when installing fasteners in order to avoid damage to parts and systems.
   
7. Install the fuel sender shield to the fuel tank and tighten the shield nuts to 53 inch lbs. (6 Nm).
8. Connect the fuel tank wiring harness to the fuel sender assembly.
9. With the aid of an assistant, position and support the fuel tank.
   NOTE Do not bend the fuel tank straps. Bending the fuel tank straps may damage the straps.
   
10. Install the fuel tank retaining strap attaching bolts and tighten the fuel tank retaining strap bolts to 35 ft. lbs. (47 Nm).
11. Install the EVAP canister retaining nut and tighten the retaining nut to 53 inch lbs. (6 Nm).
12. Connect the fuel tank filler pipe to the fuel tank and tighten the fuel tank filler pipe hose clamp to 22 inch lbs. (2.5 Nm).
13. Connect the fuel feed pipe quick-connect fitting at the inlet side of the fuel filter.
14. Connect the fuel return pipe quick-connect fitting near the fuel filter.
15. Connect the fuel tank EVAP pipe quick-connect fitting near the fuel filter.
16. Connect the fuel tank wiring harness connector at the body pass-through.
17. Lower the vehicle.
18. Add fuel and reinstall the fuel tank filler pipe cap.
19. Connect the negative battery cable.
20. Inspect for fuel leaks with the following procedure:
    1. Turn ON the ignition for 2 seconds.
    2. Turn OFF the ignition for 10 seconds.
    3. Turn ON the ignition.
    4. Inspect for fuel leaks.

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you bled the radiators, so this is a water system not steam. Sounds like you expansion tank is "slugged" or full of water, so there is no place for the water to expand when it warms up. get some water out of the expansion tank

It is called an Expansion Tank

You keep it topped up with water and when the cooling system wants some water it draws it down from there.

If the cooling system is too full it pumps the water back into the expansion tank

There should be two marks on the tank Min & Max , the water level should be on Max at all times

replace your coolant reservior cap; the rubber ring has become brittle and "steams" the coolant out as you drive

There should be 2 bleeder screws in the system, one on the expansion tank, and one on a thermostat housing. Open both screws all the way. Turn the key to the On postion (but dont start the car). Then turn the heater to max heat, but have the fan set at the lowest setting. Fill the expansion tank til coolant comes out of both bleeder screws with no bubbles. After that close the screws (be very careful not to over tighten them!!). Then completly fill the expansion tank. (dont mind the full mark, if theres air in the system, it will drop the level in the expansion tank) And thats it. Drive the car around then check the level.

First check the coolant expansion tank and radiator for correct level. The radiator should be full (take the cap off when the engine is cold, doing this on a hot engine will cause hot coolant to spray out and possibly cause major burns). The expansion tank should be marked level-wise, either min-max, or min only, or max only. If the levels are right, your level sensor is probably bad. This sensor is located somewhere in the expansion tank.