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Oil cap yes, but not fuel cap. Oil cap creates vac leak, but fuel tank is vented and shouldn't care if there is a cap or not.
If the fuel cap actually helped, it may point to an issue with fuel tank venting and fuel pressure, but It's hard to imagine that causing issues with surge at idle and nowhere else in the map. I don't know what the stock vent line is supposed to look like (my car is a bit of a mongrel), but there is a tank vent tube on the filler neck that might warrant a look.
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