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87 YJ Runs great cold. Runs rough, Dies warm

5K views 8 replies 6 participants last post by  Luckymac 
#1 ·
Bought this jeep from a family that used it strictly off road. It was not running when I bought it. The previous owner had replaced the crank sensor, plugs, wires, cap, and rotor, coil and installed a holley electric fuel pump. The sending unit in the tank has power wires on it so im not sure whats going on there but when I tested the sending unit I didnt hear a pump turn on. The jeep was not running do to a broken chassis ground which I have replaced. I can drive it for about 20 minutes and it will start breaking up at high RPM and idling rough and after a while it will die. It always will start back up and when cold fires immediately. I do not have a check engine light and have yet to see it illuminate. Any pointers you guys can give me. Not to familiar with these TBI systems
 
#2 ·
No check engine light on you year. Computer is stuck in closed loop may not be getting temp inputs to start adjusting once warmed up.

Tstat should be 195 and check you air inlet temp and water temp sensors on intake are plugged in.
 
#3 ·
Update. Tested all sensors and all test good and within spec. Installed a new coil on icm and jeep wouldn't start. Reinstalled old coil and fires up. Wiring going to icm will cause it to die if touched. Can I buy the two plugs from anywhere? I obviously need an icm also being that it can't handle the extra power needed by a new coil.
 
#4 ·
Dumb question, but here goes:

Why is there a crank sensor on a carburetor YJ? (One of the appeals of my '88 YJ was the minimal number of sensors to fail, in comparison to a FI model).
 
#6 ·
On a carb'd stock 4.2 YJ there is no crank position sensor. Only on a fuel injected 2.5 (4cyl) stock engine and a 4.0 fuel injected engine, which didn't come on an 87 YJ. A 2.5 was stock along with a carb'd 4.2. Anything other than that isn't stock from the factory.
 
#5 ·
Only a Jeep engineer can't answer that. They are reliable easy to change better than being inside a timing cover
 
#7 ·
Couple of suggestions.
1-Fill in your profile so we can know if you have a carburated 6 cyl or a fuel injected 4 cyl. (can guess you have a 4 cyl)
2-Get a Factory Service Manual
3-Sounds like you might have a bad connector. Radio Shack might still have connectors online. Harley-Davidson is another source for connectors but you need to find a friendly counterperson to search the catalogs for what you might able to use.
Good Luck, L.M.
 
#8 ·
I have replaced all sensors and let it run while using starter fluid to check for vaccum leaks there is none even checked under jeep. complete tune up. it will start and run when cold put falls on its face when warm. it makes a sound like big leak in the throttle body. and new fuel pump.
 
#9 ·
Tom, it'd be better to start a new thread and in that thread describe your symptoms and what you've done to diagnose the condition.
Plus, tell us something about your Jeep, like what year and what engine/transmission and any mods.
The OP in this thread may have been able to cure his problem or may have given up. Sometimes we never find out.

Do you have access to a Factory Service Manual?

Good Luck, L.M.
 
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