Compression loss

I've been noticing a loss of power on my 134 recently. No knocks, pings or bad engine sounds. Rebuilt the engine about 5000 miles ago and was running great until I let a friend drive it for a day. Checked compression, 120,118, 96, 120. Put a teaspoon of oil in #3 and rechecked at 120. So I'm thinking piston rings are bad. My plan is to leave the engine in the vehicle and remove the head and oil pan, pull the #3 piston, replace rings and reassemble.

Anything wrong with that plan?
Do I need to replace rod bearings at that time? only if worn?
What else to look for?

Thanks

 

Have you done a valve adjust since rebuild? If the intake is not opening properly, It won't have as much to compress. May not be it.

 

I think you have nailed the problem with the oil in the cyl.

As to the plan on fixing it, sounds like it should work just fine, be sure to get all the radiator coolant out before opening up the head.

Rod bearing only if needed, IMHO.

Dont be surprised if you find a broken ring------if so, look for a vertical scratch in the bore-----you may not be able to just hone it out

If the rings are not broken, just worn, be sure to check the skirt clearance. Too much allows the piston to rock slightly in the bore, and that will wipe out the rings.

Also, check the ring groove in the piston----needs to be square and flat. The seal is between the top of the ring, and the top of the groove.

Good luck

 

You should also check the ring end gap. It should be listed with the other engine specifications. Be sure and check it at the bottom of the stroke if there is any wear in the bore. Too much gap will let a bit more fuel/air pass but too little and it will bind in the bore and score the cylinder.

 

Before you rip 'er down, squirt some oil, in the low cylinder.
if a ring is worn the compression will bounce back up.
A old mechanic teacher taught me, the first bounce of the comprssion gauge is the valves, the last 3-4 are the rings.
If it's cracked, you may be screwed as it MAY have scracthed the cylinder too bad to hone out.
If the comprssion doen't bounce back up,
Bring the piston to T.D.C., put it in gear and blow some air into the cylinder, (remove the oil filler cap, air filter).
If it's a valve, you hear the air out the tail pipe, carb., a ring- the oil filler cap.
I've done a few rebuilds, (in the jeep), it's easy, just messier.
Make sure you remove the ridge on the top block before you go pushing the piston out, (you can rent a ridge cutter and it takes 3 seconds to cut it off. If not, and there is a ridge you could break the piston ring landing.
Make sure you make sure which way the piston facing, as well as the rod cap.
I'm anal, I'd do the rod bearings, mains, rear seal, (if nothing else check the thrust main bearing for wear whilist your there), the "earl" pump.
I check the piston gap in the cylinder at the top, middle bottom, (some bores will taper, you can use a "rigid" hone to true it if it's not too bad.
I take a piston, keep the rings on it and use that to slip the rings into the cyclinder to make sure it even. Then take the top ring off, the the oil, (some have 3 comprssion.
Make sure you know which way is up on the rings also.
Check the piston clearance on the skirt.
But that's just me, if I'm gonna lay under there and get oil all over me, I'm gonna do it all.
Joe
Best part of owning a jeep, there's always something to do to it, but it still runs!

 

only 5000 miles, just fix what is needed. and as far as replacing mains in a F134 if you have figured out a way to get past the dowels that hold that top main bearing shell in place do tell!! X2 on using compressed air to find out where the problem is, may not even have to pull a piston

 

Ok,

I finally found some time to work on it. I got #3 piston near the top on compression and blew aircompressor into the spark plug hole. Air comes out the tailpipe, but no air from oil filler neck. Moved the piston a bit and air comes out the carb. But no detectable air out of the oil filler.

Does that point to the valves needing adjustment or do I still need to tear down piston 3??

Thanks in advance.

 

The exhaust valve is leaking.

 

Make sure you,re on compression stroke on the cylinder you,re checking. If you,re on exhaust stroke, you,ll be at valve overlap and a little movement either way will open either the intake or the exhaust valves.

 

The way I'm making sure of compression stroke is.... that with a flashlight I can see the intake valve open and then close... then I crank it to piston at the top and check that it is with a bendable wire.

That should be right I think? yes?


Now I've got the exhaust valve adjustment cover off of the engine and the #3 valve tappet spins easilly as if there is zero pressure against the valve or valve spring. It is the ONLY tappet that will spin in this position.

My feeler gauge allows .016 room between the valve and tappet. but it still spins easily. Is it right or wrong?

 

Ok,

Here's my problem again. My 134 has a compression loss on cylinder #3 at 96 or so... all others are at 120 or very close. I squirted some oil in #3 cylinder and the compression came up to 120. I asked the question in my first post above. A couple of guys made a suggestion of trying compressed air into #3 while at top dead center, and check for where air comes out.

I watched with a flashlight for the exhaust valve to open and piston coming up, then intake opening and piston coming up again. stopped at top.

Took of exhaust valve cover on the side of the engine and re adjusted valve #3 to .016 with feeler guage at TDC. The tappet and nut rotate freely with fingers. Should be set right I think.

Used compressed air to check again while in gear and TDC on #3.

Here are the results:

Oil filler tube: nothing
Exhaust port on engine (manifold is off): positive small amount of air flow
Carburetor: nothing


Exhaust valve is new at less than 5000 miles ago. I believe it's properly adjusted and visually can see it seated, yet it still leaks air/ compression?
Not a lot mind you, but some.

I don't get any air from the oil filler tube that I can tell, so what I thought previously about rings being bad, must be wrong....?

I guess I'll put it back together and do another compression check.

Ideas?

 

sounds like a broke or weak valve spring or something holding the valve open, carbon, or some other form of crud

 

I second the valve spring. Personally, I'd pull the valve out and give it a good cleaning, check for carbon, etc,. Replace the spring if I could.

 

Ok, I put it all back together and rechecked compression again. 120.
I have no idea what changed. I checked twice before at 96 dry and warm, I did tinker with the valve adjustment screw and reset it, but I don't think I changed it much at all. But now it has compression at 120 all four cylinders.

(shakes head).

 

A couple of posts ago you said you adjusted the valve to .016".

That is what changed.

You are lucky to catch this when you did, sounds like the valve was just barely off the seat-----continued running like that will etch the sealing surfaces and you would have had to pull the head and do a hand lap at best, and ----well, dont dwell on the worst case scenario.