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Bad compression in cyl 6

V

vozo112

NAXJA Forum User
Location
long island
I tried researching it but couldnt find much on where to go from here.

i have a 97 xj 5speed 231tcase with 170k. it always ran like a top. in the last 5k miles i started to notice a studder. recently i started getting a p0306 code. so i changed plugs, wires, cap and rotor and sea foamed it. the light came on about 1000 miles later. i just ran a compression test. the numbers were
cyl 1: 160
cyl 2: 160
cyl 3: 165
cyl 4: 160
cyl 5: 165
cyl 6: 105

soooo. bad compression in cyl 6. and its definately lacking power and misfiring here and there. so where should i go from here? new head gasket and get the head checked out? or should i be more worried about the lower end? what ever i have to do to get it running like a top again.
 
Hold your horses man, it sometimes takes a while for someone to come along and answer.

Make sure it isn't the rings first - drop about a teaspoon of oil into the cylinder through the sparkplug port then spin it over a time or two and retest. If the compression reading goes up, your rings are on their way out, if it doesn't, it's a head gasket, head, or valve issue.
 
yeah, I'd guess valve issues unless it's been overheated. Maybe ran lean and burnt an exhaust valve?
 
What does the spark plug in #6 look like after a good run with a clean plug, oil soaked, dry black or clean, or? What did the old plug look like.

Pull the valve cover and look at the valves and springs on #6, see if anything is visibly wrong there.

You might try some gum out in the intake and MMO in the oil and run it hard to see if it changes. But that is a huge gap in pressures, and I doubt snake oil additives will do enough to close the gap as much as needed to buy any time before a hardware fix.
 
if by any chance you can get your hands on a leak down tester set cyl 6 to TDC and put 100psi in her...check and see where you hear air coming from, check tail pipe (exhaust valve) open throttle plate on the intake manifold would indicate bad intake valve issue if you hear air coming from either one. currently im running into a similiar scenario except i have cyl 1 and 6 on my 96 5spd I6 xj i did a leak down test had 24% and 27% leakage on those cylinders, put a bore scope in both of those cylinders and had a Sh*t ton of carbon causing my exhaust valve not to seat properly which in turn gave me a miss on those cylinders...not saying your vehicle is the same as mine but what i concluded after testing...give it a look see and let us know
 
Outlander560 said:
if by any chance you can get your hands on a leak down tester set cyl 6 to TDC and put 100psi in her...check and see where you hear air coming from, check tail pipe (exhaust valve) open throttle plate on the intake manifold would indicate bad intake valve issue if you hear air coming from either one. currently im running into a similiar scenario except i have cyl 1 and 6 on my 96 5spd I6 xj i did a leak down test had 24% and 27% leakage on those cylinders, put a bore scope in both of those cylinders and had a Sh*t ton of carbon causing my exhaust valve not to seat properly which in turn gave me a miss on those cylinders...not saying your vehicle is the same as mine but what i concluded after testing...give it a look see and let us know
Click to expand...

Interesting, I started to mention the carbon stuck open valve idea in my first post here. Interesting to see someone confirm it this way!!!
 
so i pulled the head off. my motor was super gummed up. the intake and the cylinders were covered in carbon build up. so i sent out the head to get checked and revamped. but i just noticed something weird... i was cleaning the block and the tops of the pistons and i noticed that piston 6 doesnt reach the top like the rest of them. all the pistons get to about 1/32" of the surface of the block. but cyl 6 only gets to about 3/16" to the surface of the block.. geez. what does this mean.... should i be worried? or should i just throw the head back on all done up and hope the problem is gone...

oh and a side note. the intake was all gummed up with carbon build up and a reddish looking oil... any idea what this might be?
 
check your bearings, wrist pin, and make sure that conrod isn't bent.

Was it wheeled or driven through flooded streets at any point by you or a previous owner? Hydrolocked?
 
kastein said:
check your bearings, wrist pin, and make sure that conrod isn't bent.

Was it wheeled or driven through flooded streets at any point by you or a previous owner? Hydrolocked?
Click to expand...

Like the man said, pull the pan and give it a good look see. What did the bore look like on #6?
 
well..... i did hydrolock it about 55k miles ago. but its run great ever since. until a little while ago. im going to try to pull the pan after i get it put back together. what do your mean the bore?
 
I bet when you hydrolocked it, it slightly compressed or kinked the conrod for #6, which is why the piston is a bit lower now. It's somewhat of a testament to 4.0 durability that it then proceeded to turn out another 55k miles before you had any real issue with it.

I'd drop the pan and double check that before putting the head back on, you can't get the pistons out the bottom but they come out the top real easy. So if you drop the pan now, you could probably just buy 1 (or possibly more if you see other kinked/bent rods, I would bet on #1 being a distinct possibility) new rod and some new rod bearings, stick em in, close it back up and run it for another few hundred thousand miles. If you wait, you may have to pull the head again.
 
yea thats exactly what i was thinking when it happened. i couldnt believe i had no problems. well i am BETTING on the durability of the 4.0 because i am gonna put it back together and hope for the best. i am going back to new york in 4 months from puerto rico so it only has to last until then because i will sell it. i only can hope i guess....
 
I want to throw something out here.

You pulled the head and that might not have been necessary.

MCCC--Mopar Combustion Chamber Cleaner available at the dealer might have removed enough of the carbon to make pulling the head unnecessary. Sea Foam might have done the job too.
 
If the rod is bent but the bore is still good, and the crank journal OK, I probably would just hone the cylinder and replace the piston and rod.
 
I think its a 4.0 thing. Ive had 3 break the ring land on number 6 in the same spot each time. 3 different motors with varying mileage and care. I think the cylinder gets a hot spot on one small section where theres no coolant veins and no air flow due to location against the firewall/trans tunnel. Thats just my guess though.
 
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