MJ Comanche Tech. Stock & Modified. MJ (86-92)
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Misfire at 1700rpm?? 1990 4.0 auto

Old 02-15-2017, 08:18 AM
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Default Misfire at 1700rpm?? 1990 4.0 auto

I'm looking at picking up a 1990 4.0 auto comanche this weekend, no frame rust and it looks really nice. Only issue is it has a misfire at 1700rpm. When I went and looked at the truck, it only did it once the whole time and i couldn't even hear it doing it. The engine is fully rebuilt by an engine shop, bored out .020", Cleggs stage 2 head and stage 1 cam. The engine only has maybe 50 miles on it after the rebuild and was also thinking maybe that could also be a factor? But honestly have no idea. I'd like to get this truck but am leary of the engine misfire. I dont want to pay for a giant headache and I know how much of a headache jeeps can be.

Last edited by Spydie; 02-15-2017 at 08:24 AM.
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Old 02-15-2017, 01:41 PM
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CPS or TPS adjustment most likelly. check out this:http://cruiser54.com/ Renix systems are rather finiky, and rely on sensor values to be just right. i say snag it, and work out the minor details through troubleshooting.
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Old 02-25-2017, 06:42 AM
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"i know how much of a headache jeeps can be" ????
No Jeeps for you!!
Really jeeps are actually a heartache not headache.
Misfire, sometimes, at 1700 rpm. Limited info, I'll assume it's when driving.
WyoC has the right track. Check out the grounding and sensor values for sure!
"no frame rust", buy it for sure.
CPS is my primary suspicion, data will tell. Get your sensors from dealer or Napa. Echlin brand works real good for me.
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Old 02-25-2017, 07:37 AM
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He's gonna dink around and not get it. Nothing to be afraid of............
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Old 02-28-2017, 02:21 PM
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Just an update, I did get it. Found it had so many vacuum leaks. He had the MAP sensor connected to one of the vacuum ports on the fuel rail, he also didn't have the vacuum canister hooked up along with a few other spots. Misfires now gone and idles way better. Can actually drive it without it backfiring every 3 seconds. Replacing the egr solenoid is next on my list. Thanks to cruisers guide I found my TPS wasn't working as it should. It looks like at some point someone put a new TPS connector on the harness and absolutely destroyed it and did a craptastic wiring job. Couldn't get a reading out of my C wire. Replaced the plug and redid the wiring job and the TPS was reading .03 and is not reading .83 like it should. It does have a new dizzy cap and wires but the previous owner put the old plugs back in. 4 of the plugs were trashed. Put new ones in and its running way better. It's still searching while idling but it at least will idle. I'm hoping the egr solenoid will fix that. Needs quite a but more work but at least now i can drive it around and enjoy it a little. I don't know how it made it the 80 miles home.

Last edited by Spydie; 02-28-2017 at 02:28 PM.
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Old 02-28-2017, 04:50 PM
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Originally Posted by Spydie View Post
Just an update, I did get it. Found it had so many vacuum leaks. He had the MAP sensor connected to one of the vacuum ports on the fuel rail, he also didn't have the vacuum canister hooked up along with a few other spots. Misfires now gone and idles way better. Can actually drive it without it backfiring every 3 seconds. Replacing the egr solenoid is next on my list. Thanks to cruisers guide I found my TPS wasn't working as it should. It looks like at some point someone put a new TPS connector on the harness and absolutely destroyed it and did a craptastic wiring job. Couldn't get a reading out of my C wire. Replaced the plug and redid the wiring job and the TPS was reading .03 and is not reading .83 like it should. It does have a new dizzy cap and wires but the previous owner put the old plugs back in. 4 of the plugs were trashed. Put new ones in and its running way better. It's still searching while idling but it at least will idle. I'm hoping the egr solenoid will fix that. Needs quite a but more work but at least now i can drive it around and enjoy it a little. I don't know how it made it the 80 miles home.
Excellent job.
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Old 03-01-2017, 06:48 AM
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Did ya clean Iac yet. A set of 746 volvo injectors will take idle and throttle response to next level.
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Old 03-02-2017, 11:36 AM
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Originally Posted by freegdr View Post
Did ya clean Iac yet. A set of 746 volvo injectors will take idle and throttle response to next level.
Not yet, I'll do that tonight. Pretty sure its the EGR as the solenoid is completely gone lol
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Old 03-02-2017, 06:12 PM
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Originally Posted by Spydie View Post
Not yet, I'll do that tonight. Pretty sure its the EGR as the solenoid is completely gone lol
Valve would not even open or close if its not hooked up. So wont affect any thing.
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Old 03-09-2017, 10:55 AM
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Well, installed the solenoid and i had a MUCH better idle. It still drops down a little every now and then but once it was warmed up I had a nice constant 500-600rpm idle range. Pretty happy with this jeep. Time to repaint the atrocity that is the paint job on the tailgate and finish my double din radio install and see how this thing drives at road/highway speed!
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