The loss left the Mariners in last place in the American League West, a game behind third-place Texas and six behind division-leading Oakland.
"For the first 5 1/3 innings, we felt we were in control of the game," manager Mike Hargrove said. "Then the roof caved in."
And the rubble fell all over Pineiro, who gave up six runs -- three in the sixth and three in the seventh -- loading the bases in each inning and watching one swing clear the bags each time.
In the sixth, he allowed a three-run double into the right-field corner by Mark Teixeira. An inning later, Pineiro put men on second and third with one out as Ian Kinsler singled and Rod Barajas doubled. Hargrove elected to fill the bases and set up a double play by intentionally walking All-Star Gary Matthews Jr. to get to fellow All-Star Michael Young, and he brought reliever Julio Mateo in to face Young.
That strategy backfired when Young, the 2005 American League batting champion, drilled Mateo's first pitch down the left-field line for another three-run double.
"It's kind of a pick-your-poison type thing with those two," Rangers manager Buck Showalter said of Matthews and Young.
"I felt if Mateo came in and threw the ball the way he can, we had a good chance of getting the ground ball," Hargrove explained. "It just didn't work."
Until the sixth, everything seemed to work for the Mariners, who grabbed a 2-0 lead on a pair of solo home runs, including the first of rookie Adam Jones' Major League career in the third.
Ben Broussard slammed his 15th homer of the season in the fifth, a drive to right-center that Matthews seemed to have in his sights. The acrobatic center fielder leaped at the warning track but went up a little too high, allowing the ball to glance off the heel of his glove and bounce over the wall.
"He was a little too good of an athlete," said a grinning Broussard, who broke an 0-for-11 slump. "I was blowing that one out. I'm blowing as I'm running. Hey, the wall-scrapers count. I'm glad it got out. Hopefully, it will get me going."
The hope for the Mariners was that Pineiro would get going after his previous start, an ugly loss to Oakland in which he allowed four earned runs in 2 2/3 innings.
And heading into the sixth, the right-hander was cruising. At that point, he had run into trouble only once, when he put two men on to open the fourth and followed with a pitch that Teixeira smoked into the gap in right-center. But Jones ran it down, made a terrific catch and got the ball to second in time to double off Young, who thought it was an easy double and was almost to third when the ball was caught.
Other than that, Pineiro was all but untouchable.
"Actually, we were pitching against the wind, so the breaking ball and sinker had very good movement today. I thought he was pitching great," catcher Kenji Johjima said through an interpreter. "Mostly, he kept the ball down. He was pitching so great until then. It was kind of surprising to give up those big innings."
Pineiro was looking so strong that Hargrove was willing to overlook his struggles in the sixth and, after consulting with pitching coach Rafael Chaves, let him start the seventh.
That, of course, turned out to be a disaster.
"With their lineup, if you give them a couple of baserunners and get the ball up in the zone, which is what happened, they'll make you pay for it," Hargrove said. "That was just one of those innings. It kind of took on a life of its own."