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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:

  1. There's not blockage in the radiator.
  2. 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.

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up vote 3 down vote accepted

My first suggestion would be to give the system a good flush, as that can cure no end of maladies. Drain the system fully (check your Haymes Manual for any obscure drain points), then use a hose to flush it through backwards (i.e. the opposite direction to the normal flow) until the water comes out clear. It is usually easier to do rad and engine seperately.

Check the condition of the rad and hoses - if they are original on a 20 year old car they will be worn out by now, the rad will have lost a fair amount of fins and so won't be cooling the water as efficiently as it used to, even if it is still flowing freely. If the hoses are starting to perish then they sill start to flake off bits of rubber into the system which can easily cause a blockage.

Replace the thermostat. New ones are dirt cheap, and they are a common failure point.

If none of those help, then yes, you probably have a water pump failure, but try the cheaper, easier things first!

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I removed and tested the thermostat and even tried running with it out, and flushed the system. There didn't seem to be any obstruction to flow, and there was no improvement. The remaining possibilities seem to just be that the radiator has lost cooling capacity, or that the water pump is not generating enough flow. –  R.. Jul 12 '11 at 21:15
    
Actually it ended up being the radiator. Flow wasn't bad, but I guess it had enough filth accumulated that it wasn't transferring heat fast enough. –  R.. Aug 1 '11 at 4:06
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