@tallpaul , Changing the vent line is more than a few hours work even for an experienced tech. There is a lot to disassemble and everything is in the way. Anyway, I just changed the vent tube (was in good condition) and upgraded to the bolted thermostats housing. One of the weak points is the metal tube that runs from back of the thermostat to the back of the engine. The tube o-rings were no longer sealing well at 50k miles, GTS. If your car is losing coolant you should inspect the engine “V” region with a borescope.

 

@ciaka did a great write up on the starter, and the you can see the steps for removing the intake manifold at the first couple. It's detailed but not difficult for an experienced garage mechanic. Didn't think it was that hard, but it had a few PITA if your car is older (brittle T, old glue that wouldn't come out). Even on my Turbo with all of the extra plumbing.

If your car is older, it's worth replacing all of intake manifold gaskets while you are in there. Good time to check and replace any gaskets on the air/oil separator as you take it off as well. Also, if your belt is showing any cracks, its easy to get at with the intake manifold off (not technically required, but that much easier). Easy maintenance item to knock off the list too.

 

Looking at the original post, have few observations.

OP did not say if coolant was low. Only said mechanic inspected car, pressure tested (best way to test for leaks), found no leaks, topped off coolant and replaced cap and seal.
If you pressure test cooling system, you WILL see leaks if there are any.
If you pressure test coolant, you will see pressure change if you have ANY leaks.

Assuming the mechanic is not dumb and blind, would see all that. Any mechanic shop has proper tools to check this.
So to me it seems likely that OP does not have a coolant leak.
One place he may want to check for seepage, is up front, by radiator, where coolant line goes into rad. Look for evidence of dried coolant.
Other than that, my first impressions are
- car coolant was filled by hand, not via venturi system fill, creating air pockets in the system. Porsche cars are bad for air pockets if you don't fill using venturi fills. Burping often does not help a lot. Only way is to fill properly.
- car thermostat is not working correctly, sticks, and temp spikes are observed on cluster.

- if you have to add even little bits of coolant,you have a leak. Fix time.
- if no leak, get venturi system and drain, refill system using venturi fill, so car has no air pockets.
- if above doesn't help, you have to get coolant out, remove intake and replace thermostat, seal, etc. I replace intake seals most every intake removal. If i know seals are near new, I may leave. Otherwise replace them. Brittle seals are best way to get CEL on cluster and winder what's wrong.


Yes, my write ups show all you may need to for your post here. Hope this helps.

 

The vent lines going towards the coolant bottle are an alternate way to remove air from the system, directly. They are easy removed from the “burping” device. I used a shop vac and a clear plastic tube to draw the air out after filling the coolant bottle and leaving the cap off. Easier to get the venturi tool and use that but I didn’t have one available.

Before using vacuum on these tubes, I used a section of radiator hose to create a vertical column to fill the engine block at the thermostat housing as much as possible. Then also filled the hose from the radiator cold leg (attaches to the left pipe of the thermostat housing.