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Unread 02-15-2008, 03:01 PM   #1
RaccoonJoe
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1995 ZJ 
 
Join Date: Sep 2007
Location: NE IN
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misfire when cold...

With the Indiana winters, these past few weeks have been rather chilly. When the Jeep sits overnight, it will misfire for a few minutes in the morning. Usually, the colder the ambient temp, the more cylinders missing. However, I have also noticed times when it was idling OK, but misfired when accelerating.

After ~2-5 minutes of idle time, the misfire clears up, and we're golden again. Any thoughts or ideas??


89, 4.0L, AW4, 232K miles

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Quote:
Originally Posted by 4.0H.O.XJ
..................
Ill have an XJ please, with a side order of 4.0, hold the catalyctic converter and the 02 sensors, thank you.
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Unread 02-15-2008, 03:37 PM   #2
tjwalker
It's the crank sensor!
 
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Just brainstorming here. Could be your ECT (engine coolant temp) sensor. Or maybe it's called the CTS on your vintage. Anyways, it should be located on the left side of the cylinder block, just below the exhaust manifold.

That sensor provides coolant temp data to the computer which in turn controls the fuel/air ratio to the engine. And yea, they can go bad and when they do, you can have poor cold performance.

The most accurate way to test them is with a multimeter, a thermometer and some cold and hot water. Compare resistance across two pins to factory specs located in your Haynes/Chilton/FSM. A quicker/dirtier way to test this sensor is to measure its resistance and watch for a change as the engine warms up. No change, or an open or closed reading would indicate a bad sensor.

Good luck and post back what you find!
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Last edited by tjwalker; 02-15-2008 at 03:54 PM..
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Unread 02-15-2008, 11:37 PM   #3
CanadianJeep86
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its winter time, your jeep is almost 20 years old and has over 200,000 miles on it and you want to know why it misfires when you first start it?

Well your first problem is its a 19 year old jeep with over 200k miles on it.
Whens the last time you gave it a full tune up and cleaned the injectors?
Not trying to be a smartass, just sayin... lol
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Unread 02-16-2008, 06:48 AM   #4
RaccoonJoe
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Quote:
Originally Posted by CanadianJeep86 View Post
its winter time, your jeep is almost 20 years old and has over 200,000 miles on it and you want to know why it misfires when you first start it?

Well your first problem is its a 19 year old jeep with over 200k miles on it.
Whens the last time you gave it a full tune up and cleaned the injectors?
Not trying to be a smartass, just sayin... lol
smartass-ness will pass me by...this time

I gave it a full tune-up when I got it (at 212K miles). new plugs, cap/rotor, everything except for the spark plug wires. I tested those, got acceptable resistance levels, so I left them alone.

Plug wires are the first thing on the replacement list, as they probably need done anyways. Then the plugs come out to check for color and gap......and the multi-meter comes out for sensor testing yay!!
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Quote:
Originally Posted by 4.0H.O.XJ
..................
Ill have an XJ please, with a side order of 4.0, hold the catalyctic converter and the 02 sensors, thank you.
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Unread 02-16-2008, 07:04 AM   #5
88XJSport
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Mine does it too. When its been 5-10 degrees in the mornings for the past few weeks, ive just gotten used to it. I cant hear the miss near the engine, i only notice the exhaust skips. When i take off though, its fine.

Ive replaced CTS, plugs, wires, map, etc, but it still does it, and i live with it. There is a massive thread somewhere on NAXJA about "renix cold miss" its worth a look if youre really that concerned.
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Unread 02-16-2008, 10:16 AM   #6
BrunoS
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cold miss could be a ton of things.

easiest fix:

1) 2 bottles of techron in fuel tank to clean injectors

2) fresh fuel filter install. you need pressure and flow to atomize fuel across an injector tip so that it will fire on a cold morning.

3) upgrade your coil to an MSD #8228. about $45 and worth every penny.

that should do the trick.
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Unread 02-28-2008, 11:12 AM   #7
RaccoonJoe
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OK, fellow Jeeple. My cold misfire is getting worse.

