So, i've pulled out the rest of my hair!
Working on a 86 XJ6. Engine was replaced (blown) by someone else
quite some time ago. Car never ran after that! Went through the
fuel pump circuit and discovered all was good, excepting the fuel
pump! (seized)
Replaced the fuel filter, pump, purged all the lines, getting clean
fuel to the rail.
Symptoms like so... Car starts, runs extremely rough for maybe 30
seconds, then dies. If you keep starter engaged car will run at
part throttle, (no higher than 1500 rpm). Once you let off the key
it dies. When it stalls there is still spark.
Put a noid light on a injector connector. Light flickers dimly
while starter engaged and car running. As soon as you let off the
key car starts to stall and the noid is BRIGHT!
Checked and swapped out main and FP relays, diode resistors seem
okay (opened, ohm test), fuel pump running as it should. Checked
throttle (idle, full) switch, checked micro switch in AFM, somewhat
checked the potenteiameter (sp, checked for change in
resistance/ohms).
Car will pretty much only start and run with throttle partially
open, starter engaged, as if running extremely rich. Dies once
starter let off.
Unfortunatly don't have the official service manual, stuff we have
is next to useless!
Thoughts?
Eric

 

Sounds to me like you have a bad ignition coil like Jose said. When it is dark outside run it and if you see a spark do a little jump between plugs it means you have an arc and need to replace the coil.