Cranks, no start - Failed Coil, weak fuel pump

Vehicle: 1996 XJ
Problem: Cranks but won't start
Solution: Failed Ignition Coil, Weak Fuel Pump, Clogged Fuel Filter

So here's the setup:

We had just got off the trail in Monache Meadows, heading back to camp. I was cruising down the road doing about 45 or so, and the engine just quits. For the first couple tries it *almost* fires, but not quite, then nothing. Cranks but won't start. After my friend figured out I wasn't behind him anymore, he showed up and towed me back to camp. Let me tell you, it's not fun being towed on a twisty mountain road with no power, but we made it back just fine. Where we were camped, we had no phone or internet access, so we had to troubleshoot the old fashioned way: Check the basics - Spark, Fuel, Timing, Compression.

So, after fiddling around with it for a while, we figure the fuel pump died. There was fuel at the rail, but the stream was very weak coming out of the Schrader valve. Unfortunately, the closest parts store was an hour down the mountain, so we lit the fire and drank beer.

The next morning, we made the trek down the mountain and Autozone graced me with a new pump and filter. Then back up the mountain to change the pump. Had the pump in and working by about 3, but no joy. Still wouldn't fire. We weren't really feeling like another 3 hour round trip, so instead we fiddled around and determined that the coil had gone bad. I didn't feel bad about changing the fuel pump, though. The fuel stream was weak, and the fuel filter literally came apart when I undid the restraining strap holding it to the frame. It was good that I changed them. I figure the bad filter caused to pump to work harder, which caused it to fail. The weak spark made it worse, and after getting very hot, the coil just quit.

Here's what we did to work it down to the coil:

First, we determined that the rotor was turning, so the timing chain didn't break. Then, we checked for spark. There was spark, but it seemed weak. Then, I determined the CPS was good by unplugging the CPS and determining that the (weak) spark went away, and then came back after plugging the CPS back in. The PCM had to be good to be sending spark signals after the CPS was plugged back in. We then verified compression by pulling a plug. The cylinder sounded pretty good and I got sprayed with gas, so we put the plug back in. That left the coil as the culprit. We pulled the coil and measured resistance across the leads. Almost 8 ohms. It's supposed to be around 1.6 to 1.8. Hmmm....coil's dead.

So after we determined what was wrong, we lit the campfire and drank beer....lots of beer....that's a different story.

The next morning, we did the 3 hour tour back to Ridgecrest and got the coil. Just to be safe, I got a CPS, also, knowing that I would return it if I didn't need it. Sure enough, I plugged in the new coil and the Jeep fired right up. After stopping at Napa to get my money back for the CPS I didn't need, it drove the 200 miles back home in 104 degree heat without so much as a hiccup.

I'm writing this up so if someone else has a similar problem, they can find this solution and try it. I searched as soon as I had signal again, but nothing pointed toward a bad coil. Whenever a 4.0 dies like that, everyone assumes CPS, but that was not the case here.

I have a very similar situation happen to me.

I was going to pickup a friend and while I was waiting in his parking garage the jeep started to sputter then backfired a few times and died. So first thing we thought was bad CPS so we broke out the DRB III and checked and the jeep had crank and cam signal and was acting syncing while cranking, we checked and we had good fuel pressure. We checked and it had a decent spark, we also found while we were trying to diagnose it if you cranked it and let off the key the motor would actually kick back and start running backwards which was very strange. We figured the timing chain might have jumped a tooth or something.

We were both very confused at this point and I had the Jeep towed over to his shop. At the shop we ran a compression test which the jeep passed with flying colors so that meant the timing chain was still in time. So we hooked up an oscilloscope to the #1 plug lead and actually watched the spark and we were seeing ignition events on the plug wire (voltage rises then rapidly falls). After banging my head against a wall for a few hours trying to figure out what was going on I looked under the driver seat of my jeep and found another ignition coil that was cracked so I figured WTH, I plugged it and bam it fired right up. It turns out the old coil had an internal short in it and was arcing internally instead of out to the plug. Since it takes less voltage to create a spark at 0 PSI then at 160 PSI it would look find when we did the spark to the fender test.