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Unread 11-23-2011, 01:11 PM   #1
DKFormula
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Misfiring diagnosis - looking for spark

02 Jeep liberty 3.7 v-6 with 106 k miles. Overheated bad a few months ago due to a broken lower radiator hose clamp. My wife was driving, it died and she restarted it several times to get it home. Changed the oil, it was black. It ran rough and misfired on cyl #2. I checked the plugs and changed them because they had very large gaps. It ran better and we have been driving it for a few months without codes or misfireing. Yesterday, I drove it and it started misfiring bad. Codes for random misfire, #2 misfire, and #3 misfire. I checked compression thinking it was shot from overheating but it was good pressure >175 psi. I checked fuel press ok. I am checking for spark but I cannot see any spark on any cylinder. I checked all but #4 cyl. Obviously some of the cylinders are sparking because it runs. What am I missing? Any other suggestions before I bring it to a dealer for a PCM check?

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Unread 11-23-2011, 06:07 PM   #2
crackerclicker
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i'd like to offer some helpful ideas, but when dealing with an overheated condition, i really don't know what to say . have you seen this thread? http://www.jeepforum.com/forum/f28/b...asket-1161343/

if you have access to a scantool that can graph fuel trim numbers that might lead us to a better conclusion. does it feel like it is misfiring at any particular rpm or load?
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Unread 11-23-2011, 06:08 PM   #3
streetglideok
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How are you checking for spark? What kind of plugs did you put in, and what did you gap them to?
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Unread 11-23-2011, 09:35 PM   #4
DKFormula
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I don't think I have a blown head gasket. No white smoke, no water in the oil and no missing coolant.
I am borrowing a very nice scan tool from a friend this weekend. I'll look at as many things as I can think of including the fuel injector pulse width.
It misfires almost continuously from idle on. Under very light loads, it sometimes feels like it is firing on all cylinders but I can't be sure. This is another thing I will check with the scan tool when I can see the cyl misfire count. I did get it to run smoothly by keeping it at a light throttle at highway speeds after the engine got good and warmed up.
While driving it home, several times I turned the engine off and restarted it. After restarting it, it would usually run better (mil light on solid) until I pushed it too hard and it got worse (mil light began blinking).
I put ACdelco plugs in it gapped at .040 inch just like the sticker says.
I am checking for spark by pulling the coil and plug, re-attaching the wire harness to the coil and inserting the plug, then grounding the plug threads with a short wire to a good engine ground (verified with an Ohm meter), turning the key and looking for a spark. Also, I pulled the fuel pump and fuel injector fuses so I don't spray fuel into the cylinders.
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Unread 11-23-2011, 10:22 PM   #5
crackerclicker
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pay particular attention to the short and long term fuel trims during the misfire. if you feel the misfire (which you should if your eng. light is blinking!!), then let us know if your trims are in the +/- 0 to 10% range or if they are more positive or negative than that. fuel trim is a great indicator of rather a misfire is air/fuel or ignition related. it is usually easier to analyze this data in a graph form over a period of time, but i know scantools with that capability aren't always available.
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Unread 11-24-2011, 07:42 AM   #6
streetglideok
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You're overcomplicating spark check. Best way to check spark, is get an actual spark tester, real cheap little tool that does wonders, as it allows you to adjust the gap and load test the coil. There should be some metal that you can lay a spark plug on, the bolts that hold the intake to the head will work, without need to run any special ground wire. The fact that you cant get a spark from any of the coils says, its not working. Next thing, acdelco doesnt make a direct crossover to the NGK plug your engine takes. Heatrange will be different etc. Really need to stick with what it calls for. Partnumber for the NGK plug is right next to the plug gap on the sticker. See which cylinders are misfiring, and look at the freezeframe data. Are the trims positive or negative, etc. Now if its a dead miss, fuel trims will be affected by that condition, but from the sounds of it, you only experience this problem under certain conditions.
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Unread 11-26-2011, 11:14 AM   #7
DKFormula
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OK, I have the scanner. I ran it for about 20 min last night while looking at the data streams. The scanner showed no misfires at all but it was running rough and chugging along feeling like it was missing. I was perplexed until I got home it smelled hot so I looked underneath and the right (pass) side cat was glowing red. So I thought I either have a plugged cat or a leaking fuel injector. I pulled the right side fuel rail and injectors and none of them were leaking, turned it over and each of the three injectors sprayed a nice pattern and did not drip.
I ran it with the scanner again this morning. The fuel trims were high on the right side. When accelerating with partial throttle open it was up to +21%. This morning drive did cause some misfires again. First came Cyl #2, not surprising, that's on the right side. But then came a cyl #3 misfire. That's funny since it is on the left bank. So, once again, I am stumped. Are cyl #2 and #3 ignition signals fired from the same output of the PCM?
I have experienced a plugged cat on a different car before. It was similar to this but different. That car had one cat and the engine barely ran. This Jeep has a sound like it is chugging so I am temped to change the cat. But I am hesitant because if it is a fuel or spark problem, it will be money wasted. I am open to ideas. Thanks
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Unread 11-26-2011, 12:28 PM   #8
DKFormula
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Ok I drilled a hole in the exhaust pipe just upstream of the cat. On the last car I had with a plugged cat, you could hear the backpressure comming out of the hole. It sounded like a small jet engine. Well, no jet engine sounds on the liberty, so I am guessing no plugged cat. The last test drive had cyl #2 #3 and #4 misfire codes set and I even noticed a few cyl #1 misfires on the counter as well. I think this is pointing to a PCM but still looking for suggestions... Thanks
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Unread 11-26-2011, 06:44 PM   #9
streetglideok
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Rule#1) Whenever you have concluded you have a bad PCM, go back and recheck whats really going on, then proceed to rule#2 if you get the same outcome
Rule#2) See rule#1.

