Hello again guys,
The patient is an '84 euro 300D (non-turbo). It is my brother's car that I've commandeered for the week whilst my '82 is down with a sick water pump.

The coolant level was low and the car would start to overheat every time I came to a stop; the temp was okay (if slightly higher than I like) when moving. There's no telling how long he's been driving it like that (mostly around town stop-and-go stuff).

I went to add some water this morning, after the car had been sitting overnight, and the rad cap (new 1.4 bar) opened with a gunshot-esque POP and shot out of my hand. That much pressure after sitting for 14 hours makes me worry about exhaust gasses in the cooling system, the head gasket, and generally more wrench turning than I care to contemplate. It doesn't have a coolant reservoir so I can't see if there's oil in the coolant, but the oil doesn't appear to be contaminated. It runs like a top with no smoke and oil consumption is very low.

I'd appreciate it if someone could reassure me that I'm just being an alarmist and that this is somehow normal and/or easily remedied.

Thanks,
Tom