On the ICM, the front yellow wire should be getting about 12 volts, and the rear gets 5 from the ECU. Here in this Utube, I ran that rear through a 3 volt LED, (to ground), to see that my CPS was telling the ECU to do whatever it is it does to tell the ICM/Coil to spark. (led's are polarity sensitive, so it won't flash if it's backwards. (the black is ground)

Also I might see if there is a crisp, sharp blue spark right from the coil it's self.
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There is a nice crisp, sharp, snappy, blue spark right off the coil wire.

 

Can you list all the stuff you have replaced since the problem began? I looked back at your posts and didn't see anywhere that you tried a new distributor cap/rotor, new plugs, new wires etc. Those are cheap tune-up items.


The Renix is fairly simple minded. I'm assuming you have the Renix fuel injection manual. If not, google it and it tells you how the system works. If you look at that, you can see there's only a few sensor inputs that control spark, fuel injector pulse width, timing etc. Most of those inputs are from the CPS, TPS and MAP so if you haven't changed it in a while, a new CPS is, in my opinion, the first thing I would replace. A TPS can be flaky too. They are basically "wear items" as far as I'm concerned.


We can't tell what your history is with the Jeep. How long have you had it, what work has been done by your or others etc. If all the electrical/sensor/ECU stuff checks out, then you have to go to other items like compression tests, valve issues etc. I'm still thinking it's electronic at this point which is why I'm suggesting you change out the easily changed items.

 

Since the problem began, I've replaced the coil with a new one, the MAP sensor was switched with a used one. The CPS is less than a year old. I'm going to replace the cap and rotor soon just to see if that helps. Also, the spark plugs are less than two years old. Not many miles on them. Anything else I think of I'll post.

 

Have you tried restarting it immediately after shut down? I'd try this hot and cold and note any difference in starting. Also, have you noticed any patterns when it's easier or harder to start?-air and engine temp, how long it was driven, how long it was parked, etc. The fact that it started at WOT would indicate that something is flooding the engine. Leaking injector, bad FPR, maybe the evap control system (charcoal canister). With the injectors off cranking at WOT something defective had to be fueling the engine for it to start, unless you had already cranked it while not at WOT first.

Edit-thinking of the possibility of the evap system, next time you park it loosen the gas cap a bit and see if it starts any better.

 

It seems to start easier when it's warmer outside, like in the 90°'s. And holding it WOT only seems to help sometimes.
Maybe the sparks are too weak and holding it WOT makes it easier because there is less fuel so the weak sparks can ignite the air fuel mix better? Just throwing thoughts out there.

 

In your shoes I might go with new wires as well, since there is a difference in your plugs firing. (Napa for that). You did gap plugs to .035? And which are they Btw? Not some goofy Platinum I hope...

 

I haven't double checked plug gap yet, have to find/buy a gauge. The plugs themselves are champion copper.

 

Pardon me? Post #83 and you haven't checked your plug gap? Usually there is one of those gapper disk deals right on the parts counter for about $1. Yea, lets go ahead and rule that one out.

 

X2 on that. Champion coppers are like $2.50 each so $15. A cap and rotor are like $25-30. Wire set, around $20. Autozone or other probably have a tune-up kit with all of the above and a plug gapper for under $100. Money might be tight but if you're working a lot, you don't have time to mess around with the simplest stuff to change out.

 

I checked them without a gauge, I was just guesstimating.

 

Grasshopper, I might eyeball something like that to within some thousands. In this case, please find one of those disk/gauge things.

A QUESTION FOR CF ! If you have 300+ K on a 4.0, and a ridiculously long crank time, might the lifters and cam be so gone that it needs to crank long until the lifters are fully pumped up? Granted a rod or piston usually fails first, but what's next if one of those two doesn't go?

 

I plan on going and buying one tomorrow and checking them then.

 

While you are there ask them if they will let you borrow a mechanical oil pressure gauge for free. Do you have a dash gauge that's nearly O at hot idle, or a light that blinks at hot idle?

 

If holding WOT injects no fuel, which I think is true, then it shouldn't be able to start unless something else put fuel into it.