Piston Rings

cherokeedon · 10-02-2007, 02:52 PM

98 Cherokee sport. Inline 6 cylender. Daughters car that overheated twice. Replaced the lower radiator hose, thought it was leaking. Replaced the water pump because it was leaking. Filled fluids, started and the thermostast was bad. Replacedd the thermostat. Started and the car warmed up, then started blowing white smoke. Replaced the head gasket, intake/exhaust gasget. Replaced the plugs, wires, distributor cap, & button. Started the car and it ran real rough still, and blew smoke. Ran a compression check and 5 cylinders were at 150 ibs. and the #1 cylinder was at 5 lbs. Does this sound like the piston rings are shot? and is it normal for just one set to go out? Thanks

diskman_1 · 10-02-2007, 04:09 PM

Was the head worked on (milled, trued, re-seated etc)? If not, you just ate a head gasket. Rings would have to be severly shot to register 5# compression. Sounds like a headgasket or valve problem. Add a few shots of oil in the cylinder in question and test again. If compression increases then it's the rings.

Rock Kustoms · 10-02-2007, 04:30 PM

overheating can cause preignition (detonation) which is destructive to EVERYTHING. 5 lbs is SHOT, like a whole in the piston, cracked block or head, burnt valve etc etc etc. if it were me i would do a leak down test, that will tell you the problem, IMO compression tests are useless because all they do is say you have a prob, not where it is.

if you dont know, a leak down test works the same way, but you use shop air and presurize the cylinder. the only trick is the motor must be at TDC of the cylinder that you are checking (compression stroke ofcourse), and you push 100 psi into the motor. if your dip stick tube blows out of the motor, its a hole in the piston, or broken rings. air cleaner, intake valve. tail pipe, exhaust valve, bubbles in the radiator, head gasket or cracked block/head.

good luck

scotch · 10-02-2007, 06:30 PM

when i do a leakdown test i prefer to pull th oil cap first.. just in case the dipstick is really well sealed or jammed. this way you wont blow out a more critical oil seal somewhere, and then you just listen to the hole in the valve cover for the leak

Rock Kustoms · 10-02-2007, 09:58 PM

yeah, youll never blow out a seal, but it will blow through the PCV, which can be confusing, because youll hear it in the air box.

i thought some more about the piston rings themselves, yes after overheating they can loose theyre spring and colapse, which can potentially be another cause, which is prolly what you were thinking originally

ORJ · 10-03-2007, 02:41 AM

5 psi would be a token register on the dial. I would call it "no compression".

In addition to what Rock Kustoms said, I would also pull the valve cover off just to make sure the #1 valves are actually moving.

I had a like situation and mine was a hole in #1 piston.

SteveTheFolkie · 10-03-2007, 07:01 AM

First - dump a couple teaspoons of oil down #1 and re-do the compression check - if the compression comes up it's probably rings. If not it could be something really simple - like carbon build up on a valve that's not allowing it to close and seal - in which case it's a simple case of pulling the head, pulling the offending valve, dressing the seat and valve and putting everything back together -

Best of luck

Steve

AZ Jeff · 10-03-2007, 11:30 AM

If you are only registering 5 lbs on a "dry" compression test, all the oil in the world (to seal the rings) will not raise the pressure measurably on a "wet" test.

At that low a number, you have a MAJOR mechanical issue, as others noted, like one of the following;

1. cracked piston or small hole in piston
2. flame cut or seriously scored cylinder wall due to cracked ring
3. cracked valve seat or hole in valve seat
4. stuck valve due to loss of spring pressure and/or bent stem
5. cracked valve
6. seriously cracked head and/or warped head

Since I assume you did a cursory inspection of everything when you took it apart, you would have noticed items 1, 2 and 5 pretty easily. That leaves items 3, 4 and 6 as possible culprits.

The only way you are going to know for sure is to remove the head again, and start doing some careful examination and inspection. Giving the head to a machine shop for magnafluxing and flatness checking would help here.

Good luck.

10-02-2007, 04:30 PM	#3
Rock Kustoms Registered User Join Date: Feb 2005 Location: Wolcott, CT Posts: 3,848 Profile Gallery Classifieds My Jeeps	overheating can cause preignition (detonation) which is destructive to EVERYTHING. 5 lbs is SHOT, like a whole in the piston, cracked block or head, burnt valve etc etc etc. if it were me i would do a leak down test, that will tell you the problem, IMO compression tests are useless because all they do is say you have a prob, not where it is. if you dont know, a leak down test works the same way, but you use shop air and presurize the cylinder. the only trick is the motor must be at TDC of the cylinder that you are checking (compression stroke ofcourse), and you push 100 psi into the motor. if your dip stick tube blows out of the motor, its a hole in the piston, or broken rings. air cleaner, intake valve. tail pipe, exhaust valve, bubbles in the radiator, head gasket or cracked block/head. good luck

10-02-2007, 09:58 PM	#5
Rock Kustoms Registered User Join Date: Feb 2005 Location: Wolcott, CT Posts: 3,848 Profile Gallery Classifieds My Jeeps	yeah, youll never blow out a seal, but it will blow through the PCV, which can be confusing, because youll hear it in the air box. i thought some more about the piston rings themselves, yes after overheating they can loose theyre spring and colapse, which can potentially be another cause, which is prolly what you were thinking originally