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Unread 01-13-2013, 04:12 PM   #1
McGR
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2007 XK Commander 
 
Join Date: Aug 2005
Location: McHenry, IL, US/IL
Posts: 267
Figured out my misfire...

A few weeks back, just before Christmas, my son was complaining that my old Grand Cherokee that I gave to him was running like garbage and eating through gas like crazy. We decided to take a closer look...

The check engine lamp was on, so I decided to check codes. When I checked the codes and came up with multiple misfire codes P0300 etc. and an oxygen sensor code P0171. I initially started chasing the obvious suspects such as spark plugs, sensors and fuel injectors to no avail. While, I did get it to run slightly better, it still idled like junk and was setting misfire codes.

Eventually, I decided to perform a power balance by disconnecting the injector leads while monitoring RPMs with my ear. My ear basically told me that I wasn't really losing much RPM when 3 & 4 fuel injectors were disconnected. After I proceeded to swap around injectors to no avail, I decided to do a compression test (I know - this should have been the first thing I did, not the last).

I really didn't think I was going to find anything as while the engine is high mileage (203K), I had always kept up with oil changes etc. . This being the case, I was somewhat shocked to find cylinders 3 and 4 running around 60 PSI. In order to further determine the cause of the low compression, I then added a little transmission fluid to cylinder 3. When I performed the next test my compression didn't seem to change much, however I did experience atomized transmission fluid coming from the cylinder 4 spark plug hole. Looks like I probably have a bad head gasket...

This morning I confirmed my suspicion. When I pulled the head, sure enough there was a gap in the gasket between 3 & 4 (see pics).

Anyway, as long as I'm tearing into the engine I'm taking this opportunity to do a semi/(half-a$$?)-rebuild on the engine. I'm going to clean the head, lap the valves, and install new stem seals. More importantly, I'm going to install new pistons, rings, rod bearings and lifters. I'm replacing the pistons because it's had fairly substantial piston slap over the last couple of years. I quiets down fairly well when warm, but the sound has always disturbed me... so, we're putting in new pistons.

This brings up a question. I'm thinking I'll simply just install new pistons without bothering with honing the bores. My thoughts are:

1. There is no way I want to pull the whole block.
2. I'm afraid if I hone the bores with the crank installed, I'll contaminate the crank and bearings.
3. I'm not too sure I can get a hone into the rear cylinders anyway.
4. My cylinder bores look to be in real good shape.
5. It is my teenage son's car. It doesn't have to last forever maybe a couple more years of local driving.

So, should I hone it? Is this a fatal mistake if I don't? Let me know what you think...








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Unread 01-13-2013, 04:27 PM   #2
busadave
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2001 WJ 
 
Join Date: Feb 2009
Location: pa
Posts: 53
either replace the head gasket and put it back together. or pull the motor and do a complete rebuild. I tried short cutting the same way you are planning to and contaminated my bearings doing the hone with the engine in the jeep. it started up ran great took it for a 1/2 hour drive and by the time i got home she was a knockin. ended up buying a reman engine and droppin it in.
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Unread 01-13-2013, 04:38 PM   #3
McGR
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busadave - that is exactly why I'm inclined not to hone the cylinders, I don't want to ruin the crank. My thoughts are to simply replace the bad parts and put it back together and put it back in service.
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Unread 01-13-2013, 04:40 PM   #4
pdpunome
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Location: , NJ
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i had the same exact gap in my head gasket. just cleaned the head and valves up put back together that was at 160k now i am at 196k and truck runs great! i didnt hone, shave, lap or anything the engine was so clean i didnt think it had to be done which i was right the piston slap or tick is typical and unless you use rotella t5 oil it will not go away nor will it cause damage that is if it is the typical tap till warm and disapears.
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Unread 01-13-2013, 04:50 PM   #5
busadave
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Just sayin. I wouldn't waste my time or money installing new pistons and rings in old, most likely out of round cylinder walls.
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Unread 01-13-2013, 06:03 PM   #6
nickblack
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^ x2
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Unread 08-24-2013, 11:10 AM   #7
McGR
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Join Date: Aug 2005
Location: McHenry, IL, US/IL
Posts: 267
I thought I'd do an update on my half-a$$ rebuild...

Well it's been about 7 months and 6,000 miles since I replaced the pistons, rings, rod bearings, head gasket and performed a valve job. I happy to say the 4.0 has been running fantastic ever since. I did eventually decide to hone the cylinders prior to installing the new pistons/rings. I was just very careful to cover up the crank bearings during the hone and I cleaned them up good afterward.

All of the prior symptoms (misfiring, piston slap etc.) have been remedied. Absolutely no issues, it runs like new. Overall, I'm very happy with my low $ rebuild. It cost a little over $400 to perform this job. I figure the engine is good for a least another 100K now... money and time well spent.
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