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Ascending: under load..2002 Jeep Grand Cherokee Limited..auto trans…

Customer Question
Symptoms: Massive power loss...
Symptoms: Massive power loss and lurching starting when ascending steep highways, but ~90% loss of power (really limping) continuing when no longer under load. <br /> <br />Vehicle: 2002 Jeep Grand Cherokee Limited w/ 4.0 Cy engine and auto trans. <br /> <br />So far, I have heard three theories, each from a skilled expert, and none seem to be provable. Two of the three agreed and commented that replacing anything until the problem is reproducable is silly. Well, it's reproducable now, but the verdict still isn't in.<br /><br />It looks like either the fuel delivery system, the ignition coil, or the ECU/EEPROM.<br /><br />The first theory was by a Jeep dealership. The Jeep was towed to them initially as I was out of town. The dealer wanted to do over $800 of fuel delivery system repairs. <br /> <br />A mechanically talented friend who has two Cherokees and does all his own repair work didn't think the dealership was correct, so I took it to my regular mechanic upon return home. (my initial post confused two separate incidents he experienced, please ignore) One issue he had was a microfracture in the ignition coil, and the other turned out to be his EEPROM fried his crankshaft and camshaft sensors.<br /> <br />My regular mechanic back in town (Mario) was able to reproduce the lurching and power-loss on his second attempt today. He had some sort of diagnosis equipment hooked to the Jeep and the data doesn't point to a clear answer yet.<br /> <br />The Jeep was initially at his shop six months ago for these symptoms, at which time Mario was unable to reproduce them. Since then, I have only commuted to/from work in town (very flat) around 7 miles each way, and I haven't experienced any massive power losses (~90%, it totally limped the time it happened). <br /><br />Mario initially thought ignition coil, or maybe fuel pump/filter but isn't convinced and doesn't like throwning darts. He also hasn't ruled out the ECU.<br /> <br />When Mario reproduced the problem today, he initially thought it was the coil, and proven so. However, he thought about it some more, and noted a few curious items. All cylinders showed a significant miss-timing in firing (and subsequent power loss), but cylinders 5, and 6 suffered the most. Given the 'coil' is 3x groups of two, why would cylinders 5, and 6 be worse than the others if the coil was at fault? Can just one set of two be more corrupt than the other two sets? Why didn't it just go completely out if it was the coil, and how did it last six more months in quiet city traffic w/o biting the dust? Etc... <br /> <br />He also noted the computer didn't throw any codes despite the massive miss-fire. This makes me wonder if the ECU thinks the firing is correct (due to it potentially being the culprit telling the coils to miss-fire?). <br /><br />So, the question could either be "how to isolate the bad component(s) before dropping $ ?", or "In your experience, lurching followed by a ~90% power loss in 2002 Jeep Cherokees (grand, limited) is caused by or often missdiagnosed as, but turns out to be"?<br /><br />I realize this question isn't exactly concise, but not sure how else to put it.<br /> <br />I should also add that one month prior to the lurching/power loss incident, the vehicle gave me the Jeep equivelent of Microsoft's BSOD on the way to work, and again just as I pulled into home. The Crankshaft sensor was replaced, and all was well again.<br /><br />PS:  Sorry for the poor formatting, it looks great in the text box I'm submitting it in...
