Lots of info on the interwebs for XJ. so much from this site. But Ive search a lot to find my problem before I made another post about it. Obliviously haven't found my problem. My issue is more of an annoyance, im going to triple check the fuel. ive ran the pump multiple time before start. It isn't light and day difference but it helps a little. But like always I appreciate any positive advice
Any difference between cold and hot starts?
You might want to check your CPS voltage. I usually get about the same results with an OHM test, the ohms get much over 150 and the ECU can miss a few pulses during cranking. 175 ohms or more an starts can get really iffy. The ohms can change depending on temperature.
A slow starter can also mess with your starts. I've heard the Renix has to be cranking a certain RPM to get a start, not sure of the RPM or maybe it just takes a certain number of pulses from the CPS? I've noticed when I've let my battery run down, starts can take a long time.
Does it ever miss shortly after a start and then stabilize? This can be a sign the CPS may be missing a few pulses. Best guess is the CPS-ECU program likely adapts to a few missed pulses, but has it's limits. A few too many missed pulses and you get a miss or an iffy start. Like I said, just a guess, but observation seems to support my guess.
Try a volt test at the larger yellow wire at the ignition coil, while it's cranking over. The starter can suck the voltage way down. It isn't set in stone, but voltage down to around 9 volts and the spark is good enough, if it gets down to around 6 volts starts can get iffy, especially in cold weather.
I ran an ohm test on my CPS wiring to the ECU connector and had substantial resistance. I fiddled with it for most of a day. The next day I gave up and chopped the CPS wires from the ECU about 2-3 inches inside the engine bay and ran the CPS pigtail straight to my shortened wires. Jeep used to have a kit, that did about the same thing. I just relocated the connector from the harness and bypassed a bunch of wiring. I soldered and heat shrank the splice.
Like Cruiser said the Renix typically needs a long crank to start compared to some other cars, even the later HO models start quicker. Mine typically takes a slow two count maybe a fast three count to start on a warm motor.
If turning the key back to off and then to run again to get some addition prime doesn't help, it is likely to be a CPS pulse issue (CPS or wiring) or as you suggested weak spark.
My 88 pops right over, unless I've let it sit to long and it's lost a little too much prime pressure/fuel volume in the fuel rail.
For bragging rights, I let my 88 sit for almost a year. I ran the pump for awhile to purge the old fuel out of the fuel rail and the fuel lines and it popped right over first try, 2-3 seconds of cranking. I tossed the carbon core plug wires a long time ago and replaced them with solid core coil wires, I kept the stock carbon core ignition coil to cap wire, it seemed to run better that way. The new wires really helped my idle and my starts. Mine idles a little low (500 RPM), but thumps like a champ and never misses a beat.