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Had waterpump replaced and belts, but the reservoir keeps running out of water in traffic but seems okay on open road, mechanic has had a look and no visible leaks and checked gaskets and all okay. What could it be?

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Thanks will have this looked at! – Jason Hill Oct 17 '13 at 8:01

The hoses may not be sealing properly. When you're in traffic, your car gets about 20% hotter than on the open road, so the extra pressure caused by trapped steam (or hot air at any rate) may be forcing the water out of a leak that is invisible at lower temperatures.

At least, that's what I found with my Astra.

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How many times have you refilled the reservoir? It is possible that air was trapped in the system after the water pump replacement. After a couple of high speed runs the air may have worked its way out of the cooling system and into the reservoir. Some vehicles are more difficult to "burp then others. If you have filled it more than 2-3 times you have a leak. Ask your mechanic to test the radiator for the presence of combustion gasses which will indicate a bad head gasket. Did he do a pressure test of the cooling system or just look for drips? If doing a pressure test make sure to check the heater core as drip from it may stay in the heater box and not reach the ground.

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The question mentions having 'belts' changed. If the problem was not there before the belts were changed, and the problem involves overheating, then it is very likely that one of the belts changed was the cam belt. If the engine was not timed properly with a new cambelt fitted this would cause your problem.

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