Q: Overheating issue with my 1994 Honda Accord

Stop leak products will plug up cooling passageways, particularly in the radiator, and consequently OEM manufacturers such as Honda discourage use of these products. In any vehicle, if there is a "leak", of anything (oil, refrigerant, coolant, etc.) the leak has to be repaired using standard, OEM approved professional techniques and stop leak products simply do not qualify as such. In your circumstance, you have a new radiator which hopefully does not have any residue from previously applied stop leak products.

Overheating can be due to lean engine operation. That possibility can be diagnosed by YourMechanic. Trapped air in a cooling system can cause overheating, so you want to rule that out, too. Typically, there are air purge ports that are opened when refilling the cooling system. A defective radiator cap can cause overheating if the cap is unable to withstand normal system operating pressure.

If all these are ruled out, one approach you can try is use an infrared thermometer to determine the actual operating temperatures, particularly at entrance and exit of radiator and with those measurements determine whether the cooling system fan(s) are coming on at the right (factory specified) threshold. If fan operation is delayed, or the fan motor is not running at the required speed, that could be causing or contributing to the problem. There are timers, relays, and sensors involved in the cooling fan(s) systems, not to mention the condition of the fan motor and bearings.

If you want a professional to evaluate the overheating issue, and identify the causes of the problem, feel free to schedule an overheating diagnostic with a certified professional from YourMechanic.