I purchased a 91 Honda Prelude SI the otherday. Situation around purpose was this. Son owed Dad money, Honda was collecting rust and killing the grass, Dad takes Honda in place of cash. I buy Honda for $600. Car now runs, sounds good, and ready for inspection. Now for the issue. When the car idles its fine. When I drive it heats up, when I stop the temp goes down. The fan works and I changed the thermostat, now what?
You might want to check the oil and the coolant. Do you find oil in the coolant or coolant in the oil?
Have you bled the air from the cooling system. Some Hondas have a special bleed valve for this. An air pocket in the system will cause problems.
Sounds like there’s a blockage in the cooling system. At idle enough coolant is moving through the engine to keep it cool, but once you start running the engine harder, something is stopping enough coolant from moving through the engine and radiator to keep everything cool.
Maybe the radiator is blocked
or maybe a hose is blocked
or maybe the water pump isn’t moving enough water.
Could be as easy as what the last poster said.
You want to replace the radiator cap. If it’s not holding pressure the coolant will boil over at a lower temp.
This is the best answer, the rubber seal has dried out from sitting around.
A flush and clean of the cooling system could help restore the cooling systems efficiency. Walmart and auto parts stores sell the kits for under $20.
Yep, I think you have diminished cooling capacity as others have mentioned. In addition to the other ideas, you may just need a new radiator.