Greg, yes, it's all a guess. It's often a best guess, but it is still a guess.
The average guy sees 5-10 cars a day. They are anything from a 6 year old low miles creampuff to some old POS. Try to figure out EXACTLY what is causing a misfire on each and every one of them without any work! CO (Rich) is fairly easy in comparison, but any of a hundred things can cause a miss.
Could you be able to tell which houses had plumbing issues AND be able to tell the owner what they are and how much it will cost just by walking thru it for 10 minutes?
