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Old 09-29-2013, 10:15 PM   #1
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93 Wrangler backfired, died, and won't start

I have had this YJ for about a year and a half and it has been almost trouble free. It has had a rattling noise in the engine since I got it and I thought it was probably lifters. Now I'm thinking maybe it has been slack in my timing chain. Since I have had it, it has died or cut out for a couple seconds on multiple occasions, but has always started right back and this only happened once or twice every six or eight weeks. No common denominator I am aware of when it would happen.

Now for the current situation: Yesterday, while driving on the highway at about 60 mph, it cut out, backfired (first time it has done that), then died. It will turn over great, but will not start. I tested the coil resistance and it was within specs and providing good spark. I checked a spark plug and it was firing good as well. The fuel pump is priming and I smell the fuel as it is spraying into the cylinder I removed the spark plug from, so I suppose it has adequate pressure. The number one cylinder compression pushes my makeshift "thumb" away at the moment the spark plug sparks, so I'm told that is a shade-tree test for proper timing. I replaced the CPS, dist cap, and rotor. Also replaced plugs and wires within past year. Any advice?

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Old 09-30-2013, 08:18 AM   #2
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2.5L or 4.0L??? Either one I would lean toward a timing issue. Is it throwing any codes? Timing for FI engines is through the computer. Have you checked the fuses/relays? Check to see if it is throwing any codes to help narrow down the cause.

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Old 09-30-2013, 10:02 AM   #3
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If you have spark and fuel it should run, may not run good but still run. But if you slipped timing.... that's another problem. Pull #1 plug out, roll it to TDC and then look at your rotor under the cap and see which plug wire it's pointing to.
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Old 09-30-2013, 11:17 AM   #4
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It does not show any trouble codes. Is there a particular fuse or relay that would cause the no start, but still have spark and fuel? I
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Old 09-30-2013, 11:20 AM   #5
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It does not show any trouble codes. Is there a particular fuse or relay that could cause the no start, but still have fuel and spark?
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Old 09-30-2013, 11:54 AM   #6
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This hasn't 't happened on my jeep but that very same thing has happened on two old motorcycles. One had the coil wire come loose & the other had blown a main fuse. In each case they died & backfired at the same time. I figure they quit running because they lost electricity, but the fuel in the system managed one more bang. Correct me of I'm off base.


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Old 09-30-2013, 04:57 PM   #7
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This is a 4.0L and as stated, I'm getting fuel and fire to the spark plugs. I'm not experienced at checking the timing, but from what I can see the number one spark plug is firing during the "compression stroke?"...it fires at the exact moment my thumb is popped off the plug hole by the compression. I've had two techs tell me that's a positive test that the timing is ok. I'm open to more opinions, because I know we can all get it wrong sometimes. I believe any related fuse or relay, such as the ASD, would cut power to the fuel or ignition and I could not have fuel and/or spark. I'm open to correction on that assumption as well. If the PCM is bad, would I still have both spark and fuel? My gut tells me it's the timing chain, but I don't want to start yanking stuff off and find out it was something I overlooked. Help!!!
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Old 10-01-2013, 06:22 AM   #8
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If you have spark and fuel it should run, may not run good but still run. But if you slipped timing.... that's another problem. Pull #1 plug out, roll it to TDC and then look at your rotor under the cap and see which plug wire it's pointing to.
I've read your last reply. And I under stand what you said. Your thumb pops away from the hole off when the #1 plug sparks. Like I've stated above is true also. Are you positive you're getting fuel?
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Old 10-01-2013, 06:26 AM   #9
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It does not show any trouble codes. Is there a particular fuse or relay that could cause the no start, but still have fuel and spark?
There is an automatic shut down relay in the fuse box in front of the battery but you stated that you have spark. I'm really curious about your fuel side.
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Old 10-01-2013, 09:40 AM   #10
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What about a vacuum leak. Have you checked for disconnected/damaged hoses?
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Old 10-01-2013, 12:54 PM   #11
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What about a vacuum leak. Have you checked for disconnected/damaged hoses?
I'm no expert by any means but I believe a vacuum leak would just make it run bad...
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Old 10-01-2013, 03:40 PM   #12
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All kinds of stuff run off the vacuum. You said it backfired for the first time and a vacuum leak is one of the things that can cause this. Have you tried to push start it? May be able to get it started that way to troubleshoot the problem.
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Old 10-01-2013, 04:01 PM   #13
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All kinds of stuff run off the vacuum. You said it backfired for the first time and a vacuum leak is one of the things that can cause this. Have you tried to push start it? May be able to get it started that way to troubleshoot the problem.
I like that idea, give it a push start.
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Old 10-01-2013, 04:50 PM   #14
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Thanks for your replies. I definitely have fuel and fire. Since it won't start, my plugs are getting wet with fuel which confirms the fact I'm getting fuel and of course I can see the good fire at the spark plug. I have searched for bad vacuum hoses, but have not found any. I will search again though, because I agree that could be an issue. I can't push start it, because it's an automatic. I know, I know, it's not as fun, lol. I'm pretty sure the ASD relay shuts down voltage to the fuel pump instead of spark, but either way, I have both. Does anyone know if the PCM can be bad and still provide spark and fuel?
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Old 10-05-2013, 06:37 PM   #15
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Still needing HELP! Continued to have good spark and fuel, but could not convince myself the timing was 'exactly' right, so I went ahead and changed out the timing chain and gears. The old chain had a little more than the acceptable 1/2" of slack, but did not seem to have jumped out of time. It's back together now definitely in time, definitely has spark at the plugs, and definitely has fuel in the cylinders. It still will not start. Cranks over just fine, but will not start. I know I have asked this a couple times already, but no one has provided an answer. What about the PCM? Could it be bad even though I have spark and fuel? Any other ideas or possible culprits?
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Old 10-05-2013, 06:52 PM   #16
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you need to check the fuel pressure. without the proper fuel pressure the injectors will not fire correctly.
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Old 10-05-2013, 09:27 PM   #17
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Well have you been able to diagnos the problem? Did you get it running? Update please!
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Old 10-06-2013, 08:31 PM   #18
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Here's an easy way to test if it's a fuel pressure issue, just blast a 5 second shot of starter fluid into the intake and give her a crank. If she starts and dies, baddabing.
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Old 10-09-2013, 09:13 PM   #19
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Still down! Have strong blue spark at each plug. Have 39 psi fuel pressure. Injectors pass node test. Compression is 120 on each cylinder. All timing marks line up correctly and the rotor points to #1 plug wire while #1 cylinder is at TDC. New distributor, new CKP, new plugs (properly gapped). Starting fluid makes no difference. Does anyone know anyone that might know someone who can answer the question: could the problem be the PCM?
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Old 10-09-2013, 10:57 PM   #20
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Could your spark plug wires. Be out of order?


