McLaren boss Eric Boullier admits Honda's reliability on the final day of testing in Bahrain was "a bit strange" given its struggles during the opening day and the preceding grand prix weekend.
Boullier dubbed Wednesday, the final day of Bahrain's in-season test, as the team's most successful day of 2017 so far, with Stoffel Vandoorne compiling 81 laps. It came after the engine manufacturer hit trouble after just two laps on Tuesday, which followed a miserable Bahrain Grand Prix weekend blighted by MGU-H issues.
Though encouraged by the final day's mileage, Boullier does not understand the reasons behind Honda's sudden turnaround.
When asked on Wednesday if Vandoorne's successful day hinted that the Bahrain weekend would be an anomaly for Honda, Boullier replied: "No, because it's very difficult to understand what's going on. We obviously changed the MGU-H four times and [Tuesday] morning, two laps, MGU-H gone, changed the engine, did 17 laps. [Wednesday] we had a small alert but we couldn't find anything wrong physically on the car and then since then we've run faultlessly, without any problems.
"So I think it's going to be complicated for Honda to understand what's wrong, it could maybe be coming from some batch problems. I don't believe it is a car build problem but we can't find what's wrong... We can do two laps or we can do two days [worth of running]."
He was then asked if the heat was a factor, and said: "It is a little bit less hot today, by the way. Maybe these five or six degrees are good enough, but I don't know. It's a bit strange."
Boullier -- who said McLaren used the gremlin-free final day to run the car with aggressive settings -- is pleased the team finally had the chance to gather a day's worth of data.
"We had good days last year as well I think. But today we are following the test plan and a lot of information has been understood and solutions tested on the car. So a very good test in terms of data and results."