Have you ever noticed iridescent rainbow stains on your driveway after it rains? What you’re looking at is the thin-film interference principle at work, and a sign that someone’s car is leaking. But before you can address the source of the dripping, you need to figure out where it’s coming from and what it is. One of the easiest ways to do that is with a large piece of cardboard and a little detective work. I enlisted the help of two mechanics—Jason Farrell, an ASE-certified master technician and the editor of Mechanic’s Diary , and John Lim, a mechanic and the owner of JB Motor Works—who walked me through the steps of the cardboard test and how to identify the leaking fluid.
How to Diagnose Leaks Under Your Car | Autoblog Wrenched
While the cardboard test is fairly simple, there are a few things you should know to get the most out of the process.
Parking: When you're done driving, "try to park on a flat surface to prevent fluids from running off the cardboard," Lim says.
Wait: As a safety precaution, Farrell recommends waiting until the engine has cooled before starting.
