Carlos Sainz Jr set to revert to previous engine for Austrian GP

In the closing stages of Q1, the Spaniard pulled off track shortly after leaving the pits with smoke pouring from the rear of his car.

"When I went into the pitlane I could already hear that the engine was not following the throttle," he said.

"Then I looked in my mirror and it was full of smoke.

"Now obviously we're investigating what it was."

It came after Toro Rosso changed engines between final practice and qualifying, installing a previously-used unit from his pool due to a problem in the morning.

Sainz said the problem with the first engine was not severe but it was "just faster" to change it than to fix the issue given the short gap before qualifying.

"The second one looks a bit bigger issue."

Sainz said the team believes the first problem was to do with "an actuator of the turbine" and when asked if the team would revert to that unit once fixed he said: "Probably, yes."

It compounded a miserable day for Toro Rosso with Daniil Kvyat crashing out after suffering suspension failure over the exit kerb at Turn 8.

Sainz had hoped to make Q3 and rued the missed opportunity, particularly as the team is lagging in its battle with Force India in the constructors' championship.

"After P3 I thought it [Q3] was possible," he said. "We made a lot of changes for qualifying apart from the engine.

"We were much stronger and looking at the lap times it was definitely achievable.

"The lap time I had in my pocket, we knew we had two or three tenths and that would have put us in Q3.

"But we keep missing these chances.

"When running on the inter on a drying track, I know I can do something more special than on just dry.

"We keep missing these opportunities and we just lie here with a Force India and McLaren at the top.

"That's not good news and definitely not a good day for us, another missed opportunity."