[Paul Workman]

One of the best tools I ever bought for my Z was a scanner that reads out OBD-1 (for both my 90 and Ami's 91 ZR-1 cars coupled with the FSM to provide normal reading parameters and locations for sensors. Without a method of scanning these older cars, there are just too many variables and troubleshooting becomes a crap shoot.
Find a shop with OBD-1 capability - or better yet, buy your own scanner and put it in the glove compartment. Otherwise, you'll be chasing around and "replacing parts until the problem goes away".

My wife's 91 Z would often start then stutter and stall on cold startup. The IAC voltage was a tad off and once it was set and an idle adjustment made (cammed motor), the issue with her 91 was resolved. But, the scanner told of the IAC voltage issue AND identified a lazy O2 sensor as the issue(s) in her case. A scanner is THEE approach to troubleshooting and will pay for itself in short order in frustration and wasted $ for parts thrown at the problem(s). (And, things REALLY get interesting when there multiple issues, and/or if a new replacement part is "DOA"!

Good luck. Get it scanned!