The gas-cap-fix, and the mysterious disappearing coolant!
 

On a previous thread I talked about a fueling issue and a strong recurring fuel smell in the car. My shop can't get to it until Wed. :(
I went ahead and picked up a new gas cap at the dealer (mine was original) and both problems immediately went away. Could be coincidence.... Either way, I'm currently back on the road for $16.

The coolant issue has been there from day one (8 months ago for me). It just flat disappears at the rate of about a gallon every 4,000 miles. No leak, no overheating, no boiling over, nothing. I have had the cooling system gone over twice and they can find no evidence at all of a leak anywhere. System pressure tested, engine leak-down tested and the coolant tested for contamination. Everything checks out just fine. Car always runs cool even being driven all over the city every day in 105+- degree weather.
The only other unusual thing I notice is that the rear trunk gets incredibly hot as I'm driving around all day. How hot? You can't touch anything metal, and a plastic bottle I once put in there partially melted.
I don't know if steam is escaping from the coolant tank into the trunk or if there's a missing heat shield or insulation blanket in the engine compartment that is superheating the trunk and the coolant tank. :confused:
A search didn't reveal any issues this quirky, so any suggestions would be greatly appreciated! :)

My trunk gets pretty hot too when I drive, but not to the point to melt a plastic bottle.
I read this was normal though...

Did they test for leaks when the car was hot? I had a very elusive leak that only expanded enough to leak once the engine and coolant had warmed up for a while.

Have you noticed any condensation inside the rear trunk? Specifically, around the coolant cap?
What coolant are you using? The Porsche coolant dries as a white residue to help identify leaks, any signs of a white residue anywhere?

My first thought would be some kind of very steady and slow leak from the coolant tank or cap (Do you have the latest coolant cap?). That would explain the coolant loss and the heat. The trunk should not be getting that hot.

Any chance of an air bubble in the coolant system? Doesn't an air bubble expand pushing coolant out? If your not seeing steam condensation in the trunk - maybe look in the frunk. I would go look into under the windshield cowling towards the heater core. Maybe the leak is coming out of the cowling area.

Ever look at the radiators? Has the system been burped multiple times and only then closed up? Or just filled up?

Multiple possibilities

coolant cap
coolant bottle
hose connecting coolant bottle to engine
pressure release valve itself
water pump
hoses to radiator
radiator
heater core
heater hoses

any connection

cylinder heads, gasket, block

I had coolant level going down a little as well. I noticed there was condensation over the oil/coolant caps.My car has a rubber gasket around the cap area.

I changed the coolant cap and the condensation stopped happening as well as the level change

Have you changed the coolant cap? Mine was the newest version and looked fine, but I changed it anyway

Thank you for the replies!
I'll try to address the specifics.
I put on the newest cap a couple of weeks ago but it didn't change the behavior. There is no moisture or signs of condensation in the trunk. I checked under the carpet and it's dry. No residue visible at least in the easy to check places. The shop had checked it both cold and hot. radiators have been checked, but I'm not sure about the heater core. All lines were checked and one loose clamp was found. Replacing it unfortunately didn't fix the issue.
I don't think the bottle has been removed to be checked out. The system was purged at least once. Water pump is fairly new.
Yes, using the Porsche approved coolant. ;)
Just a maddeningly evasive habit here.

Where did you buy your coolant cap? Does the part number end in "04"?? Are you sure it's really-really tight, and NOT cross threaded???

I got it at the Porsche dealer, and I always "back thread" it a bit to make sure it doesn't get cross threaded going on. I batten it down pretty tight. ;)
I'll have to check the part number.