There are a few Cummins engines that have been known to have issues. The most common one is the 6.7L Cummins engine in the 2007.5-2009 Dodge Ram 2500/3500 trucks. This engine is known for having oil leaks, turbo problems, and injector issues. Another problem Cummins engine is the 5.9L in the 2003-2004 Dodge Ram 2500/3500 trucks. This engine is known for having head gasket failures and cracked blocks. If you are looking at buying a used truck with a Cummins engine, make sure to do your research and know which ones to avoid.