Yes, the cap should have opened to relieve the pressure.

Instead of buying a new expansion tank maybe you can use a good sealant to stop leakage since the pressure it is under is relatively low. At least till you rectify the problem.

If it was the head their should be mayo under the oil cap or oil in the coolant or some other sign of it leaking.

Hope This Helps!

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William
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1987 420 SEL - Like a fine wine only better with age