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Old 10-09-2013, 10:59 PM   #21
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Bad fuel.



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Old 10-10-2013, 07:02 AM   #22
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Have you tried starter fluid? When was the last time you put gas in the tank. You have already eleminated most of the obvious causes. Just thinking you have some bad fuel. Try some starter fluid down the TB and see if it will fire.
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Old 10-10-2013, 07:27 AM   #23
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Originally Posted by OldSparky View Post
Still down! Have strong blue spark at each plug. Have 39 psi fuel pressure. Injectors pass node test. Compression is 120 on each cylinder. All timing marks line up correctly and the rotor points to #1 plug wire while #1 cylinder is at TDC. New distributor, new CKP, new plugs (properly gapped). Starting fluid makes no difference. Does anyone know anyone that might know someone who can answer the question: could the problem be the PCM?
Looks like he has tried starting fluid... This could be way off, but I've seen plugs that have fired on the bench but wouldn't fire under compression. I can't imagine that you would loose more than 1.

I am still a firm believer that if you have spark and fuel it should at the very least run crappy! Maybe swap out the plugs.
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Old 10-10-2013, 12:31 PM   #24
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Went back and read your original post. Although you replaced he CPS, it may have been a bad one. Check it. It sends info to the ECM.
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Old 10-10-2013, 06:22 PM   #25
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The CKP (crank position sensor) tests good, within specs of 200 ohms + or - 75 between terminals A and B. The plug wires are in order and the plugs are new and properly gapped. Also, the fuel (pumped some out into a clean bowl at the fuel rail) is not contaminated. I ran nearly half a tank of the same gas with no problems. Still no takers on the PCM question? I'm out of ideas.
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Old 10-10-2013, 08:43 PM   #26
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What do y'all think about this? Even though I'm getting proper fuel pressure, could the fuel/air ratio be off, creating a rich condition, flooding the cylinders and causing the no start? Wouldn't this also explain the initial backfiring when it died? Cylinder(s) over loaded with fuel..."boom"...caused by a faulty input sensor or PCM itself which caused the lack of air to mix with the fuel? Is that plausible?
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Old 10-10-2013, 10:50 PM   #27
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I have had incidents in the past where the plugs got "wet" and would no longer spark. Not your PCM answer, but I would swap ALL plugs.

IF you are suspecting a RICH condition, prop open the TB with a screwdriver or some other device that WILL NOT fall into the intake and then crank her over. Might just catch and run, but poorly.

If that does not work, keep TB open and spary in some starting fluid just before cranking..... be careful and NOT spary WHILE cranking....
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Old 10-28-2013, 08:33 AM   #28
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Sorry for the delay in getting back on here, but I've been busy driving my Jeep! During this breakdown, I was told over and over that if you have fuel, fire, and compression (air) then it will start. I confirmed a blue spark at each spark plug and tested the coil's primary and secondary resistance as well as a good 1/2 inch blue spark at the coil wire. When I finally gave up on myself, three friends, and the forum here, I called in a local tech who offered to make house call. After 10 mins of inspection, he said my coil was bad. I explained again that it passed the test and was providing spark to the plugs. He said yes, but not enough spark. He said the spark at the coil wire should be a good 2 inches! What??? I doubted, but we replaced the coil and in a few minutes he had Old Sparky up and running. Lesson learned: never disregard the basics, even if you have tried and tested them. Go back and check it all again...and again if necessary. Thanks for all the suggestions guys! I'll be back, but hopefully not any time soon. I want to be ridin'!!!
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Old 10-28-2013, 10:02 AM   #29
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Glad you got it fixed!

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