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jasonsos 05-01-2012 08:24 PM

Howdy to all Jeepsters HELP: very ODD missing
Howdy to all Jeepsters!
I'm a new member as of today, 5/01/12. I will post a Bio of myself later on if that's all right, because right now I have a 'VERY' odd problem with my 2000 Grand Cherokee Laredo, 4.0 in-line 6 cylinder automatic with over-drive that neither my well trusted mechanic nor members of another forum I belong to can figure out: at a 55 mph stomp (floor-boarded) the over-drive (aka passing gear) kicks in, but just before it shifts out near 70--80 mph, there is a major 'miss out' like a power loss, or like if someone turns the key off--then quickly back on again, that is followed by a few regular kind of slight misses and then it shifts out into over-drive around 80 mph.
* My mechanic and I both thought it could be one of the spark coils or an 'RPM regulator,' but it does not have this missing out from a dead stop floor boarding through 1st, 2nd and 3rd gears. It ONLY starts this odd missing between 55mph till it shifts without letting off on the pedal. And, that first miss is a 'MAJOR' bumping kind of miss.
Has anyone ever had this problem, and if so, would you please share what this problem is before we start throwing parts at it that this disabled Veteran can't afford? Thank you kindly. Jasonsos

2005JGC 05-01-2012 10:51 PM

Re: Howdy to all Jeepsters HELP: very ODD missing
Welcome to the forum Jason!

I would be interested to see what your fuel pressure is... your pretty much chugging fuel at that rpm and throttle application and I wonder if your getting all the fuel you need. If its a hard hit I would agree it would more likely be a spark issue. Its hard to say without having it to hook a fuel pressure gauge up, hook a scantool up, monitor misfires and do a general inspection.

does it go back to normal once you let off and get back down to about 55?

jasonsos 05-03-2012 11:33 PM

Re: Howdy to all Jeepsters HELP: very ODD missing
Thanks 2005JGC: (name?) You have a point we hadn't thought of! But the first big 'bump' that feels like a sharp loss of spark just might be a loss of fuel pressure! My mechanic would know how to test the fuel pressure, I sure don't since being medically retired; my fender-bending days are behind me. *We changed the spark plugs 7 weeks ago BEFORE I had noticed this odd problem, so I think we could rule out a bad plug or it would be missing out all the time. Right? But the 'spark coils' could have one of them bad, but Gary, my mechanic, didn't want to, "throw parts at it," until we figured out what it could be. ** And to answer your last question: yes, after it shifts into over-drive it runs as smooth as a cat on a hot tin roof at 75 and over, and in the lower gears; there is no missing at all. Just flooring at MPH and just before the final gear (OD) shifts. If I let off the gas, it will not miss and then slowly I give it gas past where the miss would occur. I will have Gary do those tests you suggest! Thanks a ton, and I'll post and let everyone know if that works. He's gone for the Nascar and will be back next week May 8. Thanks again! Jasonsos

2005JGC 05-04-2012 12:33 AM

Re: Howdy to all Jeepsters HELP: very ODD missing
He may not find anything with my suggestions, your symptoms are odd... its hard to know without driving it for myself if your speed and transmission gearing have anything to do with it or if it simply a load percentage and under the right set of factors would happen at a lower speed in any gear. You may be looking at a crankshaft position sensor. Those are VERY common on the 4.0l and they can be intermittent where they cut out eventually leaving you stranded (sometimes starting the failure with the leaving you stranded ;) . You may be dealing with a worn oil pump drive or bad camshaft position sensor (oil pump drive is what your camshaft position sensor is mounted on.

I have also seen some random spark misfires from a failing coil pack on these. Are you sure you got all the boots properly over the spark plugs when you reinstalled the coil rail? if you didn't you would likely have much worse problems but possibly.

jasonsos 05-04-2012 09:10 AM

Re: Howdy to all Jeepsters HELP: very ODD missing
2005JGC: Hey! My Jeep looks as nice as yours! White too! I forgot to mention mine is a 2000 Grand Cherokee Laredo--4DR--2WD--only one previous owner (the wife, a school teacher)--garage kept--they never towed anything, no towing package or even a trailer hitch, until I bought it in late January. OK. to answer: my mechanic Gary, has done all the work. He replaced the spark plugs, so I can only hope he re-install the 'coil rail' properly, and having retired from Jeep after 25 years, having his own shop for 5 years now. He has won Awards for off-road designing and fabrications and racing as well, so I have every trust in his ability. Now, he did think it might be one of the coils in the rail miss-firing, but knowing me, he stated he didn't want to "throw parts at it" just to find out what that odd problem is. * NEW NEWS: yesterday, I drove the back country roads into town and noticed as I turned onto this road, I began going up a small hill at around 25mph, then I gave it a bit of gas while in 2nd gear and then--the same thing happened as the high speed over-drive floor boarding: there was a big miss--then a few misses-and then 3rd gear kicked in smoothly and the missing stopped. I pressed on at normal driving up to the speed limit of 55mph and the Jeep ran great. No missing out, but that was the first time there was that missing when it down-shifted to gain speed. **As for the 'load percentage' you mention; I haven't a clue. Gary never mentioned that, which it could be a problem??? And GADS! If it is as you say about the camshaft sensor, I'll ask Gary to test that, but I can't get stranded, so you got me on pins and needles--but not to worry: it only makes me MORE concerned to get to the bottom of this problem! Thanks very much. Wish you'd leave your name?

2005JGC 05-04-2012 10:58 AM

Load is just physically how much load is on the engine, like wot in a taller gear is going to be more load on the engine than 1st gear at 1/2 throttle, sometimes spark blowout can be induced with a higher load on the engine. BTW, It sounds like you have a great mechanic working on this for you! Just by the picture you paint he sounds MORE than capable of getting you back to 100%.

Andrew its my name. ;)

Sent from my SCH-I500 using Tapatalk 2

jasonsos 05-08-2012 05:35 AM

Re: Howdy to all Jeepsters HELP: very ODD missing
Thanks Andrew! That was my Grandfather's name on my father's side. He hated for anyone to call him 'Andy' and corrected them on the spot if they did. And I agree with you: I have a great mechanic who has become a friend! His wife and he both are very nice people, and he always has time to say hello if I drop by, usually to ask about this and the other problems he fixed. When he installed a new radiator (dumb luck there: I replaced the radiator cap, which was the original, and the new one blew a hole in the radiator with the new change in pressure!) I had him replace the sparkplugs, which were also the original ones (there was no area for a gap! He had never seen any plugs like those and we both wondered how the engine even started!) and he replaced that one large drive belt and didn't charge me labor to do that since all of the coulling was off anyway. But he has a 2005 in-line 6 engine that he suggested we could try putting the 'spark coil band' from it on mine where we could see if that would fix the problem. He rebuilt that engine from a wrecked Jeep to sale it someday. Nice guy huh? Anyways, thanks for your input which I will relay to him this week and see what he thinks. This is my 3rd Jeep, so I'm a Jeepster from way back. Take care and good riding!

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