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Old 10-14-2010, 01:20 PM   #1
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All 6 cylinders misfiring??

2001 Sahara 4.0L auto with 120K miles. Dealer "fixed" a problem I was having where the jeep was just shutting down while driving. Replaced the crank and camshaft position sensor and PCM a couple of weeks ago.

Now it is "bucking" or "lurching" when I am on the highway and at about 60 to 70mph. Takes it about 15 or so minutes of driving to start to do it... Codes say P0300 through P0306. A misfire in every cylinder! When it starts having this fit it will only accelerate to about 60mph. I can pull over and kill the engine, wait five or ten minutes and it runs fine again until hitting 60 or 70mph then the same thing happens all over again.

I changed out the plugs (put Champion platinums in...the kind that Jerry Bransford said were good to go) and correctly gapped them .035. Jeep is still doing this.. Fuel pressure is fine also.

Thinking maybe I have a bad coil rail...any way to check this or anyone have any other ideas?

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Old 10-14-2010, 01:29 PM   #2
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Check that they didn't get a spark plug wire swapped.

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Old 10-14-2010, 01:34 PM   #3
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Check that they didn't get a spark plug wire swapped.

I have a coil rail. No plug wires.
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Old 10-14-2010, 01:46 PM   #4
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have you looked at the timing chain/belt? It's not common, but a loose chain can look like bad crank/cam sensors, and I've seen them have intermittent problems sort of like what you're describing...
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Old 10-14-2010, 01:48 PM   #5
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I know this sounds wierd but it fixed another jeep at the shop, remove the driver airbag and then remove the steering wheel clockspring. Clear all the currrent codes and drive the jeep, if the problem still exists it probably needs a new coil rail, if the problem is no longer present you will need a new steering wheel clockspring. Please respond if this helps, Also be careful driving without an airbag and disconnect the battery and dissapate any capaciters by touching the negative cable to the positive for a minute or so.
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Old 10-14-2010, 01:53 PM   #6
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Have not checked the timing belt yet. Gotta do some research and see where it is.

And the clockspring? I would not know how to even get the airbag out without causing myself a black eye or busted lip getting it out of there...lol.
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Old 10-14-2010, 02:24 PM   #7
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I thought the 6 cyl's had a chain?
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Old 10-14-2010, 03:40 PM   #8
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They do. I just have to figure out where it is. HA! I need a damn manual for this thing..
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Old 10-14-2010, 03:47 PM   #9
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It's right there on the front, just gotta pull off a bunch of stuff to get to it... It isn't a minor/easy thing to check, but they do get old... not likely to be stretched badly at 120K, but it could be.

The airbag and clockspring would probably be easier to try first
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Old 10-14-2010, 03:52 PM   #10
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Are you sure when they replaced the PCM that they didn't reset the rev limiter to some ridiculously low number?
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Old 10-14-2010, 04:20 PM   #11
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...you checked the catalytic converter? They make you bog at higher speeds when they're clogged. Weird but true - had it happen. 60mph was my top end.
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Old 10-14-2010, 04:25 PM   #12
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...you checked the catalytic converter? They make you bog at higher speeds when they're clogged. Weird but true - had it happen. 60mph was my top end.
But would it cause the computer to throw codes for misfiring?
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Old 10-14-2010, 08:13 PM   #13
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I have no idea. Backpressure might cause it, but I'm really not sure. The symptoms sound the same as a cat, though...

I may have no idea on this one, lol.
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Old 10-14-2010, 08:20 PM   #14
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that really sounds like a timing issue....prob. the chain.
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Old 10-14-2010, 08:46 PM   #15
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I'm a mechanic & a wrangler owner. I had a similar problem with a Ferd Tortoise 3.0L.
Peformed a tune-up, etc.. 2 months later a random misfire code would appear. Way to soon for that to happen. It turned out to be bad wires even though they were only 2 mos. old. If all 6 plugs are misfiring, it must be something common to them all. Such as, the coil rail. The higher the RPM the greater the likelihood of a misfire occuring. Does the check eng light flash or is it steady?
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Old 10-14-2010, 08:57 PM   #16
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The CEL flashes at first and then is steady. Can kill the engine and restart and it is steady also.
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Old 10-14-2010, 09:00 PM   #17
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It flashes at first and then is steady.
And.. That would make sense to me. All cylinders are misfiring so something common to all of them such as the rail. And it ONLY happens with higher RPM'S. Can kill it and putter around town no problem. Get it out and open it up for a little while and it happens..
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Old 10-14-2010, 09:25 PM   #18
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Getting ready to sell this damned thing. The engine compartment and undercarriage is CLEAN.. An older couple hauled this Jeep behind their motorhome for the past few years. It is driving me nuts and I need reliable transportation..thinking a I6 was bulletproof..
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Old 10-14-2010, 09:42 PM   #19
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Could be the coil rail like you suspect. But not likely - when it fails it's only 2 cylinders affected, not all 6. It has 3 coils, each independently feeding 2 cylinders each. Someone with very good hearing may hear it "snapping."

Most likely the cat - plugged exhaust - typical symptoms.

Hook a vacuum gauge to manifold vacuum.
Note the reading at idle.
Rev it to about 2500 while watching the gauge, it should remain steady slightly lower than it was at idle. Hold it up there for a minute or so. It should not slowly sink. Rev to 3500, same thing.

