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Unread 04-21-2010, 08:35 PM   #1
1990YJnewbie
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2004 TJ Wrangler 
 
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need help--no spark when hot but runs fine cold

I've got a 1990 4.2 straight six that will start and run fine until the engine warms up. It has new cap, rotor and plug wires. New coil, new ignition control module and a remanufactured distributor. What else would cause a no fire condition when the engine gets up to operating temperature?

Any help would be appreciated.

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Unread 04-21-2010, 08:47 PM   #2
moonshinefuel
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You are certain it is not getting a spark, no spark at all when hot?
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Unread 04-21-2010, 09:12 PM   #3
moonshinefuel
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only thing I came up with is a bad coil power transistor. I am not sure how that pertains to a YJ, but it is a causitive factor. Other than that, do a wiggle test of any and all wires that pertain to the ignition and spark when running, you want the engine to shut off after you wiggle a wire, then there would be the problem.
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Unread 04-21-2010, 09:22 PM   #4
beachedjeepyj
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Are you not getting spark from the plugs or the coil, if your not from the coil my bet is on the icm (ignition control module) when they go bad the usually only want to start depending on temperature. Mine only wanted to start when it was hot, but its more common to only start when they are cold.
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Unread 04-21-2010, 09:35 PM   #5
KA24DET95
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Just because a part is "NEW" it does not mean it's bad i see it all the time at the shop. Especially if you bought it from autozone or oreily. Get a chiltons or a hanes manual and a volt meter and check the parts yourself just to be sure.
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Unread 04-22-2010, 05:31 AM   #6
1990YJnewbie
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It is not getting a spark--have tried the coil lead and spark plug leads. No spark. Let it cool down over night and it starts right up. Haven't done the nutter bypass because I have to pass emissions. Could a bad ECM be causing this? Should I do the bypass with a couple of jumper wires to make sure the ECM is bad?

Where is the coil transistor on the 4.2l?

I have tried two different ignition control modules and the problem is still the same. It will start and run for a few minutes but after that--no spark until it cools back down.
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Unread 04-22-2010, 06:34 AM   #7
Que89YJ
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Cracked/ bad coil. Check the voltage to the coil hot and see if the supply voltage is there. BTW. Many on the forum have done the Nutter and still passed emissions....Oh your in Cali.....never mind. Bad experiences when I modded my VW out there.
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Unread 04-22-2010, 07:47 AM   #8
1990YJnewbie
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Well, I replaced the coil but I have read that I should only have nine volts to the coil. The new coil I installed had a sticker on the side that said only nine volts should be supplies. I haven't hooked up a volt meter to the coil to check it. Test light showed it had voltage. I can't find any resistors mounted on the firewall, but one book I looked at said the YJ uses inline fusible links to reduce the voltage at the coil. I've never had a fusible link go bad only when it was hot--they usually blow out completely.

I have really appreciated the help I have gotten from this forum so far on this project. Thanks,
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Unread 04-22-2010, 09:28 AM   #9
Que89YJ
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So, you have voltage at the coil but no spark cold, but you need to check it hot when the issue happens.
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Unread 04-22-2010, 10:32 AM   #10
1990YJnewbie
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I have spark when it is cold--no spark after the engine gets to operating temperature. I have not checked the voltage at the coil with a volt meter after the engine quits. It may be next week before I get to work on it again due to work and my son's camping trip this weekend.
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Unread 04-22-2010, 11:04 AM   #11
KA24DET95
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Try this first i am looking for a step by step procedure as we speak.



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Unread 04-27-2010, 01:26 PM   #12
1990YJnewbie
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So I tested the coil last night and it tested fine. I swapped out the new ignition control module with the old module and it had spark. I bought it at Autozone with a lifetime warranty so I exchanged it for another new one last night. I will install it tomorrow night and hopefully this will make the problem stay away for awhile.
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Unread 04-27-2010, 04:18 PM   #13
ctm
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Here's another possibility ...that you are losing your distributor ground when the clamp and the engine block heat up. Remove and clean the distributor hold-down clamp. clean the area on the block where it clamps down also.

Better yet add a dedicated ground wire to the distributor black wire. The clamp ground is known to be a trouble spot. See JeepHammer posts in the CJ forum regarding grounds.
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