At the rain-test facility at Nissan Technical Center (NTC) we meet with Wataru Funasaka, and he casually remarks, "I can gauge the rainfall intensity just by standing in it."

"See it, feel it, touch it, and you'll understand it." One can almost sense a feeling of a sixth sense at work here when one hears Funasaka speak about water, a skill honed and perfected after years of dedication and commitment to this craft.

Just before the start of mass production of a new vehicle, water leakage was discovered inside the door of a final-stage prototype. "It was a significant leak," recalls Wataru Funasaka, Technical Expert for Door System Performance Evaluation (Water Sealing), whose job is to ensure vehicles are protected against water intrusion into the cabin. Only a little over a dozen prototype vehicles existed at that stage. The leak was found in one vehicle. Just one vehicle. Yet if that incident represented a failure rate approaching 10%, it was something that simply could not be ignored by the team. The search for the root cause began, and the team spent the next few days investigating.

Funasaka leads a globally connected team based in Japan. While ensuring the quality of vehicles produced domestically, he has also trained numerous technical engineers at manufacturing plants around the world, many of whom he considers his students. These technical engineers apply the same rigorous standards to water-sealing performance wherever Nissan vehicles are built.

From Mexico to other manufacturing plants worldwide, they conduct the same tests up to the start of mass production and verify that all vehicles meet Nissan's requirements. Together, this team safeguards the watertightness of Nissan vehicles around the globe.

Developing both the people and the technical expertise needed to maintain those standards worldwide is also part of Funasaka's role.

"Delivering the same level of quality for every Nissan vehicle, everywhere in the world. This is what we strive for every day," says Funasaka. "To ensure customers around the globe can trust and be satisfied with Nissan quality, I also focus on developing the people who will carry those standards forward. Across our teams in the United States, the United Kingdom, Thailand, Mexico, China, and beyond, we share the same testing methods, the same mindset, and the same commitment to quality.

Rain is not the only source of water intrusion. Water can also enter around window frames, doors, sunroofs, and other openings during vehicle washing. For customers, a leak-free vehicle is simply expected. That is precisely why Nissan subjects its vehicles to extensive testing designed to simulate real-world situations experienced by customers around the world.

・Heavy rain.
・Driving through severe downpours.
・Washing with household pressure washers.
・High-pressure automated car washers at gas stations.

Each scenario is carefully recreated, and only vehicles that meet Nissan's stringent water-sealing standards are released to customers. These demanding tests are not limited to a few premium models. From the Kei car Roox to the all-new Kicks and all-new Elgrand, Nissan applies the same water-sealing standards across a remarkably broad range of vehicles.

This reflects a fundamental principle of Nissan manufacturing:

Customers expect every vehicle to be leak-free, regardless of size, price point, or market. Nissan therefore holds every single vehicle to the same standard. However, water sealing requires balance.

"There is also the issue of door closing performance," says Funasaka. "If you focus solely on making the vehicle completely airtight, the door becomes difficult to close. Excessive airtightness can also create pressure changes that cause discomfort, such as a popping sensation in your ears when opening or closing the door."

While a power sliding door could address some challenges, but this introduces its own design constraints.
Take the Nissan Z, for example, which uses frameless (sash-less) doors. This presents unique challenges in balancing a sporty design with weather sealing. Since the Z33 generation, Nissan has maintained cabin sealing by designing the edge of the side window glass to fit tightly into the rubber weatherstrip when the window is fully closed. To enable smooth door operations, the glass automatically lowers by about one centimeter when the door is opened or closed, a mechanism known as partial down. This illustrates how design, usability, engineering, and water-sealing performance must all work together to prevent leaks.