Toro Rosso's final day of the first Bahrain Formula 1 test of 2014 was held back by a "major problem" with its Renault engine, according to Jean-Eric Vergne.
The Frenchman completed only 19 laps on Saturday, spending most of the day in the garage while his team worked with Renault to get the car up and running.
"We had a major problem today," Vergne told reporters in the Bahrain paddock immediately after the session, before adding the issue was on the "engine side".
"We all know there are some problems from the beginning so there is nothing new," he added.
"I will not talk negative because I don't feel like it and everybody knows it so I don't need to say anything else.
"I'm positive. We still had major problems but they were able to fix some of them. I know it's going to take time."
Bahrain test day four report
Vergne said that he feels Toro Rosso has produced a good car this year, and he believes its problematic start is holding back the STR9's potential.
"I've been really pleased with the car behaviour," he said. "We've got a good package.
"When the engine will be there I think we will look more or less OK. The team has done a good job to give me a chassis that I like."
He added that he has faith in Renault's ability to solve its issues as soon as possible.
When asked by AUTOSPORT if next week's test is too soon for all the problems to be fixed, Vergne said: "It's always too soon, I guess.
"We need to drive as much as possible. You only see the problems of a new car if you drive.
"Next week is not the first race, so maybe if we still have problems it doesn't matter.
"All that matters is we arrive in Melbourne with a good package and we are able to race.
"I have big faith in the people working in the team and the Renault people. They know what the problems are, and I'm confident they can do something about it."