Low Engine Compression

[Sulu10]

I just ran a cold compression jeep on my jeep, found compression of 155,150,60,60,120,120 at each cylinder. What does that mean and what is the repair?

Background:
Bought a 4.0 straight 6, 97' TJ Sport 3 years ago. So far I've replaced the floorplans, replaced 3" lift with 1.5" lift, replaced fuel skid, fuel pump, body rust holes, rewired the headlights, replaced the roll cage, installed steps and replaced the steering column. It's had the death wobble as long as I had it and always had it, and never made it to highway speeds well.

Recently I installed a frame splice (required removing most of the guts) near the rear passenger tire, and in the process noted the exhaust was completely disconnected at the catalytic converter. I welded the system back together and went over the weld with JB. Everything back together and it was great for a while then threw a check engine a week later. OB2 scanner couldn't find a system when plugged in, but cycling the key read code 43; misfire. New plugs, wires and a distributer didn't help. Disconnecting, then reconnecting the battery turned off the light for about 10 miles. I thought it was a damaged O2 sensor suddenly participating in the exhaust system. On a whim I tried my buddy's compression gage and discovered the readings above. I don't have much of a mechanical history and have been learning the whole way. I'm hoping this is just a new experience for me, but don't know where to start diagnosing, let alone repairing.

 

[Link]

Squirt a little oil in the low compression cylinders and check the pressure again. Does the pressure rise? Probably rings. Does the pressure not rise? Probably a valve issue. Not a 100% accurate test but a good guide to where to look next.

 

[Sulu10]

Oiled, retested, //60/65/140/140. Pressure on 5 & 6 rose, but not much change on 3 & 4. Thanks for the advice!

 

[JKP]

Probably a blown head gasket between 3 and 4.

 

[williambmac]

How many miles on the engine?
I'm not sure what your mechanical capabilities are, but you should probably do a leak down test to cylinders 3 and 4.
I'm pretty sure there was a tsb (technical service bulletin) on the 97-98 TJ's for weak valve springs not allowing the exhaust valves to completely seat.
There was also another tbs for carbon build up on the exhaust valves.

While it could also be a head gasket as @JKP mentioned, I would want to rule out a valve spring/carbon issue prior to removing and replacing a head gasket.

 

[Link]

Definitely going to need to pull the head. At a min it'll probably need a valve job and new gaskets. Might have to do an inframe rebuild....or at worse an out of frame rebuild. What did the spark plugs look like? When you get the head off...look at the piston tops. If you see a lack of carbon around the outer portion of the pistons, you will likely need piston rings. Rotate the engine with the head off and look inside the cylinders. Any major scoring?

Honestly, I think there's a good chance it just needs a head gasket set and a valve job.

 

[B1Toad]

You don't say how many miles on the engine but if reasonably low you could probably get away with a head gasket like everyone is saying. Be sure to have the head checked for cracks though. two adjacent cylinders with the same low compression is a sure sign there is a leak between those two from a blown gasket or a cracked head. The 97 heads are not really prone to cracking like the 2000 but when they crack it's between 3 & 4.

 

[swtrailboss]

Look for excessive needle bounce when testing the #3 &#4 cylinders compared to all the other cylinders testing good. Definitely sounds like a head gasket issue. Sometimes if its leaking between two adjacent cylinders you will get the excess fluctuation on the compression gauge during the test. Good Luck!!

 

[Sulu10]

How many miles on the engine?
I'm not sure what your mechanical capabilities are, but you should probably do a leak down test to cylinders 3 and 4.
I'm pretty sure there was a tsb (technical service bulletin) on the 97-98 TJ's for weak valve springs not allowing the exhaust valves to completely seat.
There was also another tbs for carbon build up on the exhaust valves.

While it could also be a head gasket as @JKP mentioned, I would want to rule out a valve spring/carbon issue prior to removing and replacing a head gasket.

279,454 miles young. When I swapped out the spark plugs none seemed sooty, and the gaps were all about right...
Never torn apart an engine before... but theres always a first I suppose!

