Diagnosing a failing oil cooler starts with monitoring the delta (variance) between engine coolant temperature and engine oil temperature. With the engine up to operating temp, both parameters should stay within 15 degrees of each other. Generally speaking, a 3 to 6-degree delta is considered normal, while a difference of 10 degrees or more warrants monitoring. Here you can see that oil temp is a toasty 41 degrees warmer than coolant temp, which is indicative of a plugged oil cooler. It’s important to note that you cannot rely on your dummy gauges to catch a temperature discrepancy. Aftermarket gauges and monitors from Edge Products, Scan Gauge, or SCT are commonly used, as is the OEM-supported Ford IDS.