So far, in the past 4-6 months (15K miles) we've done the following:
new plugs, cap, rotor, wires
new fuel pump/filter (both in tank, and in line)
removed/cleaned intake manifold (cleaned fuel injectors at the same time)
replaced cracked/broken exhaust manifold with one that's been welded back together
new TPS
new coolant, thermostat, temp sender (gauge)
dual radiator e-fans
swapped steering column/ignition switch from parts rig
2.5" exhaust from downpipe into Thrush Turbo muffler, dynomax tailpipe (2.5") *no cat*

There's been lots of other things changing, but they don't have any direct effect on engine operation. This AM, it was 12*. I let the Jeep idle for about 2 minutes before leaving for work....and I noticed it was misfiring continuously until the temp gauge hit about 160. After that, she seemed to smooth out a bit.

Like 88xjsport said....you can't hear this miss from the engine, only from the exhaust. Mine is also much more evident if the motor is loaded (acceleration, driving into head wind, towing, etc....)

Anxiously awaiting any ideas......


*edit* I'm also seeing some strange issues at cold idle. She'll idle about 800-900, then my tach bounces off 0 for a few sec, then back up to ~750-ish. We do this for about 3-5 minutes, depends on ambient air temps....
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Quote:
Originally Posted by 4.0H.O.XJ
..................
Ill have an XJ please, with a side order of 4.0, hold the catalyctic converter and the 02 sensors, thank you.
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Unread 02-28-2008, 12:02 PM   #8
jeeptown
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I would do a compression check on it. You may have a burnt exhaust valve with that kind of miles. My 98 has 180k+ and it also misfires on start up. I keep mine well tuned and the problem never seems to change. She's just plain worn out.
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Unread 02-28-2008, 12:43 PM   #9
tjwalker
It's the crank sensor!
 
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Remove and clean the idle air control (IAC). Clean it and where it sits thoroughly of all carbon with a sensor safe solvent. The coolant sensor I was referring to in an earlier post isn't the sender for the gauge, rather it's the sensor that provides an engine coolant temperature input to the computer, which will then enrich the air/fuel mixture delivered by the injectors when the engine coolant is cold along with a few other related cold controls. And x2 on checking compression. Something easy to do...
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Unread 02-28-2008, 01:32 PM   #10
RaccoonJoe
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When cleaning the IAC, should I be spraying this down with brake cleaner? 'Lectra-Clean? Soak it in Seafoam or kerosene? I've got a spare, but I'd rather not jack this one up.......

It's been a while since I've done a compression check. Can I do that with the motor hot, or will I have to worry about jacking up the spark plug threads?

Also....towards the coolant sensor.....If I'm understanding this correctly, it's on the lower part of the block, underneath the exhaust manifold?

And the final question.....Could this be caused by a bad O2 sensor? Or does the Renix not utilize the O2 sensor until the motor warms up? I'm honestly not sure if we pulled out the sensor when we were MIG/TIG welding (don't remember which) the manifold together....that could possible burn the sensor out, correct?

*edit* I'm fishing for ideas/places to start for this weekend.
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Quote:
Originally Posted by 4.0H.O.XJ
..................
Ill have an XJ please, with a side order of 4.0, hold the catalyctic converter and the 02 sensors, thank you.
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Unread 04-11-2013, 11:30 AM   #11
RARECJ8
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Holy thread from the past. Get it resolved?

Completed a 4.6 stroker project and for first 500+ miles ran great. Now all of a sudden, very rough idle on start up and cannot drive it until it fully warms up. Try to give a bit of throttle and it stumbles, sputters and getting massive lean misfires from under hood. SCary. Replaced/relocated air temp sensor to cold air intake box. New coolant sensor. Run codes and shows...nothing. tightened intake bolts. no vacuum leaks. new fuel filter. yadda yadda..

Once its fully warmed up, it runs like a racecar but don't even plan to drive it unless and until it fully warms up. Problem is, even if it sits for about 45 minutes, it does it all over again. Yes, we've been to jeepstrokers and we're stumped. any suggestions here?
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