Fernando Alonso says his McLaren team needs to conduct a "deep investigation" into the "clear" lack of downforce affecting his Formula 1 team-mate Stoffel Vandoorne.
Vandoorne, who is yet to outqualify Alonso in 2018, has been well off his team-mate's pace in the past two F1 weekends.
The Belgian said his MCL33 was "undriveable" at Silverstone and endured his "worst Friday" at Hockenheim, before setting the slowest time in the first segment of German Grand Prix qualifying.
Alonso, who will line up 11th on the grid at Hockenheim, has now backed Vandoorne publicly and called on McLaren to solve the issue.
"I think he's doing what he can," Alonso said. "Definitely, the last two races it seems that car has some kind of performance issue, and there are clear signs on the data that there is less downforce on that car.
"We've been changing some parts, and we need to go deep into that investigation because with both cars close to the points is always a little bit easier."
Speaking after qualifying, Vandoorne described the predicament as "very disappointing" and "hard to accept".
"I think the situation is the same as Silverstone," he added. "Since we hit the ground in Silverstone, we were by far the last car and it turned out the same today.
"I can't really tell anything more. It's frustrating. We changed a lot of parts on the car and it still doesn't work.
"Definitely I didn't forget how to drive a car. But the cars are in a similar spec, it's just that we can see an issue on the car.
"We've changed over a lot of parts already and at the moment we don't really have a solution to just make it work normally."
Vandoorne said McLaren broke curfew overnight to make changes to his MCL33, but admitted that "we're not really moving forward".
Asked whether the problem could be addressed through set-up changes, he said: "It's not linked to the set-up at all. It's something on the car that doesn't work.
"We've almost changed everything on the car. We see the problem on the data. Right now, we don't have a solution."
While Vandoorne's misery continued, Alonso's 11th-place effort was McLaren's best qualifying result since the French Grand Prix.
But with Lewis Hamilton's hydraulics failure and Daniel Ricciardo's grid penalty aiding his grid position, the Spaniard reckoned McLaren was no more competitive than in prior races.
"I think it's been a similar performance," he said. "We were P13 [at Silverstone] and today without the Hamilton and Ricciardo issues we were P13, so I think we are more or less in the same position.
"But on Sundays normally we score some points and we are alive thanks to that in the championship, and tomorrow we plan to do the same, to score a couple of points and keep fighting."