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Ohhhh, we should all kick ourselves if this one hasn't been done -it probably has though...

Looking over your list, I see everything done on the ignition side EXCEPT for the IDI housing (plastic coil housing -devil's pitchfork used instead of conventional wires- absolute crap piece IMO. I have seen so many of those pieces fail-out in the early years... You couldn't even find one in the junk-yards because everybody with a Quad was buying them like they were candy, and they were like $85 and not always even available aftermarket). Got stuck running on two cylinders with my '92 GA SOHC Quad back in the day because of that thing -long cross-country trip too. I've done a few conversions swapping over to the conventional two-post GM coils and conventional wires. It is a nice upgrade, though usually not as clean-looking as OE. On my '97 Z-24 I put the coils far away from the engine to keep them cooler (and the ICM). On a '96 GA I did for a friend, I fabbed a bracket that held the coils up over the IDI cover sideways and bottom-to-bottom, and the wires tucked in underneath along the back edge of the IDI cover -very clean install there.

We tried this last year. Swapped the IDI with the known-good one on my Z26.

Replace the refuel filter if you haven't already (or if its a year old per regular maintenance) .

I ordered the necessary parts to check fuel pressure - should be here in a couple days. As soon as we get back from the beach I'll be able to tell static and dynamic fuel pressures and report back.

Got my schrader valve and adapter setup installed on the GTZ - Worked like a champ, BTW. My fuel pressure @ idle is 35psi. My fuel pressure under load jumps a few psi at initial rev and then levels back out right around 35psi again. Shouldn't it be more like ~43psi? I haven't had a chance to drive it with the gauge hooked up but I expect results to be similar.

weak fuel system. Change fuel filter with a new non fram one and recheck.

pull the vacuum line off the regulator and see if it stays solid at 43-45

Just tried it - The fuel pressure jumps to ~42ish psi when the vacuum line is removed.

We've driven the car with this vacuum line off and it didn't seem to make any difference.

This is why I honestly don't feel it's the fuel system... I've experienced weak fuel systems and they very rarely cause a skip like this... Especially when registering 35PSI at idle. The most you will usually feel from a weak system is just basic power loss... that can be scanned and found when at WOT by just looking at the O2 voltage.

The thing I fear the most is Geoff had a phantom skip after his car was re-started warm... NEVER could find it and then he did his OBDII swap and it was gone... All the same engine components and electronics. So that points to either the ECU or the harness was the issue.

I wish I was a bit more familiar with the quad to suggest more options.

Brian, have you in fact tried swapping the ECU from your other car?

Pretty easy way to either confirm or eliminate that as a problem.

I had tried swapping ECU's to no avail, so ultimately my problem was in the engine wiring harness.

Hopefully that's not the case with your car Brian; as Quad wiring harnesses I'm sure aren't easy to find.

If the regulator is not to spec you can't just dismiss it. Replace it and go from there.
Pulling the vacuum line won't help the car run better.
After all, they designed it to have a vacuum line for a reason.
It might not be all the problem but it is most likely part of the problem.
An injector will spray a certain amount of fuel over a certain amount of time at a given pressure.
If you alter the pressure, it's going to spray less fuel. The ecm can compensate but if too far off
maybe it's out of the designed range of adjustment of the ecm.

How is that regulator not to spec? Why do people feel it's wrong? I told him to pull the VAC line to see if it would go to peak pressure and hold while running. His rev test it responded just the way it should with a drop in VAC then immediate increase which raised and then decreased the fuel pressure. Fuel pressure is directly related to engine vacuum so with an instant rev the vac drops to zero or close to it which causes the fuel pressure to rise, if you hold the throttle steady the vac levels off and gets higher which in turn drops the fuel pressure back down. The only time you will see 42-45PSI is at WOT conditions, or with the vacuum line removed from the regulator.

I'm sorry but troubleshooting this car in person I really disagree with all the talk of it being a fuel pressure issue alone.

OK, your right, the service manual says engine off, ignition on, pressure should be 41-47 psi,
so with no vacuum, 42 psi would be the low range of normal. (41-47psi)
It seemed 35psi running, sounded low to me.

35psi running is completely normal... that's what the vacuum line does. It drops the pressure at idle so the injectors will flow less allowing the ecu to have more control over idle quality.