Toro Rosso driver Jean-Eric Vergne believes the Faenza team is being held back by Renault's power unit during testing.
The Italian team managed to complete only 139 laps over the four days of testing in Bahrain last week, with only Red Bull, Lotus and Marussia completing fewer laps. Both Red Bull and Lotus are supplied by Renault as well.
Vergne struggled in during the final day of testing in particular, completing just 19 laps before a problem with the Renault power unit forced him back to the garage.
"Yes, sometimes it's the engine and sometimes it's ourselves. But the problem is when you stay in the garage so much. You see the problems with a new car when you drive, so that's a bit of an issue," Vergne answered when asked by crash.net if he feels that the French manufacturer is holding them back.
Vergne added, though, that progress is being made by both Toro Rosso and Renault and with four days of testing left before the season starts next month, there is still time to improve.
"I will not talk negatively because I fell like it and everybody knows it, so I don't need to say anything else," he continued.
" I'm positive. We still have major problems and they were able to fix some of them. I know it's going to take time, I knew arriving here that nothing would have been fixed [immediately].
"I have big faith in the people working in the team and also in the Renault people. They all know what the problems are and I am confident they can do something about it."