To prove that you're never too old to do something stupid, I spent part of the weekend working on the 84 Custom Cruiser. I had rebuilt the e-Qjet and was running the 307 to heat up the O2 sensor so I could properly adjust the carb. The water pump decides to take a dump at this point.
I replaced the pump and the fan clutch, which had quite a bit of wobble in the bearings (and which was likely the cause of the pump failure), and again warmed the engine to heat the O2 sensor. The car would not idle. I use an OTC Genisys to tap into the ALCL connector and the mixture control solenoid was locked at full rich. I ran the system check to ensure the O2 sensor and feed back circuit were working properly (they were). I disconnected the A.I.R. hose to be sure that a failed switching valve wasn't the problem (it wasn't). I raised the idle speed so the car would stay running. I played with the mixture screws. No luck. By 5:00 Sunday afternoon I was P.O.'d enough to call it quits.
About 8:00 that night it hit me that I had knocked the PCV hose loose while changing the water pump (actually, while accessing the bolts that hold the A/C compressor, one of many things required to come off in order to change the water pump on a 307). When I looked at the car this morning, sure enough, the hose was still disconnected. Of course, the fact that the hose was so hard that it wouldn't even stay connected to the PCV valve should have been a tipoff to replace it. There were several other questionable vac hoses on the motor, and I replaced them all.
Naturally, the car now runs great.
I don't know how many times I've been totally stumped like that, only to have the problem become obvious after a short break from the car. You would think I'd have learned this by now.