Submitted: 8 years ago.Category: Jeep
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Answered in 2 hours by:
6/25/2010
Jeep Mechanic: CDJMasterTech, Jeep MasterTech replied 8 years ago
CDJMasterTech
CDJMasterTech, Jeep MasterTech
Category: Jeep
Satisfied Customers: 1,608
Experience: Factory trained and certified Jeep Master Technician, ASE Master Technician
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You are right in that without duplication of the symptom, with test equipment attached, any repair will be a guess. In that vain, you or your mechanic needs to have a fuel pressure gauge attached, so fuel pressure can be confirmed while the problem is occurring. Also a scan tool should be hooked up, preferably with a data record feature. I need to know what the oxygen sensors are reading both during the symptom and shortly before. We need to be able to isolate the problem further before replacement of any parts. In answer to your question about the coil rail, it is possible for only one coil to fail, causing a misfire on only two cylinders. It is also possible you have some carbon tracking on one or more spark plugs, this causes a misfire when load on the engine is increased, as when load is increased, more fuel is added to the combustion chambers, requiring more electrical energy to fire the plug. At this point it becomes easier for the spark to follow the carbon track down the porcelain on the outside of the plug to ground, resulting in a misfire. It would be a good idea to take a very close look at the spark plugs and boots. When a carbon track forms, it forms concurrently on the spark plug porcelain, and the inside of the rubber boot. If you replace only the spark plugs, the track will reform. You have to replace the plugs and the boots, (coil rail on your application), to cure the problem permanently.
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Customer reply replied 8 years ago
Ty kindly for the response, and I'll ask for the O2 readings when I speak to Mario tomorrow. I'm not sure what he hooked up, but it was using some sort of diagnostic tool which should in all likelyhood include what you are after. When he was unable to reproduce the problem six months ago, he did replace the plugs, so I will also ask about the boots/rail. If it's that simple, that'd be great! Also, just to clarify, all six cylinders were miss-firing, but cylinders 5, and 6 much more-so than the others.

Is it safe to say in your experience it's the coil or coil rail causing the problems, and not the ECU or fuel delivery system? Since $ is tight, I'd at least like to play the percentages. I'm leaving in a couple of weeks on a long road trip, and I'd hate for this to crop up just as I'm hitting a highway w/ no cell service which is the case here in Northern Nevada (Carson City). Murphy is lying in wait.
Jeep Mechanic: CDJMasterTech, Jeep MasterTech replied 8 years ago
Understand that I am working only from your description and information, but with what I have, I would say that the odds are pretty good that the coil rail and plugs are the problem. If the vehicle were in my shop I would recommend replacement of these parts first, as they are relatively inexpensive compared to hours of diagnostic time that could be required to confirm they are bad. It is unlikely to be a problem with the PCM, (Powertrain Control Module), but anything is possible. There is a small diode that is mounted to the block just below the coil rail, be sure to replace this also. It is a dealer only part, but less than $10. This diode absorbs the voltage spikes from the coils, and I have seen them fail and cause strange misfire conditions.
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Customer reply replied 8 years ago
Awesome!, I accepted the answer, and will follow up tomorrow w/ the O2 readings as requested just looking for a quick response on your take of the O2 levels. I'll provide feedback then. Just curious, if the O2 is low, or high, does that point to the fuel delivery system? Ty again!
Jeep Mechanic: CDJMasterTech, Jeep MasterTech replied 8 years ago
If the oxygen sensor voltages are low or high just before the failure occurs, then that would indicate a problem with the fuel system, most likely a lean condition. This is why we want to also have a fuel pressure gauge hooked up, as the most likely cause of a lean condition would be low fuel pressure caused by a failing fuel pump. Just get back to me whenever you can with the information.
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Customer reply replied 8 years ago
I spoke with the mechanic, and the O2 levels were normal prior to the missfires. Mario felt pretty confident the fuel system is OK. He did however find the following TSB and after reading it, it sure sounds to me exactly what I experienced. I'd be interested to know if this is an issue you encountered often? http://www.wjjeeps.com/tsb/tsb_wj_0900303.pdf Thank you again for your assistance.
Jeep Mechanic: CDJMasterTech, Jeep MasterTech replied 8 years ago
No, the TSB you are revering to is not going to be your problem. The problem in this TSB is a light misfire, not a misfire associated with 90% power loss as you stated. A mechanical engine condition like this is unlikely to be your problem, as a mechanical condition is almost never intermittent. A power loss of 90% is most likely a severe failure with the fuel or ignition system. Mario may think the fuel system is ok, but the only way to know for sure is to check it. We need to get some fuel pressure readings while the problem is occurring before we can rule out the fuel system.
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