You can make a scenario where anything is rude. Here, if he said something and you said "could it be the fuel pump" that should also be an inoffensive comment, not prompt him feeling like "how dare this fool presume to lecture me about the inner machinations of an automobile; let this mark the end of our friendship"
I dunno man, I think this is more for situations where people are having personal or emotional issues. Things that they might be trying to work through. Not for troubleshooting a broken car or computer.
