996 With Coolant Issues

[1990C4S]

I need some help diagnosing a 996 issue:

This car is a 1999 996, ~80,000 miles. No work ever done on the engine AFAIK, car has a slight tune done by previous owner. In general the car drives and pulls like any 996, maybe better, this is not a weak clapped out engine.

Zero issues driving the car 'sedately', but if I drive it hard:

- Overtemp light goes on, coolant temp goes to just under boiling
- Coolant system seems over-pressurized, never seems to blow off
- Car runs fine with zero other issues when the two points above happen
- If I let the pressure of the system the coolant returns to normal level
- I am not losing coolant
- If I drive the car and let it sit a week, the coolant system is still pressurized
- Coolant might have a slight exhaust smell, or I'm paranoid...

I am thinking small crack in the cylinder? Head gasket? Could my rad cap be the problem? If it doesn't relieve properly that might be the issue?

Probably not a good outcome here...

[JFP in PA]

Time for diagnostics instead of wishful thinking: There are tests kits to check for the presence of exhaust gases in the cooling system; the test is a go/no go test, if the engine fails the test, you have a problem.

[syncroid]

Sounds like you have an air pocket in the cooling system.