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Misfires - '02 JGC Limited...

15K views 60 replies 9 participants last post by  Kpurintun 
#1 ·
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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#4 ·
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?
 
#5 ·
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?
 
#6 ·
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.
 
#7 ·
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
 
#9 ·
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?
 
#10 ·
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.
 
#11 ·
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.
 
#12 ·
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.
 
#14 ·
I have red good experiences from other people regarding Seafoom. Its worth to try and would be a cheap solution if it works. Its not available where i live, so i have no experience with it.
 
#16 ·
I was just reading about misfire on the 4.0 engine. There it can happen that carbon buildup on the exhaust valves can prevent the valves from closing completely. It will especially happen when normally the engine RPM is below 3200. The valves should normally rotate a bit, whereby eventual carbon will be removed, but at speed below 3200 RPM the valves will not rotate and the carbon buildup starts. So it can be that the same is valid for the 4.7. Seafoam could help you out in that case.
 
#18 ·
Alright. I did the 1/3,1/3,1/3 seafoam thing. They say you can leave it in and run it.. Just keep a close eye on the oil condition. I also put it into my gas and ran it thru a line. All kinds of smoke poured out. Hopefully in that smoke was my problem.

Cross ur fingers

Kyle
 
#21 ·
do you think i should run another couple rounds of the stuff thru the vacuum lines a couple days apart?

kyle
I don't think so. When I ran it through my Mustang I actually let it stall out the car when it was pulling it into the vaccuum line. Once it stalled out, I let it sit for 5-10 minutes before starting it up again.

This way the seafoam was in the entire air system and would sit and work after it stalled.
 
#22 ·
I don't know how agressive this stuff is. I should say that when you don't get the CEL anymore, to leave it as it is. But maybe someone else knows it better and can give you a better answer.
 
#25 ·
So maybe the next step would be to check the compression. When your engine is running smooth, then most probably all cylinders are working normally, but maybe the respective cylinder which is shown as misfiring has a lower compression and consequently lower output, what gives a slight deceleration what the PCM sees as a misfire.
 
#27 ·
i just bought a compression tool....

i have until the end of may to get this figured out...

ug..
A compression test is VERY easy to do. To prep, first you need to remove all your spark plugs. Then remove the power wire to the ignition coil (or disconnect all coil packs). Unplug your injectors so you don't have fuel sprayed into the engine. Also pull the throttle cable so the TB is completely open.

Screw the gauge into the spark plug hole of the first cylinder. Jump in the car and crank the car over 4 or 5 times. Record the reading from the gauge. Screw the gauge into the next plug hole and repeat. Finally, compare your numbers to the numbers Jeep gives.
 
#28 ·
Just had same issue, combination of things happened. First off i need a new computer(pcm) it is running rich as can be and the sensors tell the comp to lean the engine out but it wont. i am missing on about 5 to 6 cyl at random. what i did is when it was missing i had no injector ground pulse from pcm on some injectors and some coils not getting ground pulse from pcm. dissconected battery took off unbolted pcm harness, put togather and the misfires switched to opposite sides of engine. anyway i need a comp. and becasue it was running so rich there was so much carbon build up, i had low compression on cyl#3. all others reading were 160-170psi
the #3 was first time 60psi then 70 psi. all missfires went away above 2000 rpms....weird.... jeep pcm has pre set determined values the pcm puts out at certain rpm ranges and thats why missfires went away above 2k rpms. i took off left head toook to maching shop and #3 valve needed a new valve job like somone else posted valvestem seal was leaking oil plus lots of carbon build up !!! looking at everything i can see the mess of carbon build up becasue engine running the richest it could for a long time and finally paid its toll on the head. it al stemmed from my computer so after head gets back from machine shop taking it to dodge to program new computer
 
#30 ·
ug!...

i am not sure... and i am probably jumping the gun...

but after like 6 or seven serious sea foam treatments... my wj pulls harder than ever. and it hasn't seen a misfire in over 50 miles. maybe it was just a shitty piece of carbon on a valve or something...

i haven't had the time to do a compression test. i have been traveling a lot.

ill let you know. i am gonna try to get the emissions test done today. if that works out.... WIN. if not... i'm crashing it into a bridge support.

kyle
 
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