I have a 1992 Civic that's been overheating, first only at speeds above 65 mph and only after an hour or so of driving, but it's getting progressively worse. It's drivable if I coast with the clutch depressed or in neutral on and off - while coasting the temp at the sensor creeps up, then if I put it back in gear (especially in a lower gear/higher rpms) the gauge quickly drops to normal operating temperature. This leads me to believe the problem is coolant circulation, perhaps blockage or a failing water pump.
Along that line of reasoning, I tried revving the engine with the radiator cap removed, and did not get coolant gushing out the top of the radiator, only a slight surge, which suggests to me that:
- There's not blockage in the radiator.
- The water pump is not producing significant flow.
Does my diagnosis seem correct? I've replaced the water pump on a similar Civic before and it's a major pain so I don't want to do that first if it might be something else simpler wrong.