Country superstar TIM McGRAW is hiding a big secret from his fiery wife Faith Hill, and sources tell The ENQUIRER that it could spark another one of her jealous outbursts.

“When they got married, Tim promised Faith that he would never lie to her, and he doesn’t,” said an insider. “But he doesn’t tell her everything. He keeps some secrets to keep the peace.”

While sources say Tim is devoted to Faith and their three daughters, her jealousy is out of control.

“Every time Tim glances at another woman, Faith jumps all over him,” revealed the insider.

 “She’s never forgotten that Tim broke up with another woman to be with her, and she’s worried that he could do it again – to her.”

And the secret that Tim is hiding now revolves around that former girlfriend, divorcee Kristine Donahue. At one point, Tim promised to marry her and help raise her then young son.

The secret also involves Tim’s troubled relationship with his late father, baseball great Tug McGraw, who refused to recognize Tim as his child for many painful years.

It’s a fascinating story that sounds like a country tune about lies, jealousy and betrayal – and you can find it ONLY in the latest issue of The ENQUIRER!