my friend had a problem like this on a similar swap. He had opted to keep his half-sized radiator from the civic shell he was working on and put in a b-series integra motor. When he switched to the full sized radiator from an integra (which I believe he had to custom fit in unless I'm mistaken), it actually fixed his problem. I don't know if you are having the same problem or if you gauge is just broken, but it's a thought.