Issue with overheating Buick rendezvous. Bought 1 yr ago, ran great for 6 months, started overheating. New water pump, flushed and descaled rad, replaced thermostat. Took to new mechanic, had 2clyinders blown out of 6. Replaced engine, same year but only 120,000 km on it, ran great for another 3 months. Now overheating again, upper rad hose very hot, lower is much cooler, fluids are full, engine not hot. Could this be a circulation issue?

I WENT 50 YEARS NO A/C. EVEN AT 110F . EVEN 1 YEAR OLD.
LOTS OF CAUSES. IF TRUE.
BUT NOT ONE WORD OF PROOF IT IT DID
220F IS NORMAL AND AF DOES NOT BOIL UNTIL 255f.
SHOW PROOFS?
1: NOT RUNNING CLEAR MIX OF 50% AF IS TOP CAUSE
17 YEAR OLD COOLANT IS NEVER EVER GOOD
NOR RUNNING ONLY WATER NOT ALLOWED.
2 BAD RAD CAP , REPLACE IT,
3: COOLANT TOO LOW (EACH MORNING COLD , CAP OFF ALWAYS FULL.
4: BAD THERMOSTAT AGAIN 16 YEARS IS NO GOOD.
5: LEAKS COOLANT TO GROUND, INSIDE ENGINE OR TO PASSENGERS TOES (FOOT WELLS) IS BAD.
6: WATER PUMP BAD, RUSTED OUT IMPELLER.
7:FAN BELT LOOSE. SLIPS. TENSION-ER BAD.
8: BAD FANS, DEAD OR OLD AND SLOW.
OR SIMPLE STUFF THE CORES BOTH CONDENSER AND RAD ENGINE ARE PACKED UP IN BUG GUTS.
SO BLAST THEM OUT WITH WATER HOSE SPRAY NOZZLE.

OR GET THE CAR SERVICED IN A REAL SHOP.
THEY DO KNOW WHAT TO DO AND CHECK.

indicates blown head gasket
do a compression test to confirm then replace head gaskets and have the head/s shaved as they will be warped

Sounds like you may have a head gasket leak. But first I would check the water level and get the air out of the system. Air is bad is water system. When bleeding the air squeeze the hoses to burp the air out and I also would suggest looking for a purge nut on the top of a water line or housing on the engine. Check the fan is working or works. And then maybe consider the thermostat was installed wrong.

Somebody been using tap water in their radiator? The vehicle is old enough to have a lot of accumulated rust and crud in the Radiator which, depending on relative water quality and location of water pump could over pressure the system and have it squirting into overflow. If the threads of the plug have antifreeze on them it's coming from the OUTSIDE not the inside.

My first question is make model and year.
Second, are you saying the engine is boiling over into the overflow tank and onto the ground ? Is the tank about half full when the engine is cold ?
Have you run the engine with the thermostat removed to test ?
Usually when you can hear the coolant moving around, you have air in the system and it is not full of liquid coolant.

Install the new thermostat (make sure it is the right temp). This will allow for your coolant to stay long enough in the radiator to cool down.

If that does not fix it, check radiator cap, belt that turns the water pump and the water pump itself along with its pulley.

There is also the possibility that the coolant temperature sensor is giving off false readings, that the coolant system is clogged or your engine fan is not operating when it should be.

If the top hose is hot and the bottom is cold, your definitely not getting circulation. You,ve already done alot of work to this vehicle so you have pretty much answered your own question. About the only thing left is the heater core. Of course, I dont want to be the one to tell you it is definitely the heater core because obviously I am not looking at the vehicle. But If i was a betting man, thats where my money would be.

You should take it to a rad shop and get the whole cooling system cleaned and flushed,then pressure tested to eliminate it before goig any furter because an y other cause of overheating is going to cost you a bundle.

Remove and raise upper hose at thermostat end and remove lower rad hose.
Fill rad with a hose and watch for flow from bottom rad hose and/ or bottom rad hose connecton.......water should fall though rad easily.
Honda rads are cheap.....any restriction....replace rad.

Remove thermostat....fill thermostat housing with hose and watch for water flow at bottom rad hose.
Flush good.

Remove heater hoses.......flush back and forth until a good flow is present "With heater control set to hot" if no or poor flow....replace heater core.

Stop leak can plug everything.

Also.......not likely causing over heating....but ....at your mileage,if you do not know if the timing belt has been changed.....CHANGE IT NOW!
Water pump is optional at this mileage.