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I have a 1994 civic dx(work car) with 157k on it. Last year I replaced the radiator, waterpump, timing belt. The fan is working. I even took out the thermostat to see if that was the problem. No smell inside the car. No water in the oil. Flushed the rad and engine and water flows fine. Here is the problem, after about 20-30 minutes of driving(highway or city) the temp guage will rise and start to bounce from 50% to 90% hot. It will peak around 90 then drop to 50 within a matter of seconds.. Also the overflow reserviour will be full and overflowing if I stop and pull over. What could be the problem? Never did any other work on the engine..

 
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I know you posted your problem back in mach 2007, but did you ever solve the problem...I have the same exact situation, but I'm losing water from the radiator. The heater core, is good, and the engine oil does not seem to have sign of radiator coolant. Please advice if you solved your problem.