+1 on the filler cap The system has to run under pressure to prevent a boil over.

How's your fan clutch? The silicone goo can drain out over the years and leave the fan to free-wheel on the shaft. Give it a spin by hand and see how easy it is.

Last thing: When's the last time you changed your coolant? It might not be a bad idea to test it with a hygrometer and make sure you've got a 50/50 mix.

__________________
"It's not about how fast you can go, but how well you can go fast."

Bob in Richmond

'97 S320 (LWB), Ruby Red Metallic, 73k miles
'97 S420V, Smoke Silver Metallic, 155k miles