A customer brought in a mid-sized wheel loader that would run perfectly for about an hour, and then the hydraulic oil temperature gauge would rapidly climb into the red zone, causing the machine to automatically derate and barely move. The shop had already replaced the hydraulic oil cooler, thinking it was plugged, but the problem persisted.
When a machine overheats with a brand-new cooler, you have to look at the airflow, not the fluid. We started the machine and let it run until the temperature began to spike. We then took a piece of tissue paper and walked around the back of the machine where the coolers are mounted. When we held the tissue in front of the hydraulic cooler, it just fluttered limply. When we held it near the engine radiator, it was sucked forcefully against the screen.
The issue was a missing or damaged air shroud. In heavy equipment, the cooling package is a sandwich: the hydraulic cooler is usually mounted in front of the engine radiator, and a large viscous fan pulls air through both. For this to work, there must be a rubber or metal seal-an air shroud-around the edges of the coolers. Without it, the fan doesn't pull air *through* the hydraulic cooler. Instead, the fan takes the path of least resistance, pulling air from the gaps around the sides of the cooler. This creates a "vortex" or dead zone where the air just spins in circles behind the hydraulic cooler without actually flowing through the cooling fins. Because the air wasn't passing over the aluminum fins, the heat had nowhere to go. We fabricated a simple replacement rubber shroud and bolted it in place. The tissue paper immediately sucked tight against the hydraulic cooler, confirming the airflow was restored, and the machine never overheated again. A ten-cent piece of rubber fixed a multi-thousand-dollar headache.
