1990 YJ 4.2 sometimes sparks and runs some time not.


Jharp59

New Member
Joined
Apr 25, 2021
Messages
5
Location
Oklahoma
1990 yj. Did a 4.2 nutter bypass, 2100 carb, new distributor cap, pickup. fires sometimes. I have driven it. Most no fire. I put a different ECM and it seems more sensitive to some issue with it sparking. When it runs it runs great. i cant see a crank position sensor anywhere. I bought Jeep with this problem. i have 12 volts to the coil and going to the distributor thru the coil wire to yop of distributor. I cant figure it out. It will start after the battery is unhooked sometimes and runs for a a bit this shuts off. Any advice?
 

Feral

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Apr 29, 2021
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Earth
The crank position sensor on my 93 4.0 is on the bell housing behind the engine. Its not too hard to get at.

I would replace the coil as well.
 

jeepjoe43

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Dec 4, 2020
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370
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Black Canyon City, AZ 85324
1990 yj. Did a 4.2 nutter bypass, 2100 carb, new distributor cap, pickup. fires sometimes. I have driven it. Most no fire. I put a different ECM and it seems more sensitive to some issue with it sparking. When it runs it runs great. i cant see a crank position sensor anywhere. I bought Jeep with this problem. i have 12 volts to the coil and going to the distributor thru the coil wire to yop of distributor. I cant figure it out. It will start after the battery is unhooked sometimes and runs for a a bit this shuts off. Any advice?
I agree with Feral, but would add inspect the wires to the connectors, too.
Back in the 80s & 90s Chrysler was sacrificing quality for weight due to CAFE regs.
My gremlin was a stressed/broken wire on the plug to the coil, cheap wire.
As I understand it, a CPS failure usually manifests when it gets hot, but don't rule it out.
 
OP
J

Jharp59

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Joined
Apr 25, 2021
Messages
5
Location
Oklahoma
Thank you all. Changed coil, same problem. Ordered a new Ignition Control Module. I’ll update as appropriate