Nick Heidfeld has admitted that he is less happy with the balance of the Renault R31 at Barcelona than he was during his first test in the car at Jerez last week.
The 33-year-old completed 41 laps, setting the fifth fastest time of the day, after taking over from team-mate Vitaly Petrov for the afternoon session.
But despite his concerns over the handling of the car, he is confident that set-up changes will improve performance.
"I was not as happy with the balance of the car here as I was in Jerez, but I only had the afternoon, which is not a lot of time to make changes," said Heidfeld. "It's not outside the window [balance-wise], but it can definitely be improved.
"You never have a car that you fully like and there are always things to improve. In Jerez, we were pretty close to a car that I like with only small things that I didn't like. Today, I was surprised it was so different, but the track and conditions are different.
"It's very interesting and we need to understand it because you don't have so much time to find the setup. And we also have to understand what part the tyres play.
"We made some improvements both mechanical and to systems and hopefully on Monday I will get a lot more laps and try more things."
Heidfeld is confident that he will have no problem adapting to the style required for a car built around old team-mate Robert Kubica, adding that he is not convinced that focusing on one driver makes much difference to its characteristics.
"It's difficult to know how much influence that has and how much it is possible to build a car around one driver," said Heidfeld. "I will just try to do my best and not think about it.
"I will just try to do my best and not think about it. The car is what it is and whoever the car is built around doesn't make any difference to my work."
Petrov will take over for tomorrow's running, with Heidfeld back in the car for the final day of the test.