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Renault has revealed that its Formula 1 testing efforts at Jerez have been hurt by reliability worries over what it calls a "stupid part" on its engine.

While rivals Mercedes and Ferrari made an encouraging start to their 2015 programmes, Renault outfits Red Bull and Toro Rosso have had a tougher time.

AUTOSPORT has learned that concerns about the durability of a small metallic shaft that is related to the Renault ERS water pump has left both its teams having to limit their maximum stint lengths.

Renault's managing director Cyril Abiteboul revealed that the problem with the part only emerged in dyno testing last week - which meant it was too late to sort out a solution for this week's running at Jerez.

ANALYSIS: The full story of Renault's problems

However, a proper fix will be in place for the next test, which starts at Barcelona on February 19.

"We know what the issue is; we know what we have to do," Abiteboul told AUTOSPORT. "It is just that logistics did not allow for this to take place here.

"It is a metallic shaft that we have to change, and we have to optimise and review the design.

"It is something that was working very well last year, but we decided to change and improve it a bit further with the overall packaging of the engine to also support Red Bull in their attempt to also have very good packaging. That is why we did not really care for that part.

"Usually you have very specific simulations, design tests, and validation protocol. But honestly we did not do it for this part because it is such a stupid part..."

UNRELIABILITY A CONSEQUENCE OF WINTER PUSH

As well as the issues with the metal part, Ricciardo had to have an engine change on Tuesday and both Red Bull and Toro Rosso have encountered battery problems related to unrefined mapping settings.

Abiteboul says that the reliability problems that Renault have encountered in testing are the consequence of its aggressive push to close down the gap to Mercedes this year.

"We are trying to keep things calm and under control," he explained. "I think strategically for this season and next season, the sort of gap that we have to face is not the sort of gap that you can recover in one winter.

"It is such an environment that we have no choice but to be a late braker. And when you are a late braker in this business you must be prepared to take some risks and see a few issues on track."