low compression on #1

Bad month for my 96 XJ. 4.0 AW4 242. Mileage: 380,000 KM

Two weeks ago - PS lines pissing all over the exhaust manifold when turning in reverse. Changed out.
Last week - mild engine warming (few degrees warmer than normal) and rattling in the pulleys - bad water pump bearings wiggly all over by hand pressure (but not leaking) - replaced water pump.
Yesterday - Dead miss on Cyl #1. Compression test shows the following:

45
145
150
150
155
135

Dropped a couple tbsp oil in #1 and retest compression - 55

Noise like an exhaust manifold leak but don't see one.

Exhaust valve problem on #1?

What is the next step. Broken spring or stuck valve? Can I diagnose or do anything about it by just pulling the rocker cover? What else is most likely?

This is my DD and only vehicle.

I have done lots of drivetrain and gear stuff on this jeep and a head gasket on a mercedes 3.2, but have never had the top off a 4.0

(I like to work on it, but can it give me at least a week off?)

Well, can't give you engine 101 in the forum, but some advice:

As the oil added to the cylinder did not raise the compression significantly, then rings are likely NOT your issue.

You now need to determine where the compression is going--into the intake, out the exhaust, into the crankcase, or into the cooling system. Now, you could "assume" that the problem is the head gasket OR head/valves, and just go ahead and pull the head, have it cleaned, checked for flatness and pressure test the valve train. However, that could be a whole lot of work for a valve stuck open due to carbon.

So, doing a leak down test would make sense, or you could try a can of Seafoam or Mopar Combustion Chamber Cleaner into the intake through the vacuum booster hose, and then recheck the compression.

Something I most always do in a situation like that is look down the spark plug hole with a good flashlight. With the piston all the way down you can often see some of the cylinder wall, though not all. You can also see most of the top of the piston, especially if you move the piston up and down a little to improve the view. On number one it is likely to be hard to get a good view, a small mirror may help out some. A broken ring will often leave scratches in the cylinder wall. A broken piston will often have chunks of metal on top of the piston.

A leak down test is a good idea also. you can often hear where the compression is escaping to. Ear over the engine oil filler hole and you can hear the piston rings passing air, they will pass a little, a lot and you have ring issues. Open the throttle plate and put your ear near the TB opening, if an intake valve is sticking open at TDC you can hear it.

If you get the motor on TDC 1, with the valve cover off you can check the rocker height. The rockers will move a little sideways at TDC, there is not much pressure on the rocker from the valve stem or the push rod. If one rocker is noticeably looser than the other you may have a valve stuck partially open.

With a compression of 45# I would not expect a hole in the piston. Pull the valve cover and watch the valves and make sure they go up and down as you crank. If not, you could have a collapsed lifter or a rounded cam. Look for busted valve springs and bent push rods. Beyond that, you are going to need to pull the head to see what is going on.

Well, the seafoam magic fix in a can did nothing. Didn't expect anything from it.

Pulling the rockercover this morning to look.

Pulled the rocker cover - pushrods look straight, valve springs look good, everything seems to be moving as expected when I turn the motor.

Pull the head and check the valve seats and piston next I guess?

I would do a leak down test and narrow it to intake or exhaust valve first. Valve could be sticking or burnt.

My 1st post here!! I have just gone through this on my 99 XJ. So time to pay it forward a little... Put the questionable cylinder on TDC of the compression stroke. Connect your compression gauge fitting to compressed air and see what happens. If you hear air out the exhaust than it is the exhaust valve. Intake =intake valve. Out the breather= hole in piston/ broken ring. If your piston moved downward to BDC, than you are most likely looking at camshaft/ lifters. How do I know? Well I just did a camshaft and lifters. Let me know if I can help.

my 88 4.0 had low compression on cylinder 6. I took the valve cover off and found no movement in the intake valve. a lifter had collapsed from years of a lifter tick and ground down the camshaft. id suggest taking off your valve cover and turn the engine over as you have compression and it didnt increase when you added oil to the cylinder. sounds like a valve train problem to me.

I am hearing air on the exhaust side, not pulled the head yet. Would it be cheaper and easier to just pull the head and throw in another one to get me moving again if this is a valve problem?

I have a 95 parts XJ with what I believe to be a good head. Is it a direct bolt on swap for my 96?

How do you know its the head? Could be a lifter, broken spring, etc.

With as quick as the problem occurred.... and since you have already pulled the valve cover for inspection; I would look toward the lifters of that cylinder.
There are oil additives that can with used to assist in freeing up a sticking lifter....
I would go that route before you go to the trouble of pulling the head and discover it was only a sticking lifter.

Well, I thought I was going to see problems with the exhaust valve or the gasket. I don't. With the head off, everything looks okay. Valve seems to be closing and sealing. Head gasket looks fine. Water passages look nice and clear. No crud in the oily parts. (rocker cover was nice and clean too)

Rockers show surprisingly minimal wear. Piston tops look good, nice and clean. With a strong light I can just barely see the glint of the ring down the outside all the way round. Walls are beautifully smooth. Only gunk buildup is in the intake manifold and intake ports, but the valves seats all seem to be clean.

I was quite surprised how nice it looks for an engine with almost 400,000 k on it.

Now what. I am going to check the lifters out tomorrow. Where else should I be going with this?

I just reread my opening post - don't think I made it clear that after replacing the water pump I ran the engine and the temperature went into the red for about 30 seconds. I had forgotten to open the heater valve after refilling and had trapped air. I got the air out properly and all was good. Misfiring did not occur for several days of normal driving later. Related? Coincidence? I had also been getting po300 and 301 codes for several months but had no detectable misfire when driving. In fact I passed smog with great numbers. The codes went away after changing the front O2 sensor at the same time I did the wp.

I deserve to be talking to myself.

After additional reflection and testing and doing a fluid leak test on the pulled head, it is what i thought it was. Exhaust port on #1 passed liquid like a seive. I think I think too much. Or am I second guessing myself?

Some of the other exhaust valves have a very slight seeping also. I am trying to decide whether to give it to a shop or pick up a tool and some compound to grind the seats and valves myself.

Any suggestion?

Take it to a shop to go over it. Using compound is not grinding or a valve job. Its simply lapping the valves in. Which is a bandaid, but not a replacement for a valve job.

I decided to swap the head. Looking closely at the valves, the exhaust on #1 was way deeper in the head than the other valves-almost level with the surface. Several were leaking with slight warpage and some of the seats did not seem completely round.

I used the 94 head from the parts jeep. Pulled it, cleaned it up, checked for leaks, lapped the valves to get a nice surface, and put it all back together.

While I had it apart, I had my many manifold cracks tig welded at a local welding shop for 40 bucks and put in a new donut. Looks good.

It sounds okay, but I am getting a fair bit of smoke coming from the manifold side of the block after running for only about 10 seconds. I have not run it for more than 15 or 20 seconds at a time so far. Could this be from the crapload of degreaser I sprayed on it before taking apart? Something else?

It sounds good, but I can't see temp and after the last time I would rather know if it was getting too hot. Broke the sender when turning the head over and then it just rounded off trying to remove. I have a new one, just need to figure out how to get the old one out.

false alarm on the smoke, tranny fluid on the cross pipe.

running better than it has in two years.

Everything A-OK

BrokenSockets said:

false alarm on the smoke, tranny fluid on the cross pipe.

running better than it has in two years.

Everything A-OK

Click to expand...

Glad you got it done! :cheers: