Smoke coming from my radiator cap

My truck had a over heating issue and we changed the water pump on it . But now i crnk it up and in only a few minutes smoke comes from the radiator cap but the gauge doesnt register over heating . I cut it off and it stops pretty quick. I thought maybe the thermostat needs to be changed? thats my next option but just curious if anyone knows what i am talking about or the issue and more importantly the fix? Please cause thats all i have.
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