PCMs do go bad, but not nearly as often as they once did, and typically, its the last resort when grasping at straws. It goes in the categories of, "it jumped time", and "that cat must be plugged".

A positive fuel trim in your case, means it is adding fuel to the precalculated fuel charge. +25 means its increasing the fuel delivery 25% over what is expected. Keep in mind, actual fuel trim is a combination of short and long term fuel trims for a given bank. If bank 1 STFT is +10%, and LTFT is +5%, you're at 15% enrichment. A severe misfire will cause it to read lean, since the air/fuel charge never burned, aka a dead cylinder. Also, its not uncommon, when encountering a bad misfire, that a few other cylinders will have low misfire counts. If one cylinder is at 250, and the rest are 1-10, you can all but ignore them. Have you checked engine manifold vacuum? Hook a guage up to the vacuum line going to the brake booster. Start the engine and let it idle. Is the needle steady, or does it flutter? Flutter indicates a problem, like a sticking, or stuck open intake valve. Vacuum that starts out about normal, but steadily decreases towards zero indicates possibly an exhaust restriction, which sounds like you ruled out. Your cat is glowing hot most likely due to a bad misfire on that bank, thats dumping raw fuel into it, and its burning it there. It will melt down, and you will be replacing it soon if the problem isnt fixed.
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Unread 11-26-2011, 08:58 PM   #10
crackerclicker
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what was the fuel trim doing on the other bank?

i agree with streetglide in that you should test the vacuum.
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Unread 11-26-2011, 09:07 PM   #11
tjkj2002
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A 12v test light and a wiring diagram of the coil drivers and injector drivers is all you need to test that part of the PCM.Noid lights can make testing the injector drivers very simple with no wiring diagram needed.


Though I'd be doing a compression test 1st.
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Unread 11-27-2011, 11:36 AM   #12
DKFormula
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The left side fuel trim was close to 0 while the right side was at 21 when the throttle was open. Right side dropped back towards zero when throttle was closed.
-- The misfire counts on #2 and #3 were nearly identical - thru the roof tripple digits. #4 was much less (low double digits) #1 was single digit misfires. I did assume that #4 and #1 misfires are secondary affects of the primary failure.
-- I have looked at minifold vacuum via the scan tool. It is looking what I consider to be normal, high teens at idle, lower numbers when I open the throttle, higher numbers when the engine is at higher speed and the throttle is suddenly closed. I don't think the scan tool is fast enough to show any "flutter" although I guess I could graph it and maybe see variation. I don't have a vacuum gage. Maybe I will pick one up this afternoon to play with.
-- I thought that my good compression test numbers ruled out any valve sealing issues. See my first post.
-- I think I will go get a test light and a vac gauge so I can dot the i's and cross the t's.
Thanks so much for the ideas and questions. I'm hoping to figure this out eventually.
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Unread 11-27-2011, 12:19 PM   #13
streetglideok
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Compression readings are important, and will show if you have a bad sealing issue, but a valve may seal under non firing compression, but will leak when the fuel charge ignites, due to significantly more pressure. They can also cause issues that a compression guage wont catch, but a pressure transducer will, but thats beyond your tooling. A vac guage isnt expensive, and will give you some idea of whats going on in the intake. The scan tool readings wont read sharp enough or fast enough to see a problem. A labscope on a map sensor would though. Your misfire occurs when you give it throttle, and your misfire counts coincide with this, as well as the sharply positive fuel trims, correct? If so, and your vac readings are stable, even with the throttle opened enough to cause misfires, then take the #2 and #3 injectors, and put them on cylinders 1 and 5. Reset the fuel trims thru the scan tool and clear codes, then redrive. See if your misfire moves.
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Unread 11-27-2011, 02:12 PM   #14
DKFormula
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I need to get back to my original question. I have no spark. I tried it again today. When I pull the coil and plug, reassemble, give the plug a good ground, and crank the engine; I see no spark on any of the cylinders. I even turned off all the lights in my very dark garage to see if maybe the spark is just faint. But I see no spark. How is this thing running at all with no spark? What else besides the PCM could cause no spark in all cylinders?
Thanks.
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Unread 11-27-2011, 06:03 PM   #15
streetglideok
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Quote:
Originally Posted by DKFormula View Post
I need to get back to my original question. I have no spark. I tried it again today. When I pull the coil and plug, reassemble, give the plug a good ground, and crank the engine; I see no spark on any of the cylinders. I even turned off all the lights in my very dark garage to see if maybe the spark is just faint. But I see no spark. How is this thing running at all with no spark? What else besides the PCM could cause no spark in all cylinders?
Thanks.
You are overcomplicating the problem. You have spark on most of the cylinders, as it runs right? Your method doesnt work, so you need to check and see if the power is being switched on and off at the coil connector, if you wont get a spark tester to do this. You either dont have a good enough ground, or likely, not good enough contact within the coil to the spark plug to get an arc. Word of warning, you must be very careful checking for power being switched at the connector, as the contacts are easily damaged. Powerprobes and testlights are well known murderers of such connectors. Paperclip sometimes work. Power is fed to one terminal(battery voltage), and the other side is grounded by the PCM whenever it wants the coil energized, and pulls the ground when it wants it to fire. Switch the coils to a different cylinder if you want to rule out a coil being a problem. Ive seen a guy fight with a vehicle for a day, because he didnt check the ignition system properly, and blindly threw parts at it all day. 15 minutes after I looked at it, easy to spot fuel delivery problem. If you cant get even known good components to work during a test, there is a problem with the test.
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