Stoffel Vandoorne's "rally" car, as he put it, was the result of a large piece of tape that was stuck in the diffuser.
Vandoorne had an unhappy last outing in 2017 when he finished down in 12th place at the Abu Dhabi Grand Prix.
The McLaren driver described his car as a "rally" car as he struggled with the handling.
McLaren have revealed the cause of the issue.
"We think that Stoffel spent the whole race with a large piece of tape stuck in the diffuser," Vandoorne's engineer Tom Stallard told Motorsport.com.
"The kind of super-sticky two-inch tape that touring car teams use to reattach the front bumper. At the end of the race, there was a 20cm piece still stuck to the left-hand diffuser fence.
"During the race, we believe this tape was much larger. We measured a loss of downforce equivalent to opening the DRS, but all around the lap, and without the drag drop.
"Also the loss on the diffuser was symmetric, but at the end, the tape was only on the left-hand side.
"Around lap 22 the diffuser partially recovered and Stoff's pace improved. We think until this point it was caught on both sides, but then snapped and was then only on the left.
"I would guess the original piece must have been around 50cm long."
He added: "Pierre Gasly, who was behind Stoffel in the first stint, said the car was incredibly oversteery, and also that he could see something flapping in the diffuser.
"Given this, I think Stoffel did an extraordinary job in not only keeping his car on the track but also in keeping six cars behind him."
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