Why is the coolant overflowing when the temp guage gets half way?

I would look else where for the problem unless the thermostat is stuck open. Thermostats work by blocking the water to the radiator until the coolant in the block has reached operating temperature and then they start to open. As soon as they read a cooler water temperature they will start to close and restrict the flow until the operating temperature is maintained . IT would make no difference which thermostat you fitted as the number indicates when it starts to open. I would be checking the temperature sender unit and the gauge and I would be checking the coolant level in the radiator at the cap not the resevoir

Softly418 had this problem...........

This could result from a thermostat which is stuck closed (but you have a new one), or a failed coolant temp sensor not allowing the radiator fans to run when they should, but I think you have a blown head gasket. Overheating is a classic sign of combustion gases getting into the coolant. The pressure also forces it out of the overflow.

Warm up your engine a little, about half, and remove the radiator cap by putting a towel over it first. Then let the engine keep idling until hot while you look down the filler neck into the radiator......carefully.

If you see white bubbles like dry ice, that is a failed head gasket. If not, have a mechanic do a test for combustion gas in the coolant with a gadget like this

http://www.youtube.com/watch?v=1_SIwHMLmkM

u may need 2 get your radiator checked make sure its not blocked inside&restricting the flow

Sounds like there is air trapped in the system. On a COLD engine, remove the radiator cap, and check the level. If it's low, slowly add coolant until the rad is filled right to the top, then cap it. Top up coolant overflow bottle if required. Start the engine. As the engine reaches operating temperature, the coolant level in the overflow bottle should rise. If it doesn't, it means that all the air is not yet 'burped' out of the system. Provided that the radiator cap is good, and that the coolant level is maintained at the proper level, the system should eventually expel all the air. Hope this helps. Good luck.

You refill the engine coolant by filling the overflow reservoir/pressure tank.Start the vehicle and keep filling until it reaches the full mark,then allow the engine to get to operating temp of 190 degrees and then top off the coolant.There is no bleeders for this engine unless you have a duramax then there is a bleeder on top of thermosatat housing that you remove with the engine off and fill the coolant until it comes out of the bleed hole.Start the duramax engine and let warm to operating temp.You may need to hold the engine speed at 2000 rpm to get the diesel to operating temp.

YOU NEED BLEED AIR OUT THE COOLING SYSTEM.WAIT TO ENGINE COOL DOWN.YOUR VECHICLE YOU HAVE TO POUR COOLANT IN THE COOLANT OVERFLOW JUG UNTIL COOLANT LEVEL STAY AT THE FULL COLD MARK.THEN START ENGINE LET IT IDLE A FEW MINUTES WATCH COOLANT IN THE OVERFLOW JUG WHEN IT DROPS ADD MORE COOLANT UNTIL COOLANT LEVEL STAY AT FULL COLD MARK.WHEN COOLANT LEVEL STOP DROPPING IN OVERFLOW JUG.SCREW RADIATOR CAP BACK ON OVERFLOW JUG WATCH ENGINE COOLANT TEMPERATURE GAUGE AS COOLANT TEMPERATURE START CLIMBING TURN OFF ENGINE LET IT SET A WHILE KEEP FROM GETTING SCALDED.TAKE LARGE RAG PLACE OVER COOLANT OVERFLOW JUG CAP SLOWLY OPEN RADIATOR CAP A LITTLE AT A TIME UNTIL PRESSURE RELEASE.REMOVE OVER FLOW JUG CAP.ADD MORE COOLANT UNTIL COOLANT LEVEL AT THE FUL COLD MARK ON THE OVERFLOW JUG.THEN SCREW CAP BACK ON THE OVERFLOW JUG.START ENGINE LET IT IDLE AGAIN WATCH ENGINE COOLANT TEMPERATURE GAUGE WHEN TEMPERATURE GAUGE START CLIMBING.TURN OFF ENGINE WAIT A WHILE.CHECK COOLANT LEVEL IN THE OVER FLOW JUG.DO THIS PROCEDURE UNTIL THE COOLANT LEVEL IN THE COOLANT OVERFLOW JUG STOP DROPPING AND STAY AT FULL COLD MARK,WHEN THE TOP RADIATOR HOSE GET HOT THERMOSTAT OPENING AIR IS OUT THE COOLANT SYSTEM. WHEN ENGINE COOLS DOWN.CHECK COOLANT OVERFLOW JUG COOLANT MUST BE IN THE COOLANT OVER FLOW JUG AT THE FULL COLD MARK WHEN ENGINE COLD. DONT .DONT OVERFILL COOLANT OVERFLOW JUG.KEEP COOLANT IN THE OVERFLOW JUG IF YOU LET COOLANT OVERFLOW JUG RUN DRY AIR WILL BE DRAW IN COOLANT SYSTEM CAUSE ENGINE TO OVERHEAT.

DID YOU BLEED AIR OUT THE COOLANT SYSTEM.TO BLEED COOLANT SYSTEM, FILL THE COOLANT SYSTEM THROUGH THE RESERVOIR TANK OR THE RADIATOR UNTIL THE FULL COLD MARK IS REACHED.BESURE TO USE 50/50 HALF WATER AND ANTIFREEZE.YOU CAN BUY ANTIFREEZE THAT IS ALREADY PREMIX 50/50.AFTER ADDING ANTIFREEZE TO RESERVOIR OR RADIATOR FILL RADIATOR TO FULL MARK OR IF YOU DONT HAVE RADIATOR CAP ON RADIATOR POUR ANTIFREEZE IN THE OVERFLOW JUG UNTIL COOLANT LEVEL STAY THE FULL COLD MARK.WAIT AWHILE UNTIL COOLANT LEVEL STOP DROPPING.NOW INSTALL THE RADIATOR OR OVER FLOW JUG CAP.START THE ENGINE AND ALLOW IT TO RUN UNTIL NORMAL OPERATING TEMPERATURE IS REACHED AND THE UPPER RADIATOR HOSE BECOME HOT.STOP THE ENGINE AND OBSERVE THE COOLANT LEVEL IN THE OVERFLOW JUG. THE COOLANT LEVEL SHOULD BE AT THE FULL HOT LINE.ALLOW THE ENGINE TO COOL UNTIL THE AMBIENT TEMPERATURE IS REACHED THEN CHECK THE COOLANT LEVEL IT SHOULD BE AT OR ABOVE THE FULL COLD LINE.ADD MORE COOLANT IF NECESSARY. WARNING BECAREFUL WHEN OPEN THE RADIATOR CAP OR JUG OVER FLOW CAP.USE A LARGE RAG OVER THE RADIATOR CAP TO KEEP FROM GETTING SCALDED.OPEN RADIATOR CAP A LITTLE AT A TIME TO KEEP FROM GETTING SCALDED.WAIT UNTIL ENGINE COOL DOWN BEFORE OPENING RADIATOR CAP OR OVERFLOW JUG CAP.WEAR SAFETY GLASSES.

temp should stay about half while driving sounds like you have a bad thermostat if guage is half way at idle do you have heat feel both heater hoses should be about same temp if one is cooler not flow coolant

U are talking about two different sending units, one is the Engine Coolant Temp sensor, the other is the temp guage sender, sounds like you have a bad thermostat, if the engine doesn't reach the proper operating temp it will set a code for the engine coolant temp sensor, and the guage will not move or move very little, replace your thermostat and both problems will go away.

change the themostat when you remove the old themostat you should see it stuck in the open postion . before i install the new one i fill the the block with coolant install the new stat run the mtr and heater core until hot iwould also replace the cap