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  #1  
Old 04-30-2010, 08:10 AM
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Question Misfires - '02 JGC Limited...

I'm lost at this point...

I have had an engine light for a while. The tester at auto zone says "random, 5 and 7..."

With just over 100k on the truck, I decided it had to be the plugs. I bought new ones from auto zone. The plugs I pulled out were burnt to nubs... Lol... Anyhow, the new ones are supposedly direct replacement copper plugs from Motorcraft. I have them gapped to the .040 the underside of the hood mentioned. After replacements... The same codes. P0300, p0305, p0307. #&@%! Noticeable increase in performance.

So then I looked at my coils. I obviously am not hurrying up to replace all of them... So I set out to determine which one it may be. So I pulled out and made sure I cleaned them off really well... They looked pretty good.

I moved 1 to 7, 3 to 5, 5 to 3, and 7 to 1.... As well as 1 to 8, 4 to 6, 6 to 4, and 8 to 2... Hoping that the problem will move... No such luck... The same codes. P0300, p0305, p0307. #&@%!

So then I looked at my injectors. I obviously am not hurrying up to replace all of them... So I set out to determine which one it may be. So I pulled out and made sure I cleaned them off really well... They looked pretty good.

I moved 1 to 7, 3 to 5, 5 to 3, and 7 to 1.... As well as 1 to 8, 4 to 6, 6 to 4, and 8 to 2... Hoping that the problem will move... No such luck... The same codes. P0300, p0305, p0307. #&@%!

So at this point, I have moved everything I can think of... Still the same f'n problem exists.
The Misfires are pretty inconsistent. Seems to come from cold starts. Warm starts and running normal is clean. There is no vibration or silly sounds… if it weren’t for that light, I’d never know…

The only things left…
Bad Knock Sensor, possible software update, possible bad wire to spark coil or injector.. I suspect it’s not misfiring in 2 cylinders, but 1. The cylinders are right next to each other both physically and in the firing order. They are so close, it may be hard to detect.
Specs – Stock Jeep Grand Cherokee Limited with the 4.7 Engine code “N”

Thoughts?

Kyle
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Old 04-30-2010, 09:22 AM
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Re: Misfires - '02 JGC Limited...

What about your Distributor Cap? Check that!
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  #3  
Old 04-30-2010, 11:12 AM
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Re: Misfires - '02 JGC Limited...

.......i'm glad one of us is having fun..
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Old 04-30-2010, 02:49 PM
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Re: Misfires - '02 JGC Limited...

As far as i know the knock sensors are only for the 4.7 HO engine.
I presume that you cleared the PCM memory first and checked for the codes after the next engine run, right?
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Old 05-01-2010, 01:42 PM
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Re: Misfires - '02 JGC Limited...

Yeah. I now know about the knock sensor absence on the standard 4.7.. I have a scanner and have cleared the codes. Then I'll walk away. Let the engine cool off.. I'll come back. It's a 50/50 shot the codes come back. But the eventually do. If not the first running but likely the second or third running. Always the same f'n codes.

If there is no knock sensor.. How does the computer determing where and when a knock or misfire happens?
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Old 05-01-2010, 05:41 PM
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Re: Misfires - '02 JGC Limited...

This engine probably doesn't have problems with engine knock, they only started with this with the higher compresion ratio HO engine and i believe the newer GC (2005 and on) have them as well.
I red in the Jeep manual that misfire is detected by the PCM by a small change in engine speed when the respective spark plug is firing.
So the problem can be ignition related, fuel injection related or maybe low compression in the respective cylinder. You had checked the injectors visually, but maybe the ultasonic cleaning will help. Or do a check of the compression.
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Old 05-01-2010, 07:35 PM
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Re: Misfires - '02 JGC Limited...

the injectors look good. very clean looking. no obvious damage..

if i had a "compression" problem.. how would i even start to fix that with out taking out a mortgage?

the jeep actually runs pretty well at basically all engine speeds. clean idle, smooth acceleration... just the start up is a bit... "lacking"

could i maybe need a new battery? its got an "07" sticker on it.. i know im starting to reach... but i need options.

i appreciate the help.

kyle
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Old 05-02-2010, 04:02 PM
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Re: Misfires - '02 JGC Limited...

Quote:
Originally Posted by Kpurintun View Post
the injectors look good. very clean looking. no obvious damage..

if i had a "compression" problem.. how would i even start to fix that with out taking out a mortgage?

the jeep actually runs pretty well at basically all engine speeds. clean idle, smooth acceleration... just the start up is a bit... "lacking"

could i maybe need a new battery? its got an "07" sticker on it.. i know im starting to reach... but i need options.

i appreciate the help.

kyle
Run a compression test. You can do it yourself and it's cheap and easy. Hopefully it will come back fine and you won't worry about it. If you do have a compression problem, it could EASILY get really expensive to fix.
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Old 05-03-2010, 10:51 AM
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Re: Misfires - '02 JGC Limited...

When you say "the startup is lacking", do you mean that the starter motor is turning the engine around slowly, or does it take longer then normal for the engine to start?
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Old 05-04-2010, 08:18 AM
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Re: Misfires - '02 JGC Limited...

the start up isn't a strong one. not like it used to be. i just noticed what looked to be more than a puff of steam from the exhaust. like there may have been oil or something left over that burned during start up.

not sure how to "run a compression test" but i have a funny feeling the rings are in need of replacement. I'm not sure i am ready for that kind of surgery. nor am i ready to pay for that kind of surgery.

i have ran mobile one for the last 75k... it has 102k.
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Old 05-04-2010, 10:45 AM
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Re: Misfires - '02 JGC Limited...

You had checked the spark plugs before, didn't you? If there would be oil passing the rings, or the valve stems, you probably would have seen it on the spark plugs as black deposite or carbon buildup. When the engine turns over slowly during start, first let your battery checked before buying an other.
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Old 05-04-2010, 01:01 PM
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Re: Misfires - '02 JGC Limited...

Quote:
Originally Posted by Frango100 View Post
You had checked the spark plugs before, didn't you? If there would be oil passing the rings, or the valve stems, you probably would have seen it on the spark plugs as black deposite or carbon buildup. When the engine turns over slowly during start, first let your battery checked before buying an other.

there was a really tiny bit of "wet" on the #7 plug when i pulled it out for the very first time after 100k. it looked like it might have been oil, or a dirty fluid. but there was so little that i couldn't really tell. after about 800 miles... i pulled them out again... and they weren't wet. they are a bit sooty. but nothing really major.
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