I was hoping the OP would let us know how he got on.

Partly as a matter of courtesy to all those who tried to help, and also to see if my opinion was right or wrong and how it was fixed.

Roger.

 

Yeah, we're all lined up to say, "I told you so!"

- Eric

 

It appears that a radiator cap corrected the problem (at least the immediate one). Contrary to popular belief (not just in this forum - this is aimed at no one in particular, many customers have tried to win this "conversation" before) a faulty radiator cap can cause overheating (among other problems like collapsed radiator hoses). The pressure that the radiator cap holds back increases the boiling temperature of the coolant. Once the boiling starts you are done. each vehicle will experience the overheat to a different degree when the cap can not hold pressure depending on how critical that few extra degrees are (approx 3 degrees per PSI if I remember correctly). I have seen cars that can go all day with no cap and I have seen some that overheat in short order with a failed cap. Are there also other concerns - yes. The vehicle could be close to the threshold due to a radiator with decreased heat rejection capacity. Bottom line is, as someone rightfully pointed out above, the radiator cap has multiple functions. If it fails ANY of them it needs replaced. Note - if the radiator cap vacuum valve fails closed it does two things - collapses the hoses on cooldown AND does not pull coolant back into the system when the vehicle cools down.