Idle again.
Snap the accelerator hard, reving it to about 3000. It should drop to near 0 while it accelerates, then hold slightly below idle reading. Then let go - let it snap closed. The vacuum should bounce up fast to the idle reading or above, then drop back to idle.

If either one of those fails, exhaust is plugged. Inspect the pipes for smashed areas before condeming the cat.

Go underneath with a laser thermometer - measure the exhaust temp before and after the cat - right on the inlet and outlets - if the cat's working, "after" should be at least 100 degrees hotter. No, I didn't say it backwards.

A muffler shop can do a backpressure test to verify it's the cat before replacing.
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Old 10-16-2010, 07:28 AM   #20
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Well, I unhooked the negative lead on the battery overnight night before last and drove it as normal on the highway and it did not misfire, no codes shown or CEL light. I wanted to clear the PCM all together... But not sure if that really has anything to do with it.

I did some research and found a similar post at jeepforum that sounded like my problem. The valves may be bad/dirty which can cause a random misfire code at high speeds and moderate RPM's. When the engine misfires it places the engine into "limp mode" (which sounds like the problem I was having). Gotta get to the valves and make sure they are rotating properly. When you reset everything like it should be there is a procedure where you heat soak for a while and then drive in low gear a few times... I'm definitely no mechanic and can't do this on my own.

I havent checked the cat yet and will more than likely have to take it to the shop to do it..
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Old 10-16-2010, 08:03 AM   #21
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sometimes a fella needs to throw the code book away, and use the old "eyes-ears-smell-taste" technique, lol. Cars all basically act the same. I would be really surprised if all the problems you think are happening just showed up, it would be a gradual thing.

A plugging cat, in my previous vehicle, indicated a serious engine oil leak due to a broken piston (didn't smoke!) - while I really hope you don't have a problem like that, with 120K on the jeep, the cat may have reached the end of its life anyway.

ETA: I just bought an '01 sport myself, 54K miles! Have fun with it!
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Old 10-16-2010, 09:20 AM   #22
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54k on an 01? Wow, that's pretty good. I bet it's in good shape..
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Old 10-18-2010, 07:52 PM   #23
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OK, update for those interested or having the same problem. I went ahead and went with my gut and bought a new coil rail. Checked the plugs again and they looked fine. Put the new coil rail on and went on about a 100 mile trip and drove it hard. I saw the CEL flash twice and go off during the trip but it drove absolutely great. Checked for codes on my odometer and it did not show any at all.

Hopefully I have fixed this mystery problem headache of mine. Time will tell..
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Old 10-18-2010, 07:55 PM   #24
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not sure about the 4.0 but my 4banger had fits simmilar to what you described and fresh o2 sensors helped, worked for me but im no mechanic.
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Old 10-20-2010, 07:56 AM   #25
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Dang! Drove another 100 miles on my way back home and halfway through the trip the CEL came on constant. But no noticeable misfire or loss of power. Checked the codes and now is saying 0300 and 0305. Another ramdom misfire and now cylinder 5. Funny thing is, its not acting like it is missing and not acting like it was before I changed the coil rail AT ALL. Arrrggghhhh....
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Old 10-20-2010, 10:44 AM   #26
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Of course you read post # 19 - and did the tests as described BEFORE you experimented with a new coil rail.

What were the results you found?

Your symptom/complaint is NOT an uncommon one.


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Old 10-20-2010, 11:43 AM   #27
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He's code happy. He didn't think our posts were any good.

Why, when I was a young whippersnapper, I drove by the seat of my pants, and the sound of parts falling off my car. Going to the autoparts store and saying "what's this?" began my long experience with cars.
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Old 10-20-2010, 12:34 PM   #28
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Of course you read post # 19 - and did the tests as described BEFORE you experimented with a new coil rail.

What were the results you found?

Your symptom/complaint is NOT an uncommon one.


.

I did, just did not post it. Took it to a muffler shop here and they hooked some sort of gauge up. Said the cat was fine. And Hambubba, I appreciate your input...but no point in being a smarta**.
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Old 10-20-2010, 01:22 PM   #29
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How is the battery and charging system doing? What is the voltage reading at the speeds it sets the codes? Does it read normal at slower speeds, then go lower at speed?

A bad battery or defective alternator could cause a misfire.

Don't go off half cocked and replace them until you post the result here.


The plugs - you obviously did NOT use the plugs the engine was designed for. Look at the underhood sticker - firewall, driver's side.

It tells you in no uncertain terms what plugs YOUR engine was designed to use.

Using plugs that were not designed to run in that engine can cause all sorts of trouble. Just because someone uses plugs designed for something else "gets away with it," doesn't mean you can.

Just because it "fits the hole" doesn't mean it will work seamlessly.

A set of the "factory recommended" plugs will set you back much less that what the coil rail "guess" cost you.
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Old 10-20-2010, 03:11 PM   #30
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I had the battery and alternator tested last week and they were fine. No idea what the voltage is at the speed the CEL comes on and "says" there is a misfire. I don't have, nor have access to, the equipment needed to check that.

The ONLY things I have replaced is the plugs and the rail. $100. Since replacing those, it has stopped the bucking and loss of power nonsense. Just showing a random misfire.

I do not have the money to go "half cocked" and replace everything I think it "could" be. Thanks for your advice though. Maybe I'll change those plugs again.

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