Nico Hulkenberg says the problem that derailed his Spanish Grand Prix qualifying came "out of the blue" despite earlier issues in practice, as Renault believed it was "on top of" the situation.
The German had made the final qualifying segment in all four of the first grand prix weekends of the 2018 Formula 1 season, but was left only 16th on the grid in Spain, his Q1 exit triggered by what Renault described as a "fuel pressure issue".
Hulkenberg had been unable to log a representative lap at all on his first run, and could only squeeze in one proper effort late on - which was not enough to for him to advance as McLaren's Stoffel Vandoorne beat him to a Q2 spot by 0.038s.
Talking to media after the session, Hulkenberg revealed that Renault had already discovered a fuel system issue in Saturday morning's practice run - and that the team "checked everything" ahead of qualifying.
"We had a little bit of a problem with the fuel system in FP3, but the guys went into the tank, checked everything," Hulkenberg said. "We thought we were on top of it, and didn't really expect any problem.
"So it is a little bit out of the blue, and we didn't expect it."
Hulkenberg confirmed that the fuel problem affected him on both of his runs in Q1.
He explained: "[It was] just fuel drop-off, so you just lose the drive, you lose power.
"It's like the engine just stops for split-seconds, but obviously you lose acceleration, you lose speed."
While Hulkenberg's qualifying ended in disappointment, teammate Carlos Sainz managed to break through to Q3 and put his RS18 on the grid.
This, allied to the two cars' solid showings in FP3, was a marked improvement on Renault's pace from earlier in the weekend, as Hulkenberg and Sainz had ended both of Friday's sessions well outside the top 10.
"The huge step forward that we've done compared to yesterday is pretty impressive, no?" Sainz said.
"I think we were not even in the top 12 yesterday and all of a sudden we're fighting for the top eight again.
"So it has to be a very positive day for us, a lot of progress made by the team, very good analysis done yesterday night to bring together a much better package for today."
Asked what exactly brought on the turnaround in Renault's form, Sainz said: "A lot of set-up changes. I cannot specify obviously but we did change quite a lot on the car.
"The track cooled down a bit, bit less windy, makes things easier for everyone out there - on top of that, the set-up changes we've done are definitely better."
Additional reporting by Scott Mitchell