Our standard Diagnostic fee was 105 plus tax at the time, and that covers 1 hour of my technicians time to diagnose the vehicle and verify your concern. Now on occasion there are situations where the issue is not something that can be determined 'from the surface' or without partial disassembly. For example, a plumber cannot tell you how much it will be to fix a water leak without taking down part of your walls or taking up part of the floors, he cannot see the damage from the surface.
In this case you had a leak that we were trying to identify, and upon inspection we were able to determine that the leak was either coming from an injector or a dummy plug. Being that these parts are under the valve covers we would have to disassemble partially to do our needed testing. It is bad business practice to work for free, and it is an industry standard to approve time for teardown as we have to compensate our technicians for the jobs they do.
We let you know that we would need to tear down to verify the problem, and how much that would cost. Some customers just want to know what the problem is and are interested in fixing it themselves. That is why we quote teardown time. The repair needed will not include any additional labor in this case, just the cost of the parts. We made this clear during our communications. I apologize if you did not understand how this process works, but there it is.
Additionally, in regards to your glow plugs, I think you might be confused as far as the verbiage related to this situation, as it was your dummy plugs that were a potential suspect as to what was wrong with your truck.
If you have any questions feel free to contact us directly with your concerns.