X1997J

I am having mis fire issues with my #6 cylinder on my 4.0

I did a compression test resulting in only 60psi on #6 cylinder (dry) wet test was around 120psi, leak down test I have air coming from the oil fill location on my valve cover. I am leaning toward worn cylinder rings. Noid light test on injector passed i also pulled the injector off the fuel rail and tested with a 9v battery and sprayed carb cleaner through it. I have good spark and even swapped spark plugs and fuel injectors to see if the miss fire moves to another cylinder and it didnt.

Some one told me to put Marvel Mystery Oil in the cylinder thru the spark plug hole and let it sit for a few days incase I have stuck piston rings and thats where I am at.

If I have worn or broke rings I was looking for some guidance or tips in replacing the rings.
Thanks!

awg

assuming the marvel oil offers no relief;

you could inspect the cylinder bore & piston top with a borescope that goes down the spark plug hole and connects to phone, they are cheap

If everything looks ok, you still would have to at least take the head and pan off

you dont say how many miles on motor

just replacing one piston in situ is very shade tree, but I have seen it done

X1997J

202,000 miles

i understand I need to pull the head and drop the oil pan to remove the piston. I think what I am trying to figure out is if I can simply pull the piston and replace the rings as long as everything looks ok. All the videos iam seeing about installing piston rings are from complete rebuilds.

exasemech

While in theory you can do what you want at minimum you would need to ream out the ridge which is undoubtedly at the top of the bore at this point and hone the bore if you want the new rings to seat.
this is of course assuming the rings are the only problem, which you don't really know at this point.
All of the above spreads grit and metal chips everywhere which are almost impossible to clean up in that situation.
About 45 years ago when I was working piece work in a rip off shop I was told to do that on some sort of old British MG or similar. I did it but knew it was less than 1/2. ***ed.

X1997J

Thanks for that info.
I know a complete rebuild or a swap is what I should really do but i don't have the money or space to perform the work. I think my real plan is to get it running and most likely sell it to someone who is willing to take the project on.

Randy Bobani

I've torn down hundreds of these engines and have never seen a significant ridge in a cylinder. Very uncommon. They all have a small amount of build up that can be wiped away with a little bit of carb cleaner and a scotch-brite pad.
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Yup, the rings lost their tension and are probably brittle. Worst case scenario is that the piston itself is broken or cracked. Time for a dingleberry rebuild.

You can't really install new rings without honing the cylinders. The rings will never break-in, the cylinders will wear out even more and you will be left in the same scenario with low compression and blow by. Do all cylinders so you never have to worry about it again, it doesn't take very long. I would order up some replacement rings, gasket set, flex hone, scotch-brite, carb cleaner, WD-40, micro-fiber towels, 10"-12" foam sanding block, 320 Grit longboard adhesive back sandpaper, RTV and your oil/coolant. You'll want to check the ring end gap of the new rings of course. The sand paper and sanding block are for cleaning up the deck surfaces for the new head gasket.

With the piston/rod assemblies removed, just lay some micro-fiber towels on the crankshaft rod journals and send that hone through. No need to worry about the hone's abrasive contaminating the oil. It's not that dirty of a job. Just stuff the lifter valley with micro-fiber.

There aren't many good videos of folks DIY honing their cylinders. They usually hold the drill like a snowflake, use way too little drill RPM and they stroke it way to slow. You have to rip that thing to get a good 30' cross-hatch and it only takes 5 or 6 strokes. Literally 4 or 5 seconds per cylinder. The best thing to watch is a Top Fuel engine rebuild between passes. If anyone tells you that you can't hone a block in-frame, tell them to go to a Top Fuel event. The most expensive engines on the planet are flex honed between each pass brother.



Good example here @ 5:06

X1997J

I’m guessing only replace the pistons if needed or should I replace them while I’m in there?

thanks for all the info and stuff to order.

Randy Bobani

I would replace them because the stock cast pistons are junk. If they look to be in good condition, you could just clean them up, no problem. On YouTube, there's an interview of an engineer for Chrysler who worked on the development of the Jeep straight 6 engine in the 80's/90's. He mentioned that the 4.0 was the most profitable engine in Chrysler's history and part of that was how they used the cheapest possible cast pistons. How you tackle this depends on whether this Jeep is a daily driver or not. If it is, and you don't know if a piston is cracked, you might want to be prepared with parts on deck.

You have to give thought to the rods and pins as well. Stock pistons/pins are press fit. If you decide to replace the pistons(or if you have to due to a cracked piston), consider getting bushed rods for easy install. Basically all new piston/rod assemblies. Free floating pins are more efficient anyway. If that is beyond the budget, get the MAP torch and hydraulic press ready. You could call or Email Tieson @ Clegg Engine sales@cleggengine.com (801) 226-3400. He could set you up with stock style pistons in either a Hypereutectic material or 4032 forgings. Russ Pottenger is another guy who sells Jeep engine parts russbnbracing@yahoo.com (626) 673-2203

X1997J

I see...what rings should I use?

DJKnightmare

X1997J
questions -
Did you get a set of the hypereutectic pistons? If so, did you find the correct rings?