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Old 03-18-2014, 06:59 PM
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Need Help Misfire after overheating..

Got an 02 grand cherokee Larado with a 4.0 (vin-code s) Water pump started leaking in january, the gasket was the culprit as the bolts apparently worked loose (discovered when I changed the pump this week) anyway it was too cold to work on it so i just added coolant as needed but as My wife was driving home the car overheated. Got it towed home, I replaced the water pump, thermostat and housing, which solved (so far so good) the coolant loss and overheating. However, it now has a misfire problem. PO300- random misfire P0301-302-303 which is a misfire on cylinders 1,2 and 3.

Are the two issues related? It runs very badly now, I haven't done a compression test yet (I've ordered a guage to do so) but the old fashioned method tells me there is still plenty of compression, I do not believe the engine was damaged when it overheated as she pulled off right away and shut it down. Plugs look good, but I have one of those long sealed coil packs so no real way to test it that im aware of. No coolant in the oil, or the exhaust as far as I can tell, but a head gasket could be the problem- but no hydrolock or anything out the back or in the crankcase??

any guesses as to what the problem might be? From what I've read, it's become a game of switch out the -very-expensive-parts till ya get the right one.
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Old 03-18-2014, 08:49 PM
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Re: Need Help Misfire after overheating..

There is a service bulletin for the 4.0 engine (but it seems to be applicable on the 4.7 as well), which mentiones multiple misfire conditions on the 4.0 due to carbon buildup on the valve stems (the valves become sticking due to the carbon and donīt close completely, causing the misfire). The carbon buildup happens when keeping the engine at low revs and driving short distances. When taking the engine at higher revs, the valves start rotating and eventual carbon will be removed. Here a video where you can see the valve (springs) starting to rotate, eventhough its here at a quite high rev. (watch the red marks on the springs)
This is probably not your problem, but its worth to try. You could use seafoam instead of the mopar stuff.
Could eventual be a problem with the coil pack.
Here is the TSB:
Attached Files
File Type: pdf tsb_wj_0900303.pdf (24.6 KB, 2 views)
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Old 03-18-2014, 10:32 PM
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Re: Need Help Misfire after overheating..

Yeah I'm pretty sure that isn't the problem. And the TSB for that problem leaves me where I am already - verify all other electrical or mechanical or fuel problems associated with the misfire.

Good information otherwise tho, thank you/
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Old 03-19-2014, 06:36 AM
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Re: Need Help Misfire after overheating..

The coil pack most probably isnīt the problem as well, because due to the three individual coils design, it fires two cylinders each time. 1-6, 2-5 and 3-4 fire at the same time (while one cylinder fires at the compression stroke, the other does during the exhaust stroke and is waisted)
I donīt know which years are affected, but there seem to be problems with head cracking. So lets see first what the compression test will show.
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Old 03-19-2014, 06:42 AM
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Re: Need Help Misfire after overheating..

you may have the cursed 0331 head that is known for cracking after overheating
here are some things to read
List your 99-02 4.0's with 200K or more (no head cycl. issue) - JeepForum.com

https://www.google.com/search?q=what...sm=93&ie=UTF-8
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Old 03-19-2014, 12:23 PM
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Re: Need Help Misfire after overheating..

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Originally Posted by Bushmaster69 View Post
Yeah I'm pretty sure that isn't the problem. And the TSB for that problem leaves me where I am already - verify all other electrical or mechanical or fuel problems associated with the misfire.

Good information otherwise tho, thank you/
You can also get the affect of that TSB by just intentionally using lower gears a bit, plus mine responded nicely to a full Seafoam treatment.

The most obvious problem would be the head gasket, but it takes a pretty good overheat on the 4.0 to cause problems there. Those three cylinders are together, so that still could point to a very small problem in the gasket.

You might want to do a cylinder leak down test instead of a simple compression check.

If this all checks out, it could just be sheer coincidence and your plug wires are getting old, your plugs are getting old, your PCM is getting old, you have a mild vacuum leak, etc. Plug inspection as you do the leak/compression check might yield clues.
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Old 03-19-2014, 01:21 PM
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Re: Need Help Misfire after overheating..

OK, I dont have my compression tester yet, but i do have a dreams (nightmares when it comes to vehicles), and in one last night I remembered an old 305 I had the same problem with. - Open the cap start it up and - I have exhaust coming out of my radiator! How I ever missed that I'll never know, but I did.

Here's hoping it JUST the gasket!
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Old 03-20-2014, 03:04 PM
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Re: Need Help Misfire after overheating..

OK, I got the valve cover off and everything preceeding that, the head looks good no sign of a crack. But I have a teal question here, is it better to take the head off with the manifold attached or is there a trick to "reaching" under the manifolds to get at the bolts? Also what is the size of those 12 point head bolts, looks like 1/2 inch but i'm not to sure. advice please?
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Old 03-20-2014, 04:04 PM
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Re: Need Help Misfire after overheating..

Quote:
Originally Posted by Bushmaster69 View Post
OK, I dont have my compression tester yet, but i do have a dreams (nightmares when it comes to vehicles), and in one last night I remembered an old 305 I had the same problem with. - Open the cap start it up and - I have exhaust coming out of my radiator! How I ever missed that I'll never know, but I did.

Here's hoping it JUST the gasket!
Since you'll need to pull the heads to find out, I'd look at a valve job, all new valve seals, while you are at it...

Bummer.

I'd suggest springing for a factory service manual or at the very least a haynes before you take this on unless you have beer buddies with good knowledge of this engine.

If you have a big budget, a stroker kit would be a nice present.
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Old 03-21-2014, 04:24 AM
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Re: Need Help Misfire after overheating..

Heh, heh, beer buddies are easy, just buy some beer and they crawl out of the woodwork... Only real issue I see here as far as the job goes are those intake/exhaust bolts underneath. I picked up one of those inspection cameras, hopefully it'll let me work under there. This one has the two separate exhaust manifolds which make it even more difficult to get at the flange bolts at the exhaust pipes.

This jeep don't owe me anything but if its gonna give me fits and quit putting out, I'm not buying it a diamond ring.
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Old 03-22-2014, 03:56 AM
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Re: Need Help Misfire after overheating..

Mystery solved. I have three hanging exhaust valves, 1,2 and 3. and this motor is filled with gunk, a lot of it- Probably one of those stop leak products _ I bought this Jeep used about a year ago_. I don't know how it ever ran in the first place with so much gunk in it. The heads not cracked, and no damage to the cylinders so I think we can save it. Looks like the overheat cooked the gunk an made it thick and caused the valves to hang.
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Old 03-25-2014, 07:29 AM
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Re: Need Help Misfire after overheating..

Oops, I meant three hanging intake valves!. Turns out the misfire and overheating were related by cause and effect. The Head is warped and I'm waiting on one now.
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