91 OBD1 4.0 AW4 turns off....cps/cpk, nss, or what

[Incommando]

Driving along in the old MJ and it cuts off just like you use the key. Will restart almost Immediately. It my do this again aftsr it restarts or it may not. Both times it occurred the engine was warmed up but not overheating. I amelled excess fuel when i kicked it into nuetral to try to restart it on the fly (unsuccesfully) but not when I coast to a stop and restart it. Went to cycle the key and either my CEL bulb is out,too, or there is another issue. I will get the CEL bulb thing corrected tomorrow and/or get it scanned. A buddies 98 XJ did this a few years ago and it was the neutral safety switch replacement that corrected it. Thoughts?

[HOrnbrod]

Same problem I had a while back. Mine would cut off randomly it seemed at the worst possible places; crossing a bridge, passing on a two-lane road, etc. Did the key fault code thing, had a single fault, code 54, distributor sync sensor. Changed it and it cut the cutoffs in half. Then went ahead and replaced the CPS, and it was permanently cured. I never showed any other fault codes than the 54 code.  Hopefully you have just a burnt out bulb on the CEL diagnostic procedure. I always check for codes bi-monthly or so. It's a good PM tool.

[Minuit]

Side question, sorry if it's a bother to anyone: does the OBD1 CEL ever come on by itself? I've had plenty of stuff go on that I'd think would trigger a CEL (I've also pulled a number of codes with the key trick) but it's never come on except for during a key cycle.

 

A year or so back my truck cut off with no real pattern to it similar to what you're describing (seemed to only do it after warming up, always shut off smoothly and would restart right away). Ended up being the ignition coil. My symptoms were a little different but strikingly similar. I never smelled fuel, for instance.

 

It's also pretty well reported that a CPS can fail only when hot, although I don't imagine that would let you start it up again right away. When both of my CPSes failed, the truck would restart after the engine had cooled down (and promptly stall again once warmed up) although the 4.0 CPS seems like it can fail in an amazing number of different ways.

 

When you tried to restart it in neutral, was it turning over? My experience is that the NSS will stop it from even turning over. The CPS and sync sensor (some call it a "pickup coil") are two other logical suspects.

[HOrnbrod]

My CEL light (and the rest of the dash lights) only comes on when I turn the IGN key on during the bulb check sequence when all the dash bulbs come on prior to starting. I've never seen the CEL light come on by itself while running like the OBD2 system does.

[Incommando]

I guess i had never noticed that my CEL did not come on when I started it. I swapped this cluster in a couple of months ago to get a tach and gauges and did not notice it. There was no bulb in that socket at all. Swapped in the unneeded maint reqd bulb and everything is good to go.  No CEL light when running but it lights as it should when it is cranking/starting and I can now check codes.

 

12.... To my knowledge the battery has not been disconnected in the last fifty key cycles. A weakening battery is a possibility as I bought this truck almost 4 years ago. At the time it had a less then new looking Walmart battery in it and it is still there. It does not seem to be spinning over the ol' 4.0 like it used to, either. I may replace the battery and see if I had a low voltage issue. Heck I need a new battery anyway.

 

33.... a/c clutch relay = normal as I do not have A/C

 

35.....  Cooling fan relay. I had stuck a JY electric aux. fan in and, after testing it that it worked with a hot wire and a ground, I just plugged it into the existing wires. I never got it hot enough to see if it would turn on or not. I am just going to unplug it to eliminate that possibility and work on that later. 

 

The lack of codes leads me to think CPS because I have heard of them being bad without an 11 code on an intermittent issue. I cannot get the stalling issue to repeat, either. For $40 I am just going to replace it and then keep the old one for a spare if that was not it. A CPS isn't a bad spare to have around on these anyway. 

[HOrnbrod]

I've had a code 12 ever since I've owned mine - no worries. The code 35 could be that the aux fan relay in the PDC is bad, or even missing. You can swap these around to check.