Can I extend my tourist visa in Thailand and re-enter after visiting Malaysia?

you will be stamped in for 60 days, then on Immigration you can buy one 30-days extension for 1900.- THB. Before the 90 days are over, you need to leave Thailand. Take a holiday in Malaysia, and re-enter Thailand "visa-exempt" which means you will be stamped in for 30 days. And you can also get another 30 days extension for these 30 days visa-exempt, for another 1900.- Baht. You can get almost 150 days stay permit in Thailand out of this method.

Yes it’s possible (and easy). And you will receive another 30 days when you re-enter from KL.

You can apply one time for a 30 day extension. Don't forget the day you arrive is day 1 for when you are counting days.

If your passport is from a visa exempt country you'll receive a 30 day stamp when you return from Malaysia and you if you don't have a visa.