The engine oil cap is one of those essential items that people sometimes overlook. The primary role of oil caps is to keep the oil reservoir sealed. Crankcase venting filters are often used and should be replaced multiple times over the vehicle's lifetime. Driving without an oil cap is also risky. An engine oil cap covers the contents of the engine and reduces the risk of failure. The car can run for a few miles without issue, but the damages can cost you a lot in the long run.