After more than 217,000 miles and nearly 24 years of loyal service, we’d say the factory oil cooler had a pretty good run aboard this 1997 model year 7.3L Power Stroke. Finding a wet spot where the oil cooler housing slides into the rear header (also known as the oil cooler filter adapter or oil filter base) or front header is a telltale sign that the outer O-ring has failed. Early on in the failure, engine oil leaks past the O-rings when it’s cold, but stops leaking once warm (when the O-ring swells). Although you can get by with a leaking oil cooler for quite a while, the problem will only get worse until it’s addressed.