What could be causing my 1991 Honda Accord LX to surge in neutral?
 

I have an Automatic 1991 Honda Accord LX with now 132k. I bought it at 128k.















It has been doing it ("surging" or revving RPMS) for a little bit, it started not too long after I bought it used from a dealer.















It actually started after I lost my gas cap about a month after I had lost it. It took me a little bit to get it replaced. So for about a month or two, I've been having issues with it surging in neutral, and in park. It stops though after I get it in gear. And sometimes, but rarely, it doesn't do it at all/or as bad. I replaced the gas cap a month ago, yet it's still doing this, I thought it could've been cause of that, as told by my uncle who had a similar problem before with his son's earlier Honda. But that doesn't seem to be the problem.















I love the car, it's in great shape for it's age, and amount of miles. I just don't want to start putting a ton of money in it just yet.















Any D.I.Y. or hints of what is or might be to come?































Thank you!

I think the first thing I'd explore would be a vacuum leak. It's a fairly inexpensive problem to both diagnose and fix.