4.0 Starting Issues and Sensor Question

My '90 4.0 5 speed XJ is hard starting almost all the time in every weather condition, but it never fails to start after about 10-15 seconds of cranking the motor. What could be causing this? The plugs/wires/cap/rotor are all fairly new.

Also, is there supposed to be a sensor screwed into the right hand side (pass) of the block in the center towards the bottom? Reason I ask is because there is a threaded hole in this location on my block and nothing is screwed into it. Also, there is a pretty long wire coming out of the harness by the coil with a two prong female end connector that is not plugged into anything. Funny thing is that it seems like it would be the perfect length to hook up to a sensor screwed in the the empty threaded hole on the pass side of the block I just mentioned.

I know that the coolant temperature sensor is burried behind the driver's side motor mount block bracket and that the knock sensor is underneath it but exposed. I just put a new knock sensor in because the one on my car got broken off some how.

Thanks,
Ted

That threaded hole you're seeing with nothing in it is probably for the KNOCK sensor. Does that loose plug look like one for a fuel injector? If you pull back the covering just a bit, I bet you'll find a foil overwrap on the wiring as well.

RENIX will run just fine without the KNOCK sensor - normal output for that sensor is none anyhow. It's a piezoelectric unit, and only generates a signal when there is spark knock in the engine.

"Long" crank times (~2-5 seconds, tops) are fairly normal with RENIX - since the ECU looks for two things before it will fire the ignition and fuel injectors:
1) A valid 300rpm signal from the Crankshaft Position Sensor (mandatory)
2) A valid SYNC pulse from the Camshaft Position Sensor (optional)

The engine will start and run without the second signal - it may stumble while starting, but that's because the ECU is "guessing" at when #1 comes up to fire. The engine will not run without the first signal. Period.

My read on it is that your Crankshaft Position Sensor is probably starting to go out, and wants replacing. DO NOT get one for HO (1991-up) XJ, since it's a different unit that generates a different signal and works on a different principle. As I recall, the RENIX CPS is OEMR only, but I'd have to check to be sure (something tells me I've seen a Borg-Warner part number somewhere, but it's been a while...)

I'd replace the CPS first, it will probably give that a seeing-to.

5-90