I took a second hand car I was considering buying to my local garage (Fernwood Auto--the are amazing!) for a pre-purchase inspection. The Fernwood mechanics said that certain questions could only be answered by GM and recommended I go to GM and ask about the service procedure for carbon build up on valves, and also just ask if they had seen this issue before.
I went to Dave Wheaton I was told they cannot give me any information. When I asked if the expected cost to deal with carbon build-up woudl be in the ballpark of several hundred or several thousand dollars, I was told that they could not answer me, unless I did an inspection through them.
I realize that Dave Wheaton is running a business and cannot give free advice to anyone who walks in off the street. I also realize that it can be dangerous to give rough estimates, because unexpected things could arise. However, the contrast between my local, friendly garage and the offputting environment of Dave Wheaton, where I was not able to access a mechanic for a a 5 minute conversation was striking.
I really just wanted a quick check-in with someone about this car I was about to buy. Had they given it to me, I likely would have been a customer for many years to come. I may still end up needing to be a customer of theirs, if this car needs repairs that Fernwood can't do, but I would only go as a last resort.