Does it die, if you move the switch from start to run? Does it run real rough, then crap out at idle and smoke? Can you play with the gas pedal some and get it too run at higher RPM´s? Throwing parts at it, before a good troubleshoot, rarley produces results. I´ve had a couple of pre 90, XJ´s crap out, a various times, much more often, it´s something basic and not a sensor. Ran an 87 (hard) from new, until a year ago (rolled) only replaced, plugs, cap, cables, exhaust, u-joints, some front end parts, fuel regulator, EGR and some vacume lines. Most of the troubles, I encountered in the motor control circuits, were in a connector someplace.
Have an 88 now, the dealer replaced some sensors (and gave up), I bought it and finally found an oily connector was most of the problem. Another was a bent pin in a connector, another was, some burnt wiring, to the O2, temp. sensor and knock sensor. Could be a sensor, but often it´s something basic.
Have you checked the fuel pressure? Most any kind of pressure gauge will work, with a scale going up to a little above thirty. Eye protection, safety conciderations, are important when messing with fuel system. Have somebody standing by, to kill the switch fast, a fire extiguisher handy.
I leave a jumper, tied to my balast resistor, just in case. A bad balast resistor will often let it start, but not run for long.
