input/opinions on no-spark condition

I'm on my 3rd ignition coil in 3 years. There is some underlying ignition problem. The current coil is a brand new MSD coil (p/n 8228), installed just 30 miles before the Jeep stalled & would not start. I think the coil is functioning, but I'm getting no spark, possibly a bad crankshaft position sensor or CKS (or CPS, as some have it). I took the ign wire that runs from the coil to the distributor cap off, & I held it near the engine block while my wife cranked the engine--no spark. I know I was getting spark earlier, because it would actually start & run, then stall.

specs:
XJ 1996 4.0 2WD AW4
Accel 8mm ignition wires ~4 months old, bought because of a rough idle problem, which didn't go away (related??). Brand new Accel distributor cap (brass) & brand new Accel rotor, installed the day before the Jeep broke down. Brand new plain copper Autolite spark plugs (installed after the Jeep broke down--previous set were Champion Truck Plugs). Brand new MSD ignition coil. When I installed the new coil, it was running beautifully, plugs at .040" gap, so I tried to drive it to work. Got 30 miles, then, just as I was passing a semi on a two-lane road (high RPM), it began to shudder & misfire. I pulled over, & as I came to a stop it died.

A few times, I have gotten it to start, but it won't stay running, even if I hold my foot down on the gas.

I tested the crank position sensor. Its output was 0.2v constant while cranking the engine (measured by backprobing the black-lt. blue wire and the grey-black wire at the CKS connector). According to the manuals I've read & the forum threads I've read, the output should be higher, between 0.5v & 0.8v, and it should fluctuate. It doesn't fluctuate, but stays at 0.2v.

Battery connections are good, though the battery is weakening from all the attempts to start the jeep. Ground wires are all good, so far as I can tell. Throttlebody is clean. I tested the camshaft position sensor (by backprobing the tan wire at the CMS connector), and it fell within acceptable specs, fluctuating from 0-5 volts while the engine was cranking. I tested the MAP sensor and TPS, and they were within spec, too.

So my question is about the section in bold, above: does the 0.2v reading explain why the Jeep will sometimes almost start but never keep running? Also, would a faulty CKS explain why my Jeep has killed 3 ignition coils?
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extraneous history: 3 yrs ago, hard starts, eventually wouldn't start, towed, new ign coil. 3 months ago, shudder, bucking & misfiring on the hwy, broke down, towed, new ign coil. 2 wks ago, shuddering, bucking & misfiring on hwy, decided last weekend to replace the coil before it died altogether, thistime with a more powerful, MSD coil. Drove Jeep on Thursday for first time w/new coil, died within 30 miles, would start but stall, today won't even start & has no spark from the coil.

i don't think the coils are braking but you may have a short some where. remove the old coil and put it back in see what happens. it might be that when your working in the area you bump a wire and cause it to work then some vibrations later you have a bad connection. but this is just a thought. you may have a bad ground causing high resistants causing the constant .2 volts at the cps.

90xj06 said:

i don't think the coils are braking but you may have a short some where. remove the old coil and put it back in see what happens. it might be that when your working in the area you bump a wire and cause it to work then some vibrations later you have a bad connection. but this is just a thought. you may have a bad ground causing high resistants causing the constant .2 volts at the cps.

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I do still have the old coil. It may be wise to stick it back in. I did notice that the green & orange wire looks rough (abraded I guess), but it didn't look broken/melted/fused. The Jeep is at a shop 30 miles from where I live, so I won't get to it right away, but I take your point.

Possibly NSS and you should check the engine control fuse.

MSD is junk these days.

Well, (update) I replaced the CKS & got it running. Drove it about 10 miles & it started bucking again. Another 15 miles & it was bucking badly, lost power & had to pull over, whereupon it stalled. It restarted OK, so I slowly built up speed again, but it soon started bucking again. Made it the whole 30 miles back home, starting, driving, stalling, starting, driving, stalling, along the highway.

Now it is also backfiring.

I don't know why, but replacing the CKS helped. It will now start every time. However, I can't keep it running on the highway, and around town it bogs down. The backfiring is what gets me. It's like the timing is suddenly thrown off. The key-on, key-off computer code test keeps bringing up code 43 (misfire).

Whats your fuel pressure?

This is sounding more like a fuel filter or weak pump.

langer1 said:

Whats your fuel pressure?

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If the rain lets up, I'll let you know later today.

In the meantime, I discovered a little play in the distributor shaft I didn't notice before. I can turn the rotor/shaft 1-2mm in each direction. Don't know whether that would cause my problems.

if you have a Haynes manual check you map sensor. just to rule it out

langer1 said:

Whats your fuel pressure?

This is sounding more like a fuel filter or weak pump.

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Key-on: 41 psi at the rail.
Idle: 50.5 psi at the rail.

No fluctuations in pressure either. No problem there, I think. Oh, and the filter is just one month old. The problem is spark or timing, I think. The saga continues...

90xj06 said:

if you have a Haynes manual check you map sensor. just to rule it out

Click to expand...

MAP sensor voltage was right on, when I checked it a few days ago.

90xj06 said:

it might be that when your working in the area you bump a wire and cause it to work then some vibrations later you have a bad connection.

Click to expand...

You may have won the prize with this guess. I was reading in another forum where somebody had a bad input wire to the coil, where if you shook it while the Jeep was running it would mimic the symptoms. So I tried that, I wiggled the wires on the input connector on the coil, and the Jeep bogged down. Soo... may be time to do some amateur electrical repair. Not tonight though.

I took the coil input connector pigtail apart & it was fine: the wires were firmly attached to their respective terminals, and were not touching ea. other.

However, further along their length they were rubbing. One expects that the insulation on ea. wire would prevent shorting, but, like I mentioned, one wire's insulation is a bit abraded. Turns out I could reproduce the engine stumble at idle by tapping on the wires such that they came into contact. So...

I pushed the wires as far apart as I could get them, & the problem pretty much went away. I can't believe it. I'm still not sure this has fixed it. I've also wrapped electrical tape around each wire to beef up the insulation, & I intend to make a DIY loom to keep the wires apart in case of bumps & such.

Idle is still slightly rough when cold, but that could be normal, or it could be that the computer is re-learning its settings. I'm going to have to drive it more to determine whether this has fixed the problem.

great to hear that this giant peanut of a brain still has some sense kicking around. now that i think of it i fixed a go-ped that had a bad spark plug boot. and it would have a spark jump. and would back fire.

I am going to go out on a limb, and say that the shuddering/misfiring/stalling problem has been fixed by adding the insulating tape to the coil wires. I've driven over 200 highway miles, and 30 city miles, since. Thanks guys.

And now... (I predicted this) a new wiring problem has turned up elsewhere, I think. I'm starting a new thread on it, here and on jeepforum.com (JF guys can choose whether they want to post here or on JF).