projects and questions, new to me 1994

[conesare2seconds]

Hi everyone. Recently bought a high-mile 94 with a few needs and am putting together a project and maintenance list. Have a few questions, if you don't mind. This is my 9th C4 but first ZR-1.

- There is a knock on the passenger side of the engine. Poking around with a stethoscope seems to confirm it isn't in the engine proper, but it does conform to engine rpm. Has anyone solved a mystery noise in that area? I'm leaning toward exhaust noise (stock manifolds), maybe the PS cat substrate has broken up (a PS precat was the source of a similar knock on my old 91 L98).

- AC is inop. Compressor is supposed to have been replaced a few years and not many miles ago. It took a refrigerant charge, at first, with the AC compressor cycling. Now is showing system pressure but air at the vents is still warm. I took a tip from here and unplugged the forward pressure switch on the AC line that runs down the PS of the engine bay, and the engine cooling fans immediately came on (warm engine). Did I just diagnose a faulty pressure switch?

- On a related note, cooling is not up to par. Temps rise quickly when idling or in stop and go traffic in the current hot weather, but it stays really cool when moving. The dash gauge and the digital readout also disagree, with the gauge reading much higher than the digital readout. The plenum was off recently to replace several vacuum lines. I'm wondering about an air bubble. Make sense?

- Finally, when running up through the gears under moderate throttle, there is a miss around 3200 rpm in 2nd and 3200 and 4200 rpm in 3rd. The engine stutters momentarily, then resumes pulling. My guess is the stutter is related to the secondaries opening. With the engine off and key turned to run, the vacuum pump runs constantly, which I understand indicates a vacuum leak. Oddly, the car pulls to 6000 with plenty of power in the upper rpm range, so I'll say the secondaries seem to be activating fine.

Thanks in advance for your patience with a long post. There are a bunch of other small projects on the list, most of which I can handle from prior C4 ownership. Above are the main ones that are possibly LT5-specific. Any advice or suggestions are welcome and appreciated.
Tom