If your 928 is 20 years old or older, and still using the stock radiator and/or the stock plastic coolant reservoir, it may be time to back off the radiator system pressure a tad.
The cross-tubes in our Behr radiators grow weaker with age and can burst, and we are starting to see more radiator side tanks and expansion tanks crack also – all from the age of these components while still trying to run pressure designed for the car when it was new (13 psi)
Replace that OEM radiator cap with one of our new LevRvent-style caps at 7 psi and save yourself a boat-load of headaches and expense later. 7 psi is plenty for the early cars, still prevents it from boiling over, yet is al lot gentler on the cooling system. The lever on top provides positive latching when closed, and allows safe releasing of pressure without water-soaking your clothing.