gsfish, that reminds me of a particularly great example as well. I have lots of examples, but this one is awesome:
When I was in college, I had a job detailing cars for a car rental agency. The agency was located inside a new car dealership which also had its own mechanics and shop area. Several of the mechanics had decades of experience and ALL sorts of certifications. They were a very competent group with access to tens of thousands of dollars of diagnostic equipment.
A man I happened to know, as he was taking the same college courses as me, purchased a used Blazer from the dealership. His new vehicle developed high-idling problems with it several days later. That's a bummer, but it happens with mechanical things. He brought it back and they fixed it under warranty.
A couple weeks later same thing. He brought it back in and they fixed it again.
A couple weeks later same thing. He was becoming upset. The mechanics put their minds together and fixed it again.
A couple weeks later same thing. Now he was getting quite upset. He's missing work and school because of this. The mechanics talked about it at length and fixed it. Finally!
Nope! A couple weeks later same thing. The Blazer was worse than ever; it now idled along at a brisk 45 mph. While the owner of the Blazer and the owner of the dealership were having a mildly heated discussion, a crowd of the best mechanics stopped what they were working on and stood in a semi-circle around the Blazer discussing at great length what the problem could possibly be. After several minutes an old man, slightly hunched over, walked up to and through the crowd of the shop's best. (Only an old dude could get away with this. I can't wait to get that old so I can do stuff like that! Lol.)
Wordlessly, he looks at the engine while it is idling. He pulls a Bic lighter from his denim overalls, holds it close to where the intake manifold and the engine block meet, then proceeds to walk the flame around the seam. Eventually the flame was sucked into the engine. The intake gasket was leaking!
Without looking up he quietly pocketed his lighter, walked into the lobby, and took a seat. Less than an hour later the truck was running fine and stayed that way.
For you neophytes, this is a common condition for many vehicles, especially for this particular year and model of engine, 4.3 liter, as it used a plastic gasket instead of a real one. I wouldn't be surprised if it had a recall. Air intake leaks are super easy to observe and diagnose for almost any type of engine from chainsaws to trucks.
