Perez identified a discrepancy between what the data analysis of wind tunnel tests was telling them compared with what the team was finding when they actually went out on track. It's a similar issue to that which Ferrari wrestled with in 2012.
"I think we need to keep working with the correlation," he said. "Once we get that right we can start to improve the cars in big steps."
And Jenson Button felt that the problem was getting better at Barcelona as the team returns to the track where the F1 pre-season testing days took place.
"It is good to see that the data from the wind tunnel and what we have seen today correlates, so it is something that we can build on," said the 2009 world champion. "Whether it's this weekend that we move significantly towards the front, I doubt it. But overall it was a promising day in regards to the future.
Even so, Perez admitted that it would take "a perfect weekend" for the team to be able to score any significant points in Barcelona. Perez had actually scored more points with Sauber by this stage of the season last year than he has so far at McLaren. Neither he nor Button have come close to securing a podium finish in 2013, and McLaren is now sixth in the constructors championship behind Force India.
But Button insisted that the team wasn't going to give up and that even if the title was no longer a realistic objective there was still the motivation of going out and winning Grand Prix races.
"Whatever happens this weekend, the guys will not get down," he said. "We are very good at improving over the season, so even if this weekend is not as good as we had hoped we will still keep on pushing. This team wants to win races and that is what we will aim for.
"You have ups and downs and this team has been very strong in the last couple of years even though it hasn't clinched a title, but we've won many Grands Prix," he said. "The aim is to move forward with every race and I would say that we've definitely are moving further to the front.
"The downside is: everybody else has too."