1990 Honda Accord LX running hot after a short drive
Check the fans are coming on. Check coolant levels - typically a heater would stop working if you dropped too low, but you might be borderline. Check the type of coolant too - your owner's manual will tell you what type to use. Check your hoses, no kinked or collapsed hoses. If coolant level is good, try "burping" the radiator (bleed any potential air pockets out of cooling system)(see below for how to do that). If OK, then check thermostat. Double check the radiator they installed, that it is a suitable replacement and not some aftermarket part undersized for the task. To "burp" the radiator park car on a incline facing uphill (to get the radiator elevated above engine) remove the cap and run the engine, check/add coolant if it drops.
When it runs hot are you loosing coolant, and are the hoses very hard or still flexible? If boiling over or so hot the hoses cannot be squeezed/flexed by hand, could have a bad head gasket or cracked head or block. After following KenF"s suggestions if they don't work you need to check what I mentioned.
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