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Expansion tank

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Have always maintained my tank water level at around 50%. Couple of weeks ago as car was put away I noticed a bit of water coming from the overflow. Presumably as pressure was lifting the spring cap. Went out yesterday. Water was slightly low so put a bit in back to around 50%. Again was dripping a bit as far was put away. Engine temp fine so am quite sure I have enough water. Other than a knackered cap or filling the expansion tank too high when cold is there anything else that could cause this. Oil is fine and no water drips anywhere else. Hoping head gasket is ok. Any thoughts. 

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No idea if this is related, but I recently had the car running (whilst I was re-tuning carbs for MoT emissions). The fans kicked in, but temp kept rising - probably not surprising as it was just idling on the drive and revs too low for the water pump which is too weak at idle - and water came spewing out of expansion tank. After letting it all cool down, I topped-up the expansion tank to usual level. A rew days later I went on a 400 mile round trip on motorways, A-roads and in-town driving on a very hot day. Temp guage was going up and down like a fiddler's elbow - but no overheating and no coolant loss. I can only presume that the hottest part of the cooling system is the engine block near the expansion tank, and as such when the car is stopped and pump not turning, the water will still be very well heated in a localised area - enough to cause the overflow. I'm sure more knowledgeable people will confirm/deny. 

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If the cap is old, I'd either test it (auto parts stores in the USA offer free loaner cap and cooling system testers) or just replace it.


Of course, have a good look at the mating surfaces of the cap where the rubber contacts them.

Atwell Haines

'88 Esprit

Succasunna, NJ USA

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My tank sits about 1/3 full. If it fill it up it just spits it out back to 1/3 full. If she's happy at that level and not overheating, I'm not going to argue with the old girl.


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Pete '79 S2

LEW Miss September 2009

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