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Very predictable overheating problem


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For the past few weeks my 1998 Civic has been overheating ONLY after driving on the interstate for at least an hour or so, slowing down (say at an intersection or something), and then accelerating. The temperature then drops to normal after reaching about 55 mph, and the overheat then happens every time I slow down and then accelerate between 0-55 mph.


So far I've flushed the cooling system and replaced the thermostat. The fans, ac, heater, etc all seem to be working fine. No debris on the radiator. No bad hoses. No obvious signs of gasket failure (loss of compression, misfiring, dirty or coolant smelling exhaust, milky oil, etc). Water pump was replaced a couple years ago along with the timing belt. Has ~135k miles. I'm guessing it has something to do with the water pump, but I don't know much about this stuff, which is why I'm here. Figure the precise description should yield some insight.



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I was going to ask if you had a thermometer or probe so you could check to see if it was actually overheating or just reading that it was overheating.


You noted no signs of head gasket failure, which seems like a next logical thing to check for overheating on the expressway. A sending unit short could be causing overheating. It might not be engaging the fans at the right times, but that seems less likely at this point. My next thought would be to hard wire the fan to run constant and see what you get.

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Thanks for the replies. Probably won't get to this for a day or two. But a couple points of clarification:


1. The fan is working, so could it be the fuse?

2. I bled the system of air after doing the flush, replacing the thermostat, and filling with coolant. The gauge and coolant level were normal all week while driving to/from work. It only overheats after driving for a few hours on the freeway then slowing down and accelerating again (not before accelerating; i could idle with no overheat). The overheat then happens when accelerating until I reach 55 mph, then suddenly drops to normal. Coolant was lost when it overflowed from the reservoir.


So I'm a little confused about the functions of the ECT switch (located on the thermo housing cover), ECT sending unit, and the ECT sensor (both below the distributor cap). Am I right that the switch controls the fan, and the sensor and sender unit control the gauge? If the timing of the fan was off wouldn't it be the switch, not the sending unit?


I read somewhere that it could be the drive belts slipping. Is that easy to check/fix?

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if the fan works, it is not the fuse...


how did you bleed the system?


it is best to run the car with the cap off on an incline. put the car on ramps or on a steep driveway or something.


if this doesnt work, the thermostat may be sticking. take the thermostat out and run without one. see if it happens without the thermostat.


try the easiest solutions first. then go on to the bigger problems if the easy stuff doesnt work.

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