Flushing Cooling System. Am replacing thermostat and possibly water pump on my van. It currently uses DexCool coolant. Two questions: (1) procedure for complete flushing of cooling system?, (2) should I changeover to regular antifreeze or use Dexcool as some opinions suggest using regular antifreeze. Vehicle has 102k miles. Thanks

check the level of antifreeze or thermostate is bad

Hello! Can not tell from the question if your loosing coolant...But it sounds like an air bubble behind the thermostat is blocking the flow...Remove the thermostat...Replace housing...Buy a back flushing kit from an auto parts store...Back flush the cooling system...Make sure heat is on high...Replace thermostat and refill with 50/50...start the engine with radiator cap off...Let all the air bubble out, even after thermostat opens...Wait until upper hose gets hot then replace the cap...Top off in the recovery tank...Now, monitor temperature...If all is well back flushing and air bleeding cleared the fault...If engine overheats due to loss of coolant I'll address that next...If coolant is not being lost does the engine actually overheat? ie..bubbling into recovery tank...If not the temperature sending unit is defective...send a comment...Guru...saailer

I’m happy to help further over the phone at https://www.6ya.com/expert/david_6df67de3b14de867

Hello! The heater core is partially blocked...I would suggest flushing the cooling system with the temperature control on high...NOTE...The type of anti-freeze used in your vehicle is DexCool...If another type is used it may damage the cooling system...To flush...Drain radiator...Fill with plain water...Run engine until thermostat opens...Turn engine off...Let cool then drain...Do this at least three times or until water drains clear...Install new thermostat...Refill with 50/50 DexCool...Leave the radiator cap off as engine warms...Allow all air bubbles to escape...Then install cap...

The down side of DexCool is electrolysis...Electrical current flowing in the cooling system which can cause the anti-freeze to "clump" blocking its flow...Periodic flushing eliminates the problem...

Guru.........Saailer

I’m happy to help further over the phone at https://www.6ya.com/expert/david_6df67de3b14de867

Two things are possible. Air in the cooling system, or the system needs to be flushed. You didn't say what year your van is, but I'm sure it use dexcool antifreeze. Dexcool is the orange antifreeze recommended by GM for all their vehicles. The problem is that dex cool attacks gaskets causing gasket failures, especially the intake gaskets. It also can cause buildup in the cooling system causing overheating, or plugging up heater cores. We'll try the easy solution first.We need to get the air out of the system. First, the radiator needs to be higher than the engine. If it isn't, either use drive on ramps or raise the front end with a jack, Remove the radiator cap, put the heater controls on hottest setting, start the engine and let it run. When the coolant gets hot enough, the thermostat should open allowing the coolant to circulate. When it opens, the coolant level will drop in the radiator. Add antifreeze to make it full again. The coolant will drop again and may do so several times. Just keep adding antifreeze each time. continue to do this until the coolant level stabilizes. If the thermostat closes momentarily it will cause coolant to overflow. Not to worry, wait til it opens again and top off the antifreeze. Put the cap back on the radiator and check the heater for heat. If it' still not warm, you may have to flush the radiator.

Are you sure you burped the system of all air? Air in the system can stop the thermostat from opening and cause it to overheat.

It Depends on which engine you have, this is how it goes for both. I use a little RTV with a new ring as well.

THERMOSTAT REPLACEMENT (4.2L ENGINE)
Removal Procedure
1. Remove the necessary coolant from the radiator.
2. Remove the generator.
3. Loosen the outlet hose clamp at the thermostat housing. Remove the outlet hose from the thermostat housing.
4. Remove the thermostat housing bolts.
5. Remove the thermostat housing from the engine block.
6. Clean all of the surfaces of the thermostat housing.
7. Clean the sealing surface of the engine block.
Installation Procedure
1. Install the thermostat housing to the engine block.
2. Install the thermostat housing bolts.
Tighten: Tighten the bolts to 10 N.m (89 lb in).
3. Lubricate the inner diameter of the radiator hose with engine coolant.
4. Install the outlet hose to the thermostat housing. Secure the hose with the clamp.
5. Install the generator.
6. Fill the cooling system with specified coolant and concentration. Follow the correct cooling systen bleed procedure for your engine.
7. Inspect all sealing surfaces for leaks after starting the engine.
THERMOSTAT REPLACEMENT (5.3L ENGINE)
1. Drain Radiator and remove the radiator outlet hose.
2. Remove the water pump inlet bolts.
3. Remove the water pump inlet and thermostat from the water pump.
Installation Procedure
IMPORTANT: The thermostat is not serviceable separately. The water pump inlet and thermostat must be replaced as an assembly.
1. Install the thermostat and thermostat housing to the water pump.
2. Install the thermostat housing bolts.15 N.m
3. Install the radiator outlet hose.
4. Fill the cooling system with specified coolant and concentration. Follow the correct cooling systen bleed procedure for your engine.
5. Inspect all sealing surfaces for leaks after starting the engine.

The cooling system may need to be serviced which is one of the things that is never done. The orange antifreeze has a life of 5 years and 150,000 miles and if it has never been flushed out, there may some crystal formation causing a restricted flow of antifreeze through the system. Try flushing out the cooling system with Preston radiator flush, use the dry power flush rather then the one from the bottle. Follow the instructions and replace the antifreeze with a 50/50 (water/antifreeze) mix, if you not sure how to mix it. You can buy them premixed 50/50. Too much water will boil over and too much antifreeze will gel up causing overheating. If you like you can replace the thermostat too, but that's not that easy on the 3.4 liter engines. If you go to Auto Zone you can pick up the antifreeze, radiator flush, and thermostat with the step by step instructions needed to get the job done. Keep me posted, be glad to answer any other questions you may have and good luck friend. 

have you checked to see if you fan is kicking on for the radiator?

try doing a flushing out the coolant system the factory 150k mile coolant is really not good once you flush it i recommend using the the regular green and not the dexcool

2 gals. sounds about right for fill up after a water pump job there is coolant left in radiator,heater core &engine block and of couse the reservoir. thanks from schleck.