Burnt valve..now bad piston

timw586

Member
Joined
Jan 18, 2011
Location
Charlotte, NC
Originally had a miss on cyc 1 and P300 and P301 check engine codes on my 96' Jeep Cherokee 4.0 Compression test showed 40-45PSI on number 1, and 130-155 psi on the rest of the cyclinders. Put a dab of oil in number one and rechecked, went up 5 psi. Moved on to the leak down test, and constantly had air blowing out the intake. Yes I am sure it was TDC, and still had air coming out the intake. So i determined it had a intake valve issue. Took the valve cover off, and removed the rocker arms for cylinder number one, to ensure both valves would be fully closed. Did another leak down test, and still had air coming out of the intake. It wouldnt even hold enough air to put a percentage on it, so 100% loss. Well I got the head off this evening, and of course piston number 1 has an issue. The rest look normal, and even the head gasket looked fine. The engine has 240k miles on it, and ran like a champ besides a obvious miss at idle and check engine code. The original plan was to have a valve job and throw it back together. Yes I know the increased compression might blow out the rings on the existing pistons, but I was willing to chance it. Does it piston look bad enough warrant a rebuild / replacement? I cant figure out what would cause just the edge to chew up instead of the center. The combustion chamber looked fine, never had a spark plug issue on the cylinder either. Let me know what you guys think? Oh and I forget to include, the cylinder walls on not scratched or scarred.

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Well my current options are go with a used complete motor for $200-300 with 150k-200k miles on it (with the "craigslist promise" that it runs correctly)...But a guy local to me has a *brand new* 0630 casting head that includeds springs and valves for around $200. I was quoted $150 to check my old head and do a valve job. I think im going to go get the new head, set it on there and do a compression check. If all the numbers look good, put it back together. If not I'll hone all the cylinders, replace that one piston, throw new rings and bottom end bearings in it..and then sew it back up. Cost is of course a factor, but doing this job more than once anytime soon is also a factor.
 
I think I'd put a piston in it while i had it this far apart. It's not that much more to replaced it now vs. tearing the head back off again. What does the head look like on that cyl?
 
Someone on another forum recommended I take the head off, turn it upside down and level, and pour water into the combustion chamber to verify a leak. I got the intakes off the head, and started looking at it. The intake valve does have a small place in it where it looks to not be sealing. I tried the water test, and sure enough the water leaked thru the intake valve and out of the head. I snapped a few more pics of head gasket, i think you may be right and the gasket may have been blown right at the front. Guess I am going to go get the new head and head gasket set. Put that on and run a compression check. Should I try the water trick on the piston in question to see if it holds? I can use a shop vac to suck it back out before it rusts. I hate to get this back together and waste a head gasket and set of bolts, only to find out I still have low compression on #1. But at the same time I really dont want to pull the pan and piston. Heres some pics, let me know what you think. Oh yea, is it normal for your intake runner to be so gunked up? :beer:




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The new head is a done deal, pick it up tomorrow at 2pm. Came from this place http://cylinder-heads.com/ What would happen if i dont replace the piston? I intend to bold down the head, and then do a leak down test. If I have good compression, what could happen with the piston?

I would replace the piston, but then I would end up replacing ALL the rings on all the pistons, and all the bearings in the bottom end. And by then, I might as well pull the block and get it machined instead of half assing the rebuild.

So If I have good compression with the new head, what could happen with this piston?
 
I've been into more 4.0's than most. I see stuff like this very, very often. That being said, replace that piston.
You can get a replacement piston/rod combo from Crown Automotive reletively cheap. All you would need to do is drop the oil pan, unbolt the rod and pop it out. Then re-install the new rod/piston, torque to spec and be worry free.
You will have a color code on the pass side of the block associated with each cylinder. That color will be needed to order a replacement piston (unless machine work has been done).
Its really no more difficult than taking off the head. Do it right or you'll just have to do it again.
 
If I am going to go thru the trouble of dropping the pan and piston, should I just go ahead and hone all the cylinders andput new rings on all of the pistons? And throw all new bearings in the bottom end? And of course the rear main seal? How effective will this be without getting the crank and block machined?
 
Assuming the other cylinders were fine prior to these repairs, I would only address the cylinder in question.
Hone the suspect cylinder, replace the RMS while you are in there and call it a day IMO.
Or, just rebuild the whole thing. But I see no reason for that based on a burnt valve and a questionable piston.
 
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