This question in on behalf of an uncle who picked up this very low
mileage Z a year or so ago. It has
now developed rough running when hot. He has replaced the fuel
pressure regulator and has a fuel pressure
gauge rigged so it can be read while driving. The pressure looks good
to me, riding at about 30 psig at idle
and varying with engine load between 20 and 30.

I have no experience with the Z, but the FI looks similar to the Bosch
system on my '76 Jaguar V12. I've
advised him to test the coolant temperature sensor, but don't know
what the desistance vs. temperature
should be. We are both curious about the fan mounted on the right side
of the engine with a flat duct
taking the air up and over the valve cover and into the vee. Is that
to combat fuel rail vapor formation?
If so, could a failure of this fan or controls explain the symptoms?

TIA

Ed