It turns out the fuel pump was giving it up. Fuel pressure was initially within spec at idle and under the “simulated” WOT test procedure in the FSM that consisted of plugging the vacuum source for the fuel regulatory and looking for a rise in in fuel pressure to the 43-47 psi range.
After the fuel pressure initially checked out, I poked around some more looking at ignition connections and troubleshooting flow charts. I found nothing, so I decided to recheck the fuel pressure. This time I secured the test gauge under a windshield wiper so that I could see it while driving. Under load fuel pressure would drop, and under actual WOT is was dropping under 20 psi. The engine was leaning out and inducing the misfire and hesitation, but it was not staying in that condition long enough to set a code. By time I finished the second test, the fuel pump was down to 10 psi at idle, which is not enough to actually run the car.
To confirm that there were no electrical problems, I powered the pump directly off the battery via power probe and confirmed it would still produce no more than 10 psi. While I don’t have the adapters to deadhead the fuel pump, I did briefly power it up with the supply line disconnected tho confirm that it only put out a very gentle stream of fuel and that the fuel filter I replace 8k ago isn’t clogged up. At least a whole new sending unit is less than half the cost of the opti-spark I thought I was going to have to buy, and it had the good sense to die completely in my garage while I was troubleshooting it instead of on the interstate tomorrow morning.
Over the last week, the corvette has developed a misfire, especially between 3-4k RPMs. The problem is often non-existent when cold, usually present when hot, and gets much worse with prolonged idling when hot or on a hot restart after being parked for relatively shorter periods of time. This has all the markings of the dreaded opti-spark failure (and the car is at 78K on the original 1993 opti-spark), but the corvette is not being kind enough to trigger any ECM codes. The ignition troubleshooting charts lead me down a path that says everything is fine (and I don’t have an OBD 1 reader that would let me see the high and low resolution pulses in real time), however the corvette forums seem to have plenty of folks who have had opti-spark failures that never triggered any codes.
As a starting point, and because the car was still allegedly on the original spark plugs at 78k and 23 years old, I took the opportunity yesterday to put in some new plugs. 6 of the sparks plugs are no big deal after you move the ASR module out of the way. The other 2 are a lot less fun. Plug #6 is not fun. #8 made me think bad thoughts towards GM as I turned my good, snap-on ratchet one mother-loving click at a time. The possible range of motion on my lesser Craftsman Professional ratchet was a no go, as it was less than a click. I’ve heard the F-body cars of the 90's were even worse. I was hoping (but not expecting) to see some plugs with with big, eroded gaps or a broken insulator that would explain a misfire. Instead, I got 8 plugs that looked pretty darn good. Curiously, the records never show the plugs to have been replaced and they were the correct OEM parts, but the gaps on every plug was substantially smaller than spec. The car calls for a .05 gap, and all 8 in it were all gapped around .03-.035. Between the incorrect gap, and the better than expected condition for 78k, perhaps I am missing a service record for someone changing them out sometime in the 22 years before I owned the car. What I didn’t find was any thing that would explain the misfire.
I replaced the plugs with new, OEM Delco double platinums. I even gapped them to the correct spec. When not misfiring, the car purrs nicely. I did spot the beginnings of a bit of corrosion on the terminals in a couple of the wires, but nothing that would explain the problem. Nonetheless, I did order a new set of wires.
On the fuel-side, the fuel filter is pretty new. I changed it earlier in the year, or about 8k ago. The injectors all test in the range on resistance, they are all clicking nicely when running, and I see no signs of leakage into the vacuum system at the fuel pressure regulator. However, I don’t actually own a fuel pressure tester. While this doesn’t feel like a fuel-side problem, I’ll probably take this as the opportunity to buy a fuel pressure test kit before coughing up the dough and time to change the opti-spark.