1989 Chevy Suburban keeps overheating...have replaced radiator, water pump, fan, temp. sending unit, oxygen sensor, EGR diaphram, thermostat (x2 ), flushed motor, changed antifreeze & clutch fan(?). What else IS there?? Please help if you can..Thank you!

Are you going by the temp gauge on the dash to say it's overheating ? Did you hook up a scan tool an look at live data from the coolant temp sensor to the PCM/ECM - engine computer ? Use a infrared thermometer to check engine temp at the thermostat housing ? Don't remember how many gm instrument clusters i sent to be fixed . GM had a problem with stepper motors inside the instrument cluster , stepper motors control the gauges needles . 2003 to 2006 this was a problem . you mite think it's overheating whens it's not .
https://www.youtube.com/watch?v=oTm2oDx-_GM

recheck water pump and thermostat --check and test fans -radiator flow --and for possible head gasket failure

It could be a number of things. You need to have your cooling system trouble shoot. It could be something simple as a stuck thermostat or as extreme as a cracked head. You will need to find a mechanic that has a thorough understanding of cooling system operations.

Faulty gauge or sending unit more then likely the sending unit.

Verify the sending unit is correct. Verify it is overheating. Sometimes the sending unit and/or gauge is bad.

I WOULD SAY NO BECAUSE WATER ONLY CIRCULATES WHEN YOU TURN ON THE HEATER, BUT IF YOUR VEHICHLE CONTINUES TO OVERHEAT IT COULD BE 5 THINGS. CLOGGED RADIATOR,RADIATER HOSES.THERMOSTAT,RADIATER FAN SENSOR OR THE WATER PUMP

is your electric fan working? Is it turning on when you turn the air on? If you have 2 fans on the radiator, is the other one turning on when engine starts to get hot? You seemed to have replaced everything else outside of a new engine block. Just make sure your fans are working properly.

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check to make sure the lower rad hose isn't collapsing when engine is reved

I think you started out looking in the right places. Replacing the coolant, thermostat, relay and sensor are all fairly inexpensive.

I believe you either have blockage in the system (especially if you're using tapwater), or your water pump may be failing.

I would first either take it to a radiator shop and have them flush the cooling system out thoroughly, or else pick up a chemical flush kit and do it yourself. This involves more than just drain and refill.

If this doesn't work, your next step will probably be replacing the water pump. At almost 200k miles, it's probably about due anyway.

Have you tested the fan sending unit on the engine? disconnect that and if the fan runs (Key on) that is most likely the problem.