McLaren cannot explain why the engine problems that ruined its Bahrain Grand Prix did not reoccur during the second day of Formula 1's in-season test at the Sakhir circuit.
After ongoing MGU-H problems marred its Bahrain Grand Prix weekend, and the first day of testing on Tuesday, the F1 team was unsure what to expect on the final day of running on Wednesday.
Stoffel Vandoorne completed a full testing programme without engine problems, setting the fourth best time overall and completing 81 laps.
But with Honda having made no modifications to the specification that hit trouble after just two installation laps on Tuesday, McLaren racing director Eric Boullier said he could not explain why things had improved so much overnight.
"If we knew...this is the problem, I don't know," he said when asked about the turnaround.
"Everything has worked perfectly. We are even actually pushing a little bit in terms of settings, trying to use the opportunity, and everything is working.
"This is a proper test today, everything you plan you can test it and do back-to-back, so it's very good."
HONDA PROBLEMS 'COMPLICATED'
Honda made temporary fixes to avoid a repeat of the MGU-H problems that ruined its Bahrain race weekend, but Boullier cautioned it would be wrong to believe the Japanese manufacturer has now put reliability concerns behind it totally.
"It is very difficult to understand what went wrong," he added.
"We obviously changed the MGU-H, and yesterday morning after two laps the MGU-H was gone.
"We changed the engine and did 17 laps yesterday, but we couldn't find anything wrong physically on the car, so I think it's going to be complicated for Honda to understand what's wrong.
"It's maybe coming from a batch problem, maybe some anomalies are car related... there is no common cause."