Having failed to start in Russia two weeks ago due to an engine failure on the formation lap, Alonso departed the Barcelona circuit after a few choice words and later posted a picture on Instagram of himself playing tennis.
"The engine was not good enough. We came out of the pitlane and there was a hole in the engine, and the oil was streaming out. It blew up after 400 meters," Alonso told reporters after returning for the second 90-minute practice session.
He was last on the timesheets and reported that the engine was even slower. If the tennis photo looked like a dig at Honda, accompanied with the seemingly sarcastic comment "keeping the body active", he said that had not been his intention.
"I have very little time in these weeks, with traveling, with planes," said the 35-year-old, who will go straight from Barcelona to Indianapolis on Sunday night to prepare for his debut in the 500 on May 28.
"So when I discovered that I had two hours free, instead of being on the sofa and watching television, I went for some training. My dedication is still 100 percent to my fitness and my preparation.
"It wasn't humor to go outside the circuit to play tennis, it was preparation. People got it wrong. I went out to have some fun and escape the circuit."
McLaren, the second most successful team in Formula One history in terms of race wins, have yet to score a point in four rounds of this year's championship and are last in the standings. Honda's Formula One reputation, after two seasons of unreliability and poor performance, is meanwhile being dragged deeper into the mud.
Alonso is out of contract at the end of the season and has said his future whereabouts would depend on who could offer him a winning car.
"It's not my reputation, it's theirs (Honda's) – and it's their money, and their image," he said. "I try to drive as fast as I can but it's a much bigger problem for them."
(Reporting by Alan Baldwin, editing by John Stonestreet)