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Old 03-28-2012, 03:19 AM
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YJ Missing

94 yj 4.0L std trans
I have a problem.
When cold outside (18 yesterday am) it pops, backfires, misses then quits. Later it started but ran ruff.
When engine at operating temp it will miss. It will miss 5-10 times in 3 miles.
The only computer codes are 12 and 55.
I bought it from a guy who bought it from a guy. 1st guy chged distributor and plugs, 2nd guy changed plugs, wires, cap and rotor, water temp sensor and fuel pump. I have changd map, mat, throttle position and idle control. No chg........
I used starting fluid to chk for vac leak. I unhooked the oxy sensor no change except for service engine light came on.
I am going to do a comp test today and if ok will look for a new engine control module.
Don't know what else to do.
It was in a local shop they found nothing.
I would rather be doing body work the like to it.
Mechanically sound drive line.
Thanks in advance for your input
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Old 03-28-2012, 06:43 AM   #2
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Location: Raleigh, NC
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You're probably on the right track w/ the ECM.

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Old 03-28-2012, 07:09 AM   #3
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Could be out of time, if the previous guy changed the distributor he may not have set the timing right.

Be sure the plug wires are on in the correct order.
Check the timing.
Check Coil Pack

Running rough can be contributed to a sensor or ecm problem. A miss is cuased by something in the ingnition system or a bad fuel injector. [Plugs, Spark Plug wires, Distributor, Distributor Cap, Coil Pack, Fuel injector.]

Since the distributor was changed the first thing I would check is that it was done correctly and that the timing set correctly. Out of Time will cuase the problems you described.
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Old 03-28-2012, 05:25 PM
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master 4th

I checked the timing. I didn't see where it was off.
I also did a farmer test on the timing gear/chain. I pulled the dist cap, put a socket on the end of the crank and when i turned the crank both ways the distributor turned with it, very little slop. i do know the fuel pump pressure reg is not working properly. (It is new)
There are rubber lines under the hood, i noticed the system was not under pressure the feed line was soft. so i removed the cap from the test port on the injector bar and there was no pressure. So i started it and the feed line was somewhat pressurized.
Shut it down, I used a c-clamp to pinch the return line, started it, it ran really ruff. Shut engine off again, system was pressurized (10 min, feed line hard) until i removed the c-clamp. Which tells me the pressure regulator is not holding system pressure. I will check fuel pump press tomorrow also.
thinking out loud
With the fuel pump pres reg acting the way it is could be starving for gas (missing while underway at operating temp) but would not explain over fuel when temp outside is cold. which leads me back to ecm confused, short circuited or just messed up.

The miss is not a steady constant miss where I could shut off a fuel injector or pull a plug wire to find which cyl. It will miss 5 - 10 times, unless cold out side, in 3 miles.
New coil.
Tell me truthful, if u think i should stop thinking tell me.
Thanks for the input
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