Haas has blamed struggles with tyre temperatures for its drop in performance during the opening day of practice for the Russian Grand Prix.
Romain Grosjean complained during FP2 that "something" had been wrong with the car since the Chinese Grand Prix. The Frenchman had blamed Pirelli's "ridiculous" tyre pressures for his struggles in China, which he said had made the car undriveable.
Speaking after FP2 at the Sochi Autodrom, he said: "We need to analyse. Since Friday in China I am not happy with my car, it was the case again today. We need to understand what could possibly go wrong and hopefully change it. I think there is a lot of work in progress, a lot of question marks. A lot of data. It is hard to trust everything but by tomorrow I think we find some good solutions."
Asked if tyre pressures had changed since China, he replied: "They are the same as in China and it is certainly something I don't like, driving style wise. But I have to do a better job to adapt. It is what it is."
Teammate Esteban Gutierrez also struggled with tyre pressures and thinks Haas needs to get on top of it before Sunday's grand prix, which looks set to be a one-stop race.
"We completed the programme fairly smooth, but not so happy overall because we are struggling to get the tyres working in the right window, they seem to be very, very sensitive here, especially with these low temperatures of the track and that's our main task for the weekend. Mainly thinking about the race because it won't be a degradation factor, it will be on what window you are in with the tyres in proportion to the degradation and also the components that we are using."
Team boss Guenther Steiner said: "We are struggling a little bit with that one and we are trying to find some ideas of how to get them in the window for tomorrow ... but today it was good, we just need to get the tyres in the window. They both struggled ... once it is overheating in the back and then too much pressure in the front. I don't know if other teams have got the same problem but I think it's a maybe a little bit of experience we miss."