Hi folks,
   I'm trying to help a buddy of mine get his wife's '92 240sx back on
the road. I own one also, and I just can't figure this problem out!
First, about a month or so ago it would stumble a bit and hesitate. It
gradually got worse, to the point that it would only stay running about
30seconds, then die, and refuse to start again(untill it sat for almost
a day). Now we can not get it started at all! Obviously this is a
progressive problem that has gotten worse. Here is what we have
checked/verified:

When it was still starting/stalling, this is what we checked:
-would only stay running by goosing throttle, keeping it above roughly
2.5-3k rpm.
-removed exhaust from collector-back
-no codes from ECU
-good fuel pressure, a little high though(it was 40psi at idle,
35nominal)
--I even used a hand vac to back down the FPR pressure, but it still
would stall.
-spark on all four wires
-swapped MAF for a known working one
-swapped in a known distributor for a working one, properly timed once
installed
--cap/rotor in good condion(we have spark...)
-new O2 sensor
-new coolant temp sensor
-clearing flood would not help to re-start, including clearing the
cylinders and drying spark plugs
-vac was a bit low(15"hg compared to nominal 22+"hg).

After it refused to start anymore, this is what we checked:
-still good fuel pressure and spark on all four
-crank position sensor is working: turning the gear manually fires each
injector with key->ON
-checked the vac lines that are easy to get to(ie canister controls,
etc)
-temp. disconnected Auxilary Air Valve from filter box(in case exhaust
was getting in there).

Thoughts? I would really appreciate some ideas! Perhaps a vac leak that
has progressivly gotten worse? I'm thinking it must be air/vac related,
because we have fuel+spark+timing.

Thanks,
Jamie