 

[Link]

279,454 miles young. When I swapped out the spark plugs none seemed sooty, and the gaps were all about right...
Never torn apart an engine before... but theres always a first I suppose!

Ouch. That makes a total rebuild more likely. However, I'd plan on a head gasket set and take the head to a machine shop to be checked for flatness and the valves re-ground. Clean plugs are a good sign though.

 

[roverhi]

Take the head to a machine shop. They'll tell you what needs done to it to make better than new. Personally would have them install new guides. springs, and do the valves even if it's just a head gasket problem you've got now. You will then only need a reman. short block when the bottom end gives out from all those miles. Once again the machine shop is your friend but they will probably tell you you need an overbore, new pistons, Conn and crank bearings, oil pump, gasket set and other wear effected parts on the outside of the motor. If you do the work yourself, a rebuild isn't that costly and can be done with little more tools than a torque wrench. If you don't/can't tackle it on your own you can just buy a reman. short block, reinstall the head and drop the engine back in.

 

[Sulu10]

I can't find a lead on any shops in the area that do small work like this.

It does look like the center of the head gasket between 3/4 was burnt out! Perhaps that is why the pistons were level on 3/4? maybe that's how they are supposed to be...

 

[NashvilleTJ]

I can't find a lead on any shops in the area that do small work like this.

It does look like the center of the head gasket between 3/4 was burnt out! Perhaps that is why the pistons were level on 3/4? maybe that's how they are supposed to be...

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No, that’s definitely not how it is supposed to be. You likley found your problem. While you have it apart carefully check the cylinders and heads for any other cracks or signs of damage. Also check that your heads and block are true (flat). Nice work,

But I think your real problem is that the head gasket is clearly bent in the back…

 

[tr21triton]

if you have a NAPA parts store nearby I would stop and ask them if there's a good machine shop close. they usually know where they are....with that many miles I would have the head reworked while you have it off for sure.

 

[JKP]

The pistons move in pairs. 1 and 6, 2 and 5, 3 and 4.

There should be a machine shop in your area.

If not, Clearwater makes new heads.

 

[Link]

Yep there's your problem.

Definitely have the head worked. Valves ground...maybe new valve springs. Also, they should check the head for flatness. They can grind it down to make it perfectly flat and that's a good idea to ensure you don't get another failed head gasket. Alternatively you can buy a rebuilt head and give them yours for the core.

Pistons look good. I don't see any signs of oil burning. Is there much of a lip at the top of the cylinder? Any scratches in the cylinder bores?

Remember you need new head bolts too.

 

[tr21triton]

Might have to do an inframe rebuild....or at worse an out of frame rebuild.

is this the inframe rebuild your talking about ?

 

[Sulu10]

Yep there's your problem.

Definitely have the head worked. Valves ground...maybe new valve springs. Also, they should check the head for flatness. They can grind it down to make it perfectly flat and that's a good idea to ensure you don't get another failed head gasket. Alternatively you can buy a rebuilt head and give them yours for the core.

Pistons look good. I don't see any signs of oil burning. Is there much of a lip at the top of the cylinder? Any scratches in the cylinder bores?

Remember you need new head bolts too.

Got the new headbolts on my bench at the ready! Lip at the top of the cylinder... not sure. The clambers are beveled at the gasket? Nothing rough. No cracks that I can see...

Slight warping which can be taken down, and will supposedly run. Sounds like extreme heat pitted somewhere in the valves, so they recommended replacement (at an extra cost of course...).
I'm worried about investing more in the upper and still having issues because of the cylinders. 3/4 being at the same elevations still has be worried. Am I over reacting? Is there a way to check? Just trying to figure how deep this issue might run...

 

[tr21triton]

well you a choice fix it or don't ! do it right the 1st time or don't ! 280k miles, you have the head removed, if they recommended new valves do it now. I would order 2 sets of bolt and 2 head gasket. put it back together and if it fixes your problem, take it back apart and take the head back to the shop and have them put new valves in it then put it back together AGAIN .....

 

[OldBuzzard]

... still having issues because of the cylinders. 3/4 being at the same elevations still has be worried...

As stated above (post 15), that's normal. You should be really worried if they were not at the same level.