The courage to admit you don't know the answer

I have a 1998 Mercury Sable with 160K miles on it. During a moment of extreme poverty three years ago, we had to buy a car. The best we could do was the Sable with 125K on it and even then we had to get a loan to pay for it. The car has required regular repairs ever since it was purchased and our most recent mechanic seemed pretty good. He owned a few shops and we never saw him face to face, but he would call in the early am on the morning after we dropped the car off. He gave the diagnosis and estimate and did the work fast and we had a decent running vehicle when he finished. He recently hired a full time mechanic for the shop. The new guy just doesn't have the skills his employer does.
We dropped the car off last Friday and it had a rough idle and acceleration. The Saturday morning call from the new guy reported we needed an alternator, tension pulley, and serpentine belt. This didn't sound quite right. An alternator charges your battery, but it doesn't have a whole lot to do with how the car idles. I did believe the parts he listed were old and not performing optimally, but that was true for most of the parts in the car and I didn't believe the parts he listed were causing the problem we wanted fixed. The new guy assured us this needed to be done and so we gave in and agreed to the $500. Saturday afternoon he called to tell us the repairs hadn't corrected the problem. The problem was caused by the PSE valve and he wanted to replace it. My husband told he he wouldn't pay anymore than we already had and the new guy agreed to put the PSE valve in at no cost, but it wouldn't be done until Monday morning.
Monday morning the phone call came and the report was that the problem still existed and this time the cure would be the idle control valve. He couldn't replace it for free, but, if I picked the part up at Murray's, he would put it in for free. I said we would think about it and I would pick my car up that afternoon. It was pretty obvious that guy had no clue what was causing the problem , but instead of admit that to me he was going to keep replacing parts.
I was discussing the unresolved problem with the mechanic when I went to pick the car up because I was curious what his line of reasoning was for replacing the idle control valve. He told me his wife drives a Ford (uh-huh, um , why do I care) and they were driving to Wompers Lake, a nice lake out by Saline (get to the damn point), last summer and the rpms on the car went really high (not the same problem my car has). He told her to pull over so the car didn't blow up (I guess you have evolved high enough to have self-preservation instincts). He tinkered around and got the idle to drop, but he couldn't figure out what the probem was (sounds familiar). The next day his friend at work told him it was the idle control valve.
So, I just paid $500 to someone whose high tech method of diagnosis is to ask the hillbilly working next to him. I want to be mad. I want to demand my money back. I want to call him all sorts of names and point out what an imbecile he is. But, who's the real imbecile here? Probably the fool who trusted her car to back yard mechanic who looks like he played a mountain person in the movie "Deliverance".