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Coolant fill cap leaking?

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I suspect I have a coolant tank cap leak (the fill cap in the trunk, not the expansion tank cap) in my 2008 Boxster S (987-1). After a 5 hour drive and then sitting the night the low coolant light came on when I went to start it. The coolant was 1.5 liters low and there was some white residue and a couple of drops of coolant in the rubber fill boot. The cap and gasket look ok, but I suspect they were leaking. I never saw any indication of overheating on the temp gauge. Anyone have experience or suggestions with this problem? The cap seems rather cheaply made. I wish there was a better quality replacement.

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Thanks for the reply Loren and I did see the past thread on this, but I'm talking about a different cap. I ordered the part number you mentioned just to find out it doesn't fit because it's for the expansion tank in the engine compartment, not for the coolant filler neck in the trunk. That part number is 987-106-044-00 and apparently hasn't been improved.

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Given that upgraded part is not available, is it reasonable to expect that a replacement cap will solve the problem? As I mentioned in the initial post, there are no obvious defects in the cap, but I suspect that is where the coolant was lost. Or is this sort of thing symptomatic of some other problem? It makes me think of Wayne Dempsey's advice to replace the stock 180F thermostat with a 160